Xiaodong Xu

All Publications

• Signatures of fractional charges via anyon-trions in twisted MoTe2. Nature Li, W., Wang Beach, C., Hu, C., Taniguchi, T., Watanabe, K., Chu, J. H., Imamoğlu, A., Cao, T., Xiao, D., Xu, X. 2026; 651 (8104): 48-53

  Abstract

  Fractionalization of the electron charge e is one of the most striking phenomena arising from strong electron-electron interactions. A celebrated example is the emergence of anyons with fractional charges in fractional quantum Hall effect (FQHE) states1-13. Recently, zero-field fractional Chern insulators (FCIs)14-19, lattice analogues of the FQHE states that form without Landau levels, have been realized20,21. FCIs provide a unique platform to investigate anyons, yet their detection remains a challenge. Here we report the observation of anyon-trions, a new type of excitonic complex formed by binding a trion with a fractional charge in twisted MoTe2 bilayers. Photoluminescence spectroscopy of quantum-confined excitons reveals emergent peaks that appear only within slightly doped FCI states. The new spectral features are red-shifted relative to the trions in undoped FCIs, but share the same electric field, temperature and magnetic field dependence. These observations suggest their origin as trions binding with elementary quasi-particles, that is, anyon-trions. Crucially, the ratio of binding energies between the anyon-trions in the -2/3 and -3/5 FCI states matches the expected fractional charge ratio of e/3 to e/5. This provides strong evidence for fractional charges in FCI-an essential property of anyons. Our results address a fundamental question in FCI physics and establish trion spectroscopy as a powerful probe of fractionally charged excitations, complementary to transport- and tunnelling-based approaches.

  View details for DOI 10.1038/s41586-026-10101-w

  View details for PubMedID 41639442

  View details for PubMedCentralID 3160145

• Atomically Resolved Acoustic Dynamics Coupled with Magnetic Order in a Van der Waals Antiferromagnet. Advanced materials (Deerfield Beach, Fla.) Zhou, F., Hwangbo, K., Ha, S. S., Zhang, X. W., Chun, S. H., Park, J., Eom, I., Jiang, Q., Yang, Z., Zajac, M., Kim, S., Choi, S., Chu, Z., Oh, K. H., Su, Y., Zong, A., Santos, E. J., Cao, T., Chu, J. H., Hruszkewycz, S. O., Gedik, N., Xiao, D., Kim, H., Xu, X., Wen, H. 2026: e22280

  Abstract

  Magnetoelastic coupling in van der Waals (vdW) magnetic materials enables a unique interplay between the spin and lattice degrees of freedom. Characterizing the elastic responses with atomic and femtosecond resolution across the magnetic transition is essential for guiding the design of magnetically tunable actuators and strain-mediated spintronic devices. Here, ultrafast X-ray diffraction employed at a free-electron laser reveals that the atomic displacements, wave vectors, and dispersion relations of acoustic phonon modes in a vdW antiferromagnet FePS3 are coupled with the magnetic order, by tracking both in-plane and out-of-plane Bragg peaks upon optical excitation across the Néel temperature (TN). One transverse mode shows that a quasi-out-of-plane atomic displacement undergoes a significant directional change across TN. Its quasi-in-plane wave vector is derived by comparing the measured sound velocity and the first-principles calculations. The other transverse mode is an interlayer shear acoustic mode whose amplitude is strongly enhanced in the antiferromagnetic phase, exhibiting eight times stronger amplitude than the longitudinal acoustic mode below TN. The atomically resolved characterization of acoustic phonon dynamics that couple with magnetic ordering opens opportunities for harnessing unique magnetoelastic coupling in vdW magnets on ultrafast timescales.

  View details for DOI 10.1002/adma.202522280

  View details for PubMedID 41622543

• Observation of dissipationless fractional Chern insulator NATURE PHYSICS Park, H., Li, W., Hu, C., Beach, C., Goncalves, M., Mendez-Valderrama, J., Herzog-Arbeitman, J., Taniguchi, T., Watanabe, K., Cobden, D., Fu, L., Bernevig, B., Regnault, N., Chu, J., Xiao, D., Xu, X. 2026

  View details for DOI 10.1038/s41567-025-03167-2

  View details for Web of Science ID 001674739100001

• Optical control of integer and fractional Chern insulators NATURE Holtzmann, W., Li, W., Anderson, E., Cai, J., Park, H., Hu, C., Taniguchi, T., Watanabe, K., Chu, J., Xiao, D., Cao, T., Xu, X. 2026; 649 (8099): 1147-1152

  Abstract

  Optical control of topology, particularly in the presence of electron correlations, is an interesting topic with broad scientific and technological impact1-4. Twisted MoTe2 bilayer (tMoTe2) is a zero-field fractional Chern insulator (FCI)5-10, exhibiting the fractionally quantized anomalous Hall effect11-14. As the chirality of the edge states and sign of the Chern number are determined by the underlying ferromagnetic polarization15,16, manipulation of ferromagnetism would realize control of the Chern insulator (CI)/FCI states. Here we demonstrate control of ferromagnetic polarization, and thus the CI and FCI states, by circularly polarized optical pumping in tMoTe2. At low excitation power, we achieve on-demand preparation of ferromagnetic polarization by optical training, that is, electrically tuning the system from non-ferromagnetic to desirable ferromagnetic states under helicity-selective optical pumping. With increased excitation power, we further realize direct optical switching of ferromagnetic polarization at a temperature far below the Curie temperature17,18. Both optical training and direct switching are most effective near CI and FCI states, which we attribute to a gap-enhanced valley polarization of optically pumped holes. The magnetization can be dynamically switched by modulating the helicity of optical excitation. Spatially resolved measurements further demonstrate optical writing of ferromagnetic, and thus CI (or FCI) domains. Our work realizes precise optical control of a topological quantum many-body system with potential applications in topological spintronics, quantum memories and creation of exotic edge states by programmable patterning of integer and fractionally quantized anomalous Hall domains4,19.

  View details for DOI 10.1038/s41586-025-09777-3

  View details for Web of Science ID 001684101100031

  View details for PubMedID 41606148

  View details for PubMedCentralID 5529060

• Optical control of orbital magnetism in magic-angle twisted bilayer graphene NATURE PHYSICS Persky, E., Parisot, L., He, M., Cai, J., Taniguchi, T., Watanabe, K., May-Mann, J., Xu, X., Kapitulnik, A. 2026

  View details for DOI 10.1038/s41567-025-03117-y

  View details for Web of Science ID 001654903100001

• Nonmonotonic Band Flattening near the Magic Angle of Twisted Bilayer MoTe2 PHYSICAL REVIEW X Deng, Y., Holtzmann, W., Zhu, Z., Zaklama, T., Majchrzak, P., Taniguchi, T., Watanabe, K., Hashimoto, M., Lu, D., Jozwiak, C., Bostwick, A., Rotenberg, E., Fu, L., Devereaux, T. P., Xu, X., Shen, Z. 2025; 15 (4)

  View details for DOI 10.1103/q11l-9jy1

  View details for Web of Science ID 001680508400001

• Roadmap for Photonics with 2D Materials. ACS photonics de Abajo, F. J., Basov, D. N., Koppens, F. H., Orsini, L., Ceccanti, M., Castilla, S., Cavicchi, L., Polini, M., Gonçalves, P. A., Costa, A. T., Peres, N. M., Mortensen, N. A., Bharadwaj, S., Jacob, Z., Schuck, P. J., Pasupathy, A. N., Delor, M., Liu, M. K., Mugarza, A., Merino, P., Cuxart, M. G., Chávez-Angel, E., Švec, M., Tizei, L. H., Dirnberger, F., Deng, H., Schneider, C., Menon, V., Deilmann, T., Chernikov, A., Thygesen, K. S., Abate, Y., Terrones, M., Sangwan, V. K., Hersam, M. C., Yu, L., Chen, X., Heinz, T. F., Murthy, P., Kroner, M., Smolenski, T., Thureja, D., Chervy, T., Genco, A., Trovatello, C., Cerullo, G., Dal Conte, S., Timmer, D., De Sio, A., Lienau, C., Shang, N., Hong, H., Liu, K., Sun, Z., Rozema, L. A., Walther, P., Alù, A., Marini, A., Cotrufo, M., Queiroz, R., Zhu, X. Y., Cox, J. D., Dias, E. J., Echarri, Á. R., Iyikanat, F., Herrmann, P., Tornow, N., Klimmer, S., Wilhelm, J., Soavi, G., Sun, Z., Wu, S., Xiong, Y., Matsyshyn, O., Krishna Kumar, R., Song, J. C., Bucher, T., Gorlach, A., Tsesses, S., Kaminer, I., Schwab, J., Mangold, F., Giessen, H., Sánchez, M. S., Efetov, D. K., Low, T., Gómez-Santos, G., Stauber, T., Álvarez-Pérez, G., Duan, J., Martín-Moreno, L., Paarmann, A., Caldwell, J. D., Nikitin, A. Y., Alonso-González, P., Mueller, N. S., Volkov, V., Jariwala, D., Shegai, T., van de Groep, J., Boltasseva, A., Bondarev, I. V., Shalaev, V. M., Simon, J., Fruhling, C., Shen, G., Novko, D., Tan, S., Wang, B., Petek, H., Mkhitaryan, V., Yu, R., Manjavacas, A., Ortega, J. E., Cheng, X., Tian, R., Mao, D., Van Thourhout, D., Gan, X., Dai, Q., Sternbach, A., Zhou, Y., Hafezi, M., Litvinov, D., Grzeszczyk, M., Novoselov, K. S., Koperski, M., Papadopoulos, S., Novotny, L., Viti, L., Vitiello, M. S., Cottam, N. D., Dewes, B. T., Makarovsky, O., Patanè, A., Song, Y., Cai, M., Chen, J., Naveh, D., Jang, H., Park, S., Xia, F., Jenke, P. K., Bajo, J., Braun, B., Burch, K. S., Zhao, L., Xu, X. 2025; 12 (8): 3961-4095

  Abstract

  Triggered by advances in atomic-layer exfoliation and growth techniques, along with the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or a few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals now constitute a broad research field expanding in multiple directions through the combination of layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary subset of those directions, where 2D materials contribute remarkable nonlinearities, long-lived and ultraconfined polaritons, strong excitons, topological and chiral effects, susceptibility to external stimuli, accessibility, robustness, and a completely new range of photonic materials based on layer stacking, gating, and the formation of moiré patterns. These properties are being leveraged to develop applications in electro-optical modulation, light emission and detection, imaging and metasurfaces, integrated optics, sensing, and quantum physics across a broad spectral range extending from the far-infrared to the ultraviolet, as well as enabling hybridization with spin and momentum textures of electronic band structures and magnetic degrees of freedom. The rapid expansion of photonics with 2D materials as a dynamic research arena is yielding breakthroughs, which this Roadmap summarizes while identifying challenges and opportunities for future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

  View details for DOI 10.1021/acsphotonics.5c00353

  View details for PubMedID 40861258

  View details for PubMedCentralID PMC12371959

• Ferromagnetism and topology of the higher flat band in a fractional Chern insulator NATURE PHYSICS Park, H., Cai, J., Anderson, E., Zhang, X., Liu, X., Holtzmann, W., Li, W., Wang, C., Hu, C., Zhao, Y., Taniguchi, T., Watanabe, K., Yang, J., Cobden, D., Chu, J., Regnault, N., Bernevig, B., Fu, L., Cao, T., Xiao, D., Xu, X. 2025; 21 (4)

  View details for DOI 10.1038/s41567-025-02804-0

  View details for Web of Science ID 001450723300001

• Local probe of bulk and edge states in a fractional Chern insulator. Nature Ji, Z., Park, H., Barber, M. E., Hu, C., Watanabe, K., Taniguchi, T., Chu, J. H., Xu, X., Shen, Z. X. 2024; 635 (8039): 578-583

  Abstract

  The fractional quantum Hall effect is a key example of topological quantum many-body phenomena, arising from the interplay between strong electron correlation, topological order and time-reversal symmetry breaking. Recently, a lattice analogue of the fractional quantum Hall effect at zero magnetic field has been observed, confirming the existence of a zero-field fractional Chern insulator (FCI). Despite this, the bulk-edge correspondence-a hallmark of a FCI featuring an insulating bulk with conductive edges-has not been directly observed. In fact, this correspondence has not been visualized in any system for fractional states owing to experimental challenges. Here we report the imaging of FCI edge states in twisted MoTe2 (t-MoTe2) using microwave impedance microscopy1. By tuning the carrier density, we observe the system evolving between metallic and FCI states, the latter of which exhibits insulating bulk and conductive edges, as expected from the bulk-boundary correspondence. Further analysis suggests the composite nature of the FCI edge states. We also observe the evolution of edge states across the topological phase transition as a function of interlayer electric field and reveal exciting prospects of neighbouring domains with different fractional orders. These findings pave the way for research into topologically protected one-dimensional interfaces between various anyonic states at zero magnetic field, such as gapped one-dimensional symmetry-protected phases with non-zero topological entanglement entropy, Halperin-Laughlin interfaces and the creation of non-abelian anyons.

  View details for DOI 10.1038/s41586-024-08092-7

  View details for PubMedID 39567787

  View details for PubMedCentralID 11464376

• Trion sensing of a zero-field composite Fermi liquid. Nature Anderson, E., Cai, J., Reddy, A. P., Park, H., Holtzmann, W., Davis, K., Taniguchi, T., Watanabe, K., Smolenski, T., Imamoğlu, A., Cao, T., Xiao, D., Fu, L., Yao, W., Xu, X. 2024; 635 (8039): 590-595

  Abstract

  The half-filled lowest Landau level is a fascinating platform for researching interacting topological phases. A celebrated example is the composite Fermi liquid, a non-Fermi liquid formed by composite fermions in strong magnetic fields1-10. Its zero-field counterpart is predicted in a twisted MoTe2 bilayer (tMoTe2)11,12-a recently discovered fractional Chern insulator exhibiting the fractional quantum anomalous Hall effect13-16. Although transport measurements at ν = -1/2 show signatures consistent with a zero-field composite Fermi liquid14, new probes are crucial to investigate the state and its elementary excitations. Here, by using the unique valley properties of tMoTe2, we report optical signatures of a zero-field composite Fermi liquid. We measured the degree of circular polarization (ρ) of trion photoluminescence versus hole doping and electric field. We found that, within the phase space showing robust ferromagnetism, ρ is near unity for Fermi liquid states. However, ρ is quenched at both integer and fractional Chern insulators, and in a hole doping range near ν = -1/2. Temperature, optical excitation power and electric-field-dependence measurements demonstrate that the quenching of ρ is a direct consequence of an energy gap (pseudogap) for electronic excitations of the Chern insulators (composite Fermi liquid): because the local spin-polarized excitations necessary to form trions are strongly suppressed, trion formation at the corresponding filling factors relies on optically generated unpolarized itinerant holes. Our work highlights a new excitonic probe of zero-field fractional Chern insulator physics, unique to tMoTe2.

  View details for DOI 10.1038/s41586-024-08134-0

  View details for PubMedID 39567789

  View details for PubMedCentralID 8602625

• Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet. Nature communications Wu, H., Chen, L., Malinowski, P., Jang, B. G., Deng, Q., Scott, K., Huang, J., Ruff, J. P., He, Y., Chen, X., Hu, C., Yue, Z., Oh, J. S., Teng, X., Guo, Y., Klemm, M., Shi, C., Shi, Y., Setty, C., Werner, T., Hashimoto, M., Lu, D., Yilmaz, T., Vescovo, E., Mo, S., Fedorov, A., Denlinger, J. D., Xie, Y., Gao, B., Kono, J., Dai, P., Han, Y., Xu, X., Birgeneau, R. J., Zhu, J., da Silva Neto, E. H., Wu, L., Chu, J., Si, Q., Yi, M. 2024; 15 (1): 2739

  Abstract

  Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5-deltaGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.

  View details for DOI 10.1038/s41467-024-46862-z

  View details for PubMedID 38548765

• Spectral evidence for local-moment ferromagnetism in the van der Waals metals Fe₃GaTe₂ and Fe₃GeTe₂ PHYSICAL REVIEW B Wu, H., Hu, C., Xie, Y., Jang, B., Huang, J., Guo, Y., Wu, S., Hu, C., Yue, Z., Shi, Y., Basak, R., Ren, Z., Yilmaz, T., Vescovo, E., Jozwiak, C., Bostwick, A., Rotenberg, E., Fedorov, A., Denlinger, J. D., Klewe, C., Shafer, P., Lu, D., Hashimoto, M., Kono, J., Frano, A., Birgeneau, R. J., Xu, X., Zhu, J., Dai, P., Chu, J., Yi, M. 2024; 109 (10)

  View details for DOI 10.1103/PhysRevB.109.104410

  View details for Web of Science ID 001249295000001

• Observation of fractionally quantized anomalous Hall effect. Nature Park, H., Cai, J., Anderson, E., Zhang, Y., Zhu, J., Liu, X., Wang, C., Holtzmann, W., Hu, C., Liu, Z., Taniguchi, T., Watanabe, K., Chu, J. H., Cao, T., Fu, L., Yao, W., Chang, C. Z., Cobden, D., Xiao, D., Xu, X. 2023; 622 (7981): 74-79

  Abstract

  The integer quantum anomalous Hall (QAH) effect is a lattice analogue of the quantum Hall effect at zero magnetic field1-3. This phenomenon occurs in systems with topologically non-trivial bands and spontaneous time-reversal symmetry breaking. Discovery of its fractional counterpart in the presence of strong electron correlations, that is, the fractional QAH effect4-7, would open a new chapter in condensed matter physics. Here we report the direct observation of both integer and fractional QAH effects in electrical measurements on twisted bilayer MoTe2. At zero magnetic field, near filling factor ν = -1 (one hole per moiré unit cell), we see an integer QAH plateau in the Hall resistance Rxy quantized to h/e2 ± 0.1%, whereas the longitudinal resistance Rxx vanishes. Remarkably, at ν  =  -2/3 and -3/5, we see plateau features in Rxy at [Formula: see text] and [Formula: see text], respectively, whereas Rxx remains small. All features shift linearly versus applied magnetic field with slopes matching the corresponding Chern numbers -1, -2/3 and -3/5, precisely as expected for integer and fractional QAH states. Additionally, at zero magnetic field, Rxy is approximately 2h/e2 near half-filling (ν  = -1/2) and varies linearly as ν  is tuned. This behaviour resembles that of the composite Fermi liquid in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field8-14. Direct observation of the fractional QAH and associated effects enables research in charge fractionalization and anyonic statistics at zero magnetic field.

  View details for DOI 10.1038/s41586-023-06536-0

  View details for PubMedID 37591304

  View details for PubMedCentralID 10533412

• Signatures of fractional quantum anomalous Hall states in twisted MoTe2. Nature Cai, J., Anderson, E., Wang, C., Zhang, X., Liu, X., Holtzmann, W., Zhang, Y., Fan, F., Taniguchi, T., Watanabe, K., Ran, Y., Cao, T., Fu, L., Xiao, D., Yao, W., Xu, X. 2023; 622 (7981): 63-68

  Abstract

  The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field owing to intrinsic ferromagnetism1-3. In the presence of strong electron-electron interactions, fractional QAH (FQAH) states at zero magnetic field can emerge4-8. These states could host fractional excitations, including non-Abelian anyons-crucial building blocks for topological quantum computation9. Here we report experimental signatures of FQAH states in a twisted molybdenum ditelluride (MoTe2) bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole-filled moiré minibands. Using trion photoluminescence as a sensor10, we obtain a Landau fan diagram showing linear shifts in carrier densities corresponding to filling factor v = -2/3 and v = -3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of [Formula: see text] and [Formula: see text], respectively. Moreover, the v = -1 state exhibits a dispersion corresponding to Chern number -1, consistent with the predicted QAH state11-14. In comparison, several non-ferromagnetic states on the electron-doping side do not disperse, that is, they are trivial correlated insulators. The observed topological states can be electrically driven into topologically trivial states. Our findings provide evidence of the long-sought FQAH states, demonstrating MoTe2 moiré superlattices as a platform for exploring fractional excitations.

  View details for DOI 10.1038/s41586-023-06289-w

  View details for PubMedID 37315640

  View details for PubMedCentralID 8602625

• Spin-mediated shear oscillators in a van der Waals antiferromagnet. Nature Zong, A., Zhang, Q., Zhou, F., Su, Y., Hwangbo, K., Shen, X., Jiang, Q., Liu, H., Gage, T. E., Walko, D. A., Kozina, M. E., Luo, D., Reid, A. H., Yang, J., Park, S., Lapidus, S. H., Chu, J. H., Arslan, I., Wang, X., Xiao, D., Xu, X., Gedik, N., Wen, H. 2023

  Abstract

  Understanding how microscopic spin configuration gives rise to exotic properties at the macroscopic length scale has long been pursued in magnetic materials1-5. One seminal example is the Einstein-de Haas effect in ferromagnets1,6,7, in which angular momentum of spins can be converted into mechanical rotation of an entire object. However, for antiferromagnets without net magnetic moment, how spin ordering couples to macroscopic movement remains elusive. Here we observed a seesaw-like rotation of reciprocal lattice peaks of an antiferromagnetic nanolayer film, whose gigahertz structural resonance exhibits more than an order-of-magnitude amplification after cooling below the Néel temperature. Using a suite of ultrafast diffraction and microscopy techniques, we directly visualize this spin-driven rotation in reciprocal space at the nanoscale. This motion corresponds to interlayer shear in real space, in which individual micro-patches of the film behave as coherent oscillators that are phase-locked and shear along the same in-plane axis. Using time-resolved optical polarimetry, we further show that the enhanced mechanical response strongly correlates with ultrafast demagnetization, which releases elastic energy stored in local strain gradients to drive the oscillators. Our work not only offers the first microscopic view of spin-mediated mechanical motion of an antiferromagnet but it also identifies a new route towards realizing high-frequency resonators8,9 up to the millimetre band, so the capability of controlling magnetic states on the ultrafast timescale10-13 can be readily transferred to engineering the mechanical properties of nanodevices.

  View details for DOI 10.1038/s41586-023-06279-y

  View details for PubMedID 37532936

  View details for PubMedCentralID 10156606

• Programming correlated magnetic states with gate-controlled moiré geometry. Science (New York, N.Y.) Anderson, E., Fan, F. R., Cai, J., Holtzmann, W., Taniguchi, T., Watanabe, K., Xiao, D., Yao, W., Xu, X. 2023; 381 (6655): 325-330

  Abstract

  The ability to control the underlying lattice geometry of a system may enable transitions between emergent quantum ground states. We report in situ gate switching between honeycomb and triangular lattice geometries of an electron many-body Hamiltonian in rhombohedral (R)-stacked molybdenum ditelluride (MoTe2) moiré bilayers, resulting in switchable magnetic exchange interactions. At zero electric field, we observed a correlated ferromagnetic insulator near one hole per moiré unit cell with a widely tunable Curie temperature up to 14 K. Applying an electric field switched the system into a half-filled triangular lattice with antiferromagnetic interactions; further doping this layer-polarized superlattice tuned the antiferromagnetic exchange interaction back to ferromagnetic. Our work demonstrates R-stacked MoTe2 moirés to be a laboratory for engineering correlated states with nontrivial topology.

  View details for DOI 10.1126/science.adg4268

  View details for PubMedID 37347950

• Dipole ladders with large Hubbard interaction in a moire exciton lattice NATURE PHYSICS Park, H., Zhu, J., Wang, X., Wang, Y., Holtzmann, W., Taniguchi, T., Watanabe, K., Yan, J., Fu, L., Cao, T., Xiao, D., Gamelin, D. R., Yu, H., Yao, W., Xu, X. 2023; 19 (9): 1286-+

  View details for DOI 10.1038/s41567-023-02077-5

  View details for Web of Science ID 000999570700002

• Tunable interaction between excitons and hybridized magnons in a layered semiconductor. Nature nanotechnology Diederich, G. M., Cenker, J., Ren, Y., Fonseca, J., Chica, D. G., Bae, Y. J., Zhu, X., Roy, X., Cao, T., Xiao, D., Xu, X. 2023; 18 (1): 23-28

  Abstract

  The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors1 provides a platform to explore these exciton-magnon interactions and their fundamental properties, such as strong correlation2, as well as their photospintronic and quantum transduction3 applications. Here we demonstrate the precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. We varied the direction of an applied magnetic field relative to the crystal axes, and thus the rotational symmetry of the magnetic system4. Thereby, we tuned not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon-magnon hybridization. We further modulated the exciton-magnon coupling and the associated magnon dispersion curves through the application of uniaxial strain. At a critical strain, a dispersionless dark magnon band emerged. Our results demonstrate an unprecedented level of control of the opto-mechanical-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics5-11.

  View details for DOI 10.1038/s41565-022-01259-1

  View details for PubMedID 36577852

• Light-induced ferromagnetism in moiré superlattices. Nature Wang, X., Xiao, C., Park, H., Zhu, J., Wang, C., Taniguchi, T., Watanabe, K., Yan, J., Xiao, D., Gamelin, D. R., Yao, W., Xu, X. 2022; 604 (7906): 468-473

  Abstract

  Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena1-3. The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle1-3, electrical field4-6, moiré carrier filling7-11 and interlayer coupling12. Here we report that optical excitation can highly tune the spin-spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS2 /WSe2 moiré superlattices. Near the filling factor of -1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes9, and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter13-19.

  View details for DOI 10.1038/s41586-022-04472-z

  View details for PubMedID 35444320

  View details for PubMedCentralID 3145708

• Reversible strain-induced magnetic phase transition in a van der Waals magnet. Nature nanotechnology Cenker, J., Sivakumar, S., Xie, K., Miller, A., Thijssen, P., Liu, Z., Dismukes, A., Fonseca, J., Anderson, E., Zhu, X., Roy, X., Xiao, D., Chu, J. H., Cao, T., Xu, X. 2022; 17 (3): 256-261

  Abstract

  Mechanical deformation of a crystal can have a profound effect on its physical properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temperatures. Using this apparatus, we realize a reversible strain-induced antiferromagnetic-to-ferromagnetic phase transition at zero magnetic field and strain control of the out-of-plane spin-canting process. First-principles calculations reveal that the tuning of the in-plane lattice constant strongly modifies the interlayer magnetic exchange interaction, which changes sign at the critical strain. Our work creates new opportunities for harnessing the strain control of magnetism and other electronic states in low-dimensional materials and heterostructures.

  View details for DOI 10.1038/s41565-021-01052-6

  View details for PubMedID 35058657

• Interlayer electronic coupling on demand in a 2D magnetic semiconductor. Nature materials Wilson, N. P., Lee, K., Cenker, J., Xie, K., Dismukes, A. H., Telford, E. J., Fonseca, J., Sivakumar, S., Dean, C., Cao, T., Roy, X., Xu, X., Zhu, X. 2021; 20 (12): 1657-1662

  Abstract

  When monolayers of two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can introduce entirely new properties, as exemplified by recent discoveries of moiré bands that host highly correlated electronic states and quantum dot-like interlayer exciton lattices. Here we show the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions, in an A-type antiferromagnetic 2D semiconductor CrSBr. Excitonic transitions in bilayers and above can be drastically changed when the magnetic order is switched from the layered antiferromagnetic ground state to a field-induced ferromagnetic state, an effect attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the latter, as revealed by Green's function-Bethe-Salpeter equation (GW-BSE) calculations. Our work uncovers a magnetic approach to engineer electronic and excitonic effects in layered magnetic semiconductors.

  View details for DOI 10.1038/s41563-021-01070-8

  View details for PubMedID 34312534

• Direct visualization of magnetic domains and moiré magnetism in twisted 2D magnets. Science (New York, N.Y.) Song, T., Sun, Q. C., Anderson, E., Wang, C., Qian, J., Taniguchi, T., Watanabe, K., McGuire, M. A., Stöhr, R., Xiao, D., Cao, T., Wrachtrup, J., Xu, X. 2021; 374 (6571): 1140-1144

  Abstract

  Moiré superlattices of twisted nonmagnetic two-dimensional (2D) materials are highly controllable platforms for the engineering of exotic correlated and topological states. Here, we report emerging magnetic textures in small-angle twisted 2D magnet chromium triiodide (CrI3). Using single-spin quantum magnetometry, we directly visualized nanoscale magnetic domains and periodic patterns, a signature of moiré magnetism, and measured domain size and magnetization. In twisted bilayer CrI3, we observed the coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) domains with disorder-like spatial patterns. In twisted double-trilayer CrI3, AFM and FM domains with periodic patterns appear, which is in good agreement with the calculated spatial magnetic structures that arise from the local stacking-dependent interlayer exchange interactions in CrI3 moiré superlattices. Our results highlight magnetic moiré superlattices as a platform for exploring nanomagnetism.

  View details for DOI 10.1126/science.abj7478

  View details for PubMedID 34822270

• Switching 2D magnetic states via pressure tuning of layer stacking. Nature materials Song, T., Fei, Z., Yankowitz, M., Lin, Z., Jiang, Q., Hwangbo, K., Zhang, Q., Sun, B., Taniguchi, T., Watanabe, K., McGuire, M. A., Graf, D., Cao, T., Chu, J., Cobden, D. H., Dean, C. R., Xiao, D., Xu, X. 2019

  Abstract

  The physical properties of two-dimensional van der Waals crystals can be sensitive to interlayer coupling. For two-dimensional magnets1-3, theory suggests that interlayer exchange coupling is strongly dependent on layer separation while the stacking arrangement can even change the sign of the interlayer magnetic exchange, thus drastically modifying the ground state4-10. Here, we demonstrate pressure tuning of magnetic order in the two-dimensional magnet CrI3. We probe the magnetic states using tunnelling8,11-13 and scanning magnetic circular dichroism microscopy measurements2. We find that interlayer magnetic coupling can be more than doubled by hydrostatic pressure. In bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phase. In trilayer CrI3, pressure can create coexisting domains of three phases, one ferromagnetic and two antiferromagnetic. The observed changes in magnetic order can be explained by changes in the stacking arrangement. Such coupling between stacking order and magnetism provides ample opportunities for designer magnetic phases and functionalities.

  View details for DOI 10.1038/s41563-019-0505-2

  View details for PubMedID 31659293

• Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3. Nature Sun, Z., Yi, Y., Song, T., Clark, G., Huang, B., Shan, Y., Wu, S., Huang, D., Gao, C., Chen, Z., McGuire, M., Cao, T., Xiao, D., Liu, W., Yao, W., Xu, X., Wu, S. 2019

  Abstract

  Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a layered antiferromagnetic insulator in its few-layer form1, opening up opportunities for various functionalities2-7 in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG8-11 and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far12,13 (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism14-18. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.

  View details for DOI 10.1038/s41586-019-1445-3

  View details for PubMedID 31367036

• Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering NATURE PHOTONICS Tung, I., Krishnamoorthy, A., Sadasivam, S., Zhou, H., Zhang, Q., Seyler, K. L., Clark, G., Mannebach, E. M., Nyby, C., Ernst, F., Zhu, D., Glownia, J. M., Kozina, M. E., Song, S., Nelson, S., Kumazoe, H., Shimojo, F., Kalia, R. K., Vashishta, P., Darancet, P., Heinz, T. F., Nakano, A., Xu, X., Lindenberg, A. M., Wen, H. 2019; 13 (6): 425-+

  View details for DOI 10.1038/s41566-019-0387-5

  View details for Web of Science ID 000468752300020

• Signatures of moiré-trapped valley excitons in MoSe2/WSe2 heterobilayers. Nature Seyler, K. L., Rivera, P., Yu, H., Wilson, N. P., Ray, E. L., Mandrus, D. G., Yan, J., Yao, W., Xu, X. 2019; 567 (7746): 66-70

  Abstract

  The formation of moiré patterns in crystalline solids can be used to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In two-dimensional materials, a moiré pattern with a superlattice potential can be formed by vertically stacking two layered materials with a twist and/or a difference in lattice constant. This approach has led to electronic phenomena including the fractal quantum Hall effect1-3, tunable Mott insulators4,5 and unconventional superconductivity6. In addition, theory predicts that notable effects on optical excitations could result from a moiré potential in two-dimensional valley semiconductors7-9, but these signatures have not been detected experimentally. Here we report experimental evidence of interlayer valley excitons trapped in a moiré potential in molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2) heterobilayers. At low temperatures, we observe photoluminescence close to the free interlayer exciton energy but with linewidths over one hundred times narrower (around 100 microelectronvolts). The emitter g-factors are homogeneous across the same sample and take only two values, -15.9 and 6.7, in samples with approximate twist angles of 60 degrees and 0 degrees, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley-pairing configurations. At twist angles of approximately 20 degrees the emitters become two orders of magnitude dimmer; however, they possess the same g-factor as the heterobilayer at a twist angle of approximately 60 degrees. This is consistent with the umklapp recombination of interlayer excitons near the commensurate 21.8-degree twist angle7. The emitters exhibit strong circular polarization of the same helicity for a given twist angle, which suggests that the trapping potential retains three-fold rotational symmetry. Together with a characteristic dependence on power and excitation energy, these results suggest that the origin of the observed effects is interlayer excitons trapped in a smooth moiré potential with inherited valley-contrasting physics. This work presents opportunities to control two-dimensional moiré optics through variation of the twist angle.

  View details for DOI 10.1038/s41586-019-0957-1

  View details for PubMedID 30804526

  View details for PubMedCentralID 5681217

• Imaging quantum spin Hall edges in monolayer WTe2. Science advances Shi, Y., Kahn, J., Niu, B., Fei, Z., Sun, B., Cai, X., Francisco, B. A., Wu, D., Shen, Z., Xu, X., Cobden, D. H., Cui, Y. 2019; 5 (2): eaat8799

  Abstract

  A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe2. Here, we directly image the local conductivity of monolayer WTe2 using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe2. Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform.

  View details for PubMedID 30783621

• Imaging quantum spin Hall edges in monolayer WTe2 SCIENCE ADVANCES Shi, Y., Kahn, J., Niu, B., Fei, Z., Sun, B., Cai, X., Francisco, B. A., Wu, D., Shen, Z., Xu, X., Cobden, D. H., Cui, Y. 2019; 5 (2)

  View details for DOI 10.1126/sciadv.aat8799

  View details for Web of Science ID 000460145700010

• Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides NANO LETTERS Mannebach, E. M., Nyby, C., Ernst, F., Zhou, Y., Tolsma, J., Li, Y., Sher, M., Tung, I., Zhou, H., Zhang, Q., Seyler, K. L., Clark, G., Lin, Y., Zhu, D., Glownia, J. M., Kozina, M. E., Song, S., Nelson, S., Mehta, A., Yu, Y., Pant, A., Aslan, O., Raja, A., Guo, Y., DiChiara, A., Mao, W., Cao, L., Tongay, S., Sun, J., Singh, D. J., Heinz, T. F., Xu, X., MacDonald, A. H., Reed, E., Wen, H., Lindenberg, A. M. 2017; 17 (12): 7761-7766

  View details for DOI 10.1021/acs.nanolett.7b03955

  View details for Web of Science ID 000418393300081

• Edge conduction in monolayer WTe₂ NATURE PHYSICS Fei, Z., Palomaki, T., Wu, S., Zhao, W., Cai, X., Sun, B., Nguyen, P., Finney, J., Xu, X., Cobden, D. H. 2017; 13 (7): 677-+

  View details for DOI 10.1038/NPHYS4091

  View details for Web of Science ID 000404629900017

• Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature Huang, B., Clark, G., Navarro-Moratalla, E., Klein, D. R., Cheng, R., Seyler, K. L., Zhong, D., Schmidgall, E., McGuire, M. A., Cobden, D. H., Yao, W., Xiao, D., Jarillo-Herrero, P., Xu, X. 2017; 546 (7657): 270-273

  Abstract

  Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.

  View details for DOI 10.1038/nature22391

  View details for PubMedID 28593970

• Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides. Nano letters Mannebach, E. M., Nyby, C. n., Ernst, F. n., Zhou, Y. n., Tolsma, J. n., Li, Y. n., Sher, M. J., Tung, I. C., Zhou, H. n., Zhang, Q. n., Seyler, K. L., Clark, G. n., Lin, Y. n., Zhu, D. n., Glownia, J. M., Kozina, M. E., Song, S. n., Nelson, S. n., Mehta, A. n., Yu, Y. n., Pant, A. n., Aslan, O. B., Raja, A. n., Guo, Y. n., DiChiara, A. n., Mao, W. n., Cao, L. n., Tongay, S. n., Sun, J. n., Singh, D. J., Heinz, T. F., Xu, X. n., MacDonald, A. H., Reed, E. n., Wen, H. n., Lindenberg, A. M. 2017; 17 (12): 7761–66

  Abstract

  Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.

  View details for PubMedID 29119791

• Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit. Nano letters Yuan, H., Liu, Z., Xu, G., Zhou, B., Wu, S., Dumcenco, D., Yan, K., Zhang, Y., Mo, S., Dudin, P., Kandyba, V., Yablonskikh, M., Barinov, A., Shen, Z., Zhang, S., Huang, Y., Xu, X., Hussain, Z., Hwang, H. Y., Cui, Y., Chen, Y. 2016; 16 (8): 4738-4745

  Abstract

  Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

  View details for DOI 10.1021/acs.nanolett.5b05107

  View details for PubMedID 27357620

• Monolayer semiconductor nanocavity lasers with ultralow thresholds. Nature Wu, S., Buckley, S., Schaibley, J. R., Feng, L., Yan, J., Mandrus, D. G., Hatami, F., Yao, W., Vuckovic, J., Majumdar, A., Xu, X. 2015; 520 (7545): 69-72

  Abstract

  Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC). However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots, extreme difficulty in current injection, and lack of compatibility with electronic circuits. Here we report a new lasing strategy: an atomically thin crystalline semiconductor--that is, a tungsten diselenide monolayer--is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

  View details for DOI 10.1038/nature14290

  View details for PubMedID 25778703

• Monolayer semiconductor nanocavity lasers with ultralow thresholds NATURE Wu, S., Buckley, S., Schaibley, J. R., Feng, L., Yan, J., Mandrus, D. G., Hatami, F., Yao, W., Vuckovic, J., Majumdar, A., Xu, X. 2015; 520 (7545): 69-U142

  Abstract

  Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC). However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots, extreme difficulty in current injection, and lack of compatibility with electronic circuits. Here we report a new lasing strategy: an atomically thin crystalline semiconductor--that is, a tungsten diselenide monolayer--is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

  View details for DOI 10.1038/nature14290

  View details for Web of Science ID 000352027700038

  View details for PubMedID 25778703

• Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures. Nature communications Rivera, P., Schaibley, J. R., Jones, A. M., Ross, J. S., Wu, S., Aivazian, G., Klement, P., Seyler, K., Clark, G., Ghimire, N. J., Yan, J., Mandrus, D. G., Yao, W., Xu, X. 2015; 6: 6242

  Abstract

  Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

  View details for DOI 10.1038/ncomms7242

  View details for PubMedID 25708612

• Control of two-dimensional excitonic light emission via photonic crystal 2D MATERIALS Wu, S., Buckley, S., Jones, A. M., Ross, J. S., Ghimire, N. J., Yan, J., Mandrus, D. G., Yao, W., Hatami, F., Vuckovic, J., Majumdar, A., Xu, X. 2014; 1 (1)

  View details for DOI 10.1088/2053-1583/1/1/011001

  View details for Web of Science ID 000353649900002

• 2D-material Based Nano-photonics Majumdar, A., Wu, S., Buckley, S., Jones, A. M., Ross, J. S., Ghimire, N. J., Yan, J., Mandrus, D. C., Yao, W., Hatami, F., Vuckovic, J., Xu, X., IEEE IEEE. 2014

  View details for Web of Science ID 000369908602380

• Zeeman-type spin splitting controlled by an electric field NATURE PHYSICS Yuan, H., Bahramy, M., Morimoto, K., Wu, S., Nomura, K., Yang, B., Shimotani, H., Suzuki, R., Toh, M., Kloc, C., Xu, X., Arita, R., Nagaosa, N., Iwasa, Y. 2013; 9 (9): 563-569

  View details for DOI 10.1038/NPHYS2691

  View details for Web of Science ID 000324059500021

• Optical generation of excitonic valley coherence in monolayer WSe2. Nature nanotechnology Jones, A. M., Yu, H., Ghimire, N. J., Wu, S., Aivazian, G., Ross, J. S., Zhao, B., Yan, J., Mandrus, D. G., Xiao, D., Yao, W., Xu, X. 2013; 8 (9): 634-8

  Abstract

  As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ('valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

  View details for DOI 10.1038/nnano.2013.151

  View details for PubMedID 23934096

• Electrical control of neutral and charged excitons in a monolayer semiconductor. Nature communications Ross, J. S., Wu, S., Yu, H., Ghimire, N. J., Jones, A. M., Aivazian, G., Yan, J., Mandrus, D. G., Xiao, D., Yao, W., Xu, X. 2013; 4: 1474

  Abstract

  Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.

  View details for DOI 10.1038/ncomms2498

  View details for PubMedID 23403575

• Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Physical review letters Xiao, D., Liu, G. B., Feng, W., Xu, X., Yao, W. 2012; 108 (19): 196802

  Abstract

  We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indices. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.

  View details for DOI 10.1103/PhysRevLett.108.196802

  View details for PubMedID 23003071

• Picosecond optical spectroscopy of a single negatively charged self-assembled InAs quantum dot APPLIED PHYSICS LETTERS Kim, E. D., Truex, K., Wu, Y., Amo, A., Xu, X., Steel, D. G., Bracker, A. S., Gammon, D., Sham, L. J. 2010; 97 (11)

  View details for DOI 10.1063/1.3487783

  View details for Web of Science ID 000282032900070