Charge carriers in magic angle graphene come in eight flavors described by a combination of their... more Charge carriers in magic angle graphene come in eight flavors described by a combination of their spin, valley, and sublattice polarizations [1-4]. When the inversion and time reversal symmetries are broken by the substrate or by strong interactions, the degeneracy of the flavors can be lifted and their corresponding bands can be filled sequentially [5-7]. Due to their non-trivial band topology and Berry curvature, each of the bands is classified by a topological Chern number [8-12], leading to the quantum anomalous Hall [13-15] and Chern insulator states [7,8,16-19] at integer fillings of the bands. It has been recently predicted, however, that depending on the local atomic-scale arrangements of the graphene and the encapsulating hBN lattices, rather than being a global topological invariant, the Chern number may become position dependent, altering transport and magnetic properties of the itinerant electrons [20-23]. Using scanning superconducting quantum interference device on a tip (SQUID-on-tip) [24], we directly image the nanoscale Berry-curvatureinduced equilibrium orbital magnetism, the polarity of which is governed by the local Chern number, and detect its two constituent components associated with the drift and the self-rotation of the electronic wave packets [25]. At = 1, we observe local zero-field valley-polarized Chern insulators forming a mosaic of microscopic patches of = −1, 0, or 1. Upon further filling, we find a first-order phase transition due to recondensation of electrons from valley to ′, which leads to irreversible flips of the local Chern number and the magnetization, and to the formation of valley domain walls giving rise to hysteretic global anomalous Hall resistance. The findings shed new light on the structure and dynamics of topological phases and call for exploration of the controllable formation of flavor domain walls and their utilization in twistronic devices.
2017 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), 2017
In this work we have measured the far-field emission patterns of In As quantum dots embedded in a... more In this work we have measured the far-field emission patterns of In As quantum dots embedded in a GaAs tapered nanowire and used an open-geometry Fourier modal method for determining the radial position of the quantum dots by computing the far-field emission pattern for different quantum dot locations.
The celebrated phenomenon of quantum Hall effect has recently been generalized from transport of ... more The celebrated phenomenon of quantum Hall effect has recently been generalized from transport of conserved charges to that of other approximately conserved state variables, including spin and valley, which are characterized by spin- or valley-polarized boundary states with different chiralities. Here, we report a new class of quantum Hall effect in ABA-stacked graphene trilayers (TLG), the quantum parity Hall (QPH) effect, in which boundary channels are distinguished by even or odd parity under the systems mirror reflection symmetry. At the charge neutrality point and a small perpendicular magnetic field $B_{\perp}$, the longitudinal conductance $\sigma_{xx}$ is first quantized to $4e^2/h$, establishing the presence of four edge channels. As $B_{\perp}$ increases, $\sigma_{xx}$ first decreases to $2e^2/h$, indicating spin-polarized counter-propagating edge states, and then to approximately $0$. These behaviors arise from level crossings between even and odd parity bulk Landau levels...
2) field effect transistors (FET) devices and develop an effective gas annealing technique that s... more 2) field effect transistors (FET) devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3-4 orders of magnitude. Temperature dependence measurements reveal two transport mechanisms: electron-phonon scattering at high temperatures and thermal activation over a gatetunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates. Moreover, this suspended MoS 2 device structure provides double surface access for ionic liquid gating. We are able to extract the dielectric constant of the ionic liquid, and the latest experimental results will be presented.
La these a porte sur l'etude par spectroscopie magneto-optique de boites quantiques magnetiqu... more La these a porte sur l'etude par spectroscopie magneto-optique de boites quantiques magnetiques et de nanofils semiconducteurs. Ces nanostructures a base de semiconducteurs magnetiques dilues sont elaborees et etudiees dans l'equipe depuis une quinzaine d'annee. L'interet de ces structures repose sur la possibilite de controler et d'etudier tres finement le couplage d'un petit nombre de spins localises avec des porteurs libres. La premiere partie de la presente these a porte sur l'etude des conditions de formation de polarons magnetiques (orientation d'un petit ensemble de spins par l'intermediaire d'un porteur ou d'une paire electron-trou) dans des boites quantiques CdMnTe. La formation de polarons magnetiques encore assez mal comprise avec les systemes 0D tels que les boites quantiques auto-assemblees. Une serie d'echantillons originaux de boites quantiques magnetiques auto-assemblees (concentration en manganese intermediaires) a ete...
The coexistence of superconducting and correlated insulating states in magic-angle twisted bilaye... more The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene 1-11 prompts fascinating questions about their relationship. Independent control of the microscopic mechanisms that govern these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing the separation distance between the graphene and a metallic screening layer 12,13. We observe quenching of correlated insulators in devices with screening layer separations that are smaller than the typical Wannier orbital size of 15 nanometres and with twist angles that deviate slightly from the magic angle of 1.10 ± 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 tesla, giving rise to quantized Hall states with a Chern number of 2. Our study suggests re-examination of the often-assumed 'parent-and-child' relation between the insulating and superconducting phases in moiré graphene, and suggests a way of directly probing the microscopic mechanisms of superconductivity in strongly correlated systems.
Rashba spin-orbit coupling in atomically thin InSe is varied layer by layer and by tuning out-of-... more Rashba spin-orbit coupling in atomically thin InSe is varied layer by layer and by tuning out-of-plane electric field.
In transition metal dichalcogenides layers of atomic scale thickness, the electron-hole Coulomb i... more In transition metal dichalcogenides layers of atomic scale thickness, the electron-hole Coulomb interaction potential is strongly influenced by the sharp discontinuity of the dielectric function across the layer plane. This feature results in peculiar non-hydrogenic excitonic states, in which exciton-mediated optical nonlinearities are predicted to be enhanced as compared to their hydrogenic counterpart. To demonstrate this enhancement, we performed optical transmission spectroscopy of a MoSe2 monolayer placed in the strong coupling regime with the mode of an optical microcavity, and analyzed the results quantitatively with a nonlinear input-output theory. We find an enhancement of both the exciton-exciton interaction and of the excitonic fermionic saturation with respect to realistic values expected in the hydrogenic picture. Such results demonstrate that unconventional excitons in MoSe2 are highly favourable for the implementation of large exciton-mediated optical nonlinearities, potentially working up to room temperature.
The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of supe... more The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of superconducting, magnetic and topologically nontrivial phases. The existence of all these phases in one material, and their tunability, has opened new pathways for the creation of unusual gate tunable junctions. However, the required conditions for their creation – gate induced transitions between phases in zero magnetic field – have so far not been achieved. Here, we report on the first experimental demonstration of a device that is both a zero-field Chern insulator and a superconductor. The Chern insulator occurs near moiré cell filling factor = +1 in a hBN non-aligned MATBG device and manifests itself via an anomalous Hall effect. The insulator has Chern number C= ±1 and a relatively high Curie temperature of Tc ≈ 4.5 K. Gate tuning away from this state exposes strong superconducting phases with critical temperatures of up to Tc ≈ 3.5 K. In a perpendicular magnetic field above B > 0.5 ...
Superconductivity often occurs close to broken-symmetry parent states and is especially common in... more Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators 1. When twisted close to a magic relative orientation angle near !°, bilayer graphene has flat moiré superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics 2-5 , notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moiré band filling factors # = %, ±!, ±(, ±), and superconductivity below critical temperatures as high as ~ 3 K close to-2 filling. We also observe three new superconducting domes at much lower temperatures close to the # = % and # = ±! insulating states. Interestingly, at # = ±! we find states with non-zero Chern numbers. For # = −! the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moiré flat bands, including near charge neutrality.
Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with it... more Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum 1. Likewise, in the optical domain, a synthetic spin-orbit coupling is accessible, for instance, by engineering optical anisotropies in photonic materials 2. Both, akin, yield the possibility to create devices directly harnessing spin-and polarization as information carriers 3. Atomically thin layers of transition metal dichalcogenides provide a new material platform which promises intrinsic spin-valley Hall features both for free carriers, two-particle excitations (excitons), as well as for photons 4. In such materials, the spin of an exciton is closely linked to the high-symmetry point in reciprocal space it emerges from (K and K' valleys) 5,6. Here, we demonstrate, that spin, and hence valley selective propagation is accessible in an atomically thin layer of MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a wire-like device, where we can clearly trace the flow, and the helicity of exciton-polaritons expanding along a channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley selective expansion of the polariton cloud without neither any applied external magnetic fields nor coherent Rayleigh scattering. Unlike 17-02-00383.
Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-... more Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-state platform for quantum optics. Within the nanowire section, the emitter position simultaneously determines the strength of the light-matter interaction, as well as the coupling to potential decoherence channels. Therefore, to quantitatively understand device performance and guide future optimization, it is highly desirable to map the emitter position with an accuracy much smaller than the waveguide diameter, on the order of a few hundreds of nanometers. We introduce here a nondestructive, all-optical mapping technique that exploits the QD emission into two guided modes with different transverse profiles. These two modes are fed by the same emitter and thus interfere. The resulting intensity pattern, which is highly sensitive to the emitter position, is resolved in the far-field using Fourier microscopy. We demonstrate this technique on a standard microphotoluminescence setup and map the position of individual QDs in a nanowire antenna with a spatial resolution of ±10 nm. This work opens important perspectives for the future development of light-matter interfaces based on nanowire antennas. Beyond single-QD devices, it will also provide a valuable tool for the investigation of collective effects that imply several emitters coupled to an optical waveguide.
Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics... more Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 µm-long, insulating channel only when it is biased into the ν=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of ferromagnetic and antiferromagnetic spintronics. An important goal of spintronics research is to identify mechanisms that minimize dissipation in devices that seek to exploit the action of spin currents. In magnetic insulators, spin-currents can be carried dissipatively by magnon quasiparticles 1-3. In the case of systems with easy plane magnetic order, they can also be carried collectively in the form of dissipationless spin supercurrents 4-9. While magnon transport is much less efficient in an ideal antiferromagnetic insulators (AFMI) than that in ferromagnetic insulators in the absence of a thermal gradient, spin superfluidity is theoretically expected to be a possibility in both cases. Although the potential of AFM materials 5,6,10 as active spintronic components that can be
Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination... more Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination. In this paper, an interesting and surprising phenomenon in YAG transparent ceramics is reported. UV light leads to significant changes in the microstructure of open volume defects and nano clusters as well as in the optical properties. Lightinduced lattice relaxation is suggested as the mechanism behind this intriguing behavior. The complex F-type color center with broad absorption bands is caused by the aliovalent sintering additives (Ca 2 ∕Mg 2) and Fe ion impurities. Two individual peaks in the thermoluminescence spectra illustrate both shallow and deep level traps. From positron annihilation lifetime data, vacancy clusters and nanovoids are detected and characterized, although these free-volume defects could not be observed by high-resolution transmission electron microscopy. The solarization induced by UV irradiation is associated with a change in the structure and size of defect clusters due to lattice relaxation. Therefore, this work shows how UV irradiation leads not only to a change in the charge state of defects, but also to a permanent change in defect structure and size. It significantly affects the optical properties of YAG ceramics and their performance in lasers and other optical applications. These results are crucial for advancing transparent ceramics technology.
As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide i... more As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI and demonstrate that the main degradation pathway of CrI is the photocatalytic substitution of iodine by water. While simple encapsulation by AlO, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.
Exciton polaritons constitute a unique realization of a quantum fluid interacting with its enviro... more Exciton polaritons constitute a unique realization of a quantum fluid interacting with its environment. Using selenide-based microcavities, we exploit this feature to warm up a polariton condensate in a controlled way and monitor its spatial coherence. We determine directly the amount of heat picked up by the condensate by measuring the phonon-polariton scattering rate and comparing it with the loss rate. We find that, upon increasing the heating rate, the spatial coherence length decreases markedly, while localized phase structures vanish, in good agreement with a stochastic mean-field theory. From the thermodynamical point of view, this regime is unique, as it involves a nonequilibrium quantum fluid with no well-defined temperature but which is nevertheless able to pick up heat with dramatic effects on the order parameter.
As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy... more As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulating few-layer BP transistors with mobility up to 55,000 cm 2 /Vs, we extract LL gaps over an exceptionally wide range of B for QH statesat filling factors ν =-1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν ~ -4/3 and an additional feature at - 0.56 ± 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.
Charge carriers in magic angle graphene come in eight flavors described by a combination of their... more Charge carriers in magic angle graphene come in eight flavors described by a combination of their spin, valley, and sublattice polarizations [1-4]. When the inversion and time reversal symmetries are broken by the substrate or by strong interactions, the degeneracy of the flavors can be lifted and their corresponding bands can be filled sequentially [5-7]. Due to their non-trivial band topology and Berry curvature, each of the bands is classified by a topological Chern number [8-12], leading to the quantum anomalous Hall [13-15] and Chern insulator states [7,8,16-19] at integer fillings of the bands. It has been recently predicted, however, that depending on the local atomic-scale arrangements of the graphene and the encapsulating hBN lattices, rather than being a global topological invariant, the Chern number may become position dependent, altering transport and magnetic properties of the itinerant electrons [20-23]. Using scanning superconducting quantum interference device on a tip (SQUID-on-tip) [24], we directly image the nanoscale Berry-curvatureinduced equilibrium orbital magnetism, the polarity of which is governed by the local Chern number, and detect its two constituent components associated with the drift and the self-rotation of the electronic wave packets [25]. At = 1, we observe local zero-field valley-polarized Chern insulators forming a mosaic of microscopic patches of = −1, 0, or 1. Upon further filling, we find a first-order phase transition due to recondensation of electrons from valley to ′, which leads to irreversible flips of the local Chern number and the magnetization, and to the formation of valley domain walls giving rise to hysteretic global anomalous Hall resistance. The findings shed new light on the structure and dynamics of topological phases and call for exploration of the controllable formation of flavor domain walls and their utilization in twistronic devices.
2017 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), 2017
In this work we have measured the far-field emission patterns of In As quantum dots embedded in a... more In this work we have measured the far-field emission patterns of In As quantum dots embedded in a GaAs tapered nanowire and used an open-geometry Fourier modal method for determining the radial position of the quantum dots by computing the far-field emission pattern for different quantum dot locations.
The celebrated phenomenon of quantum Hall effect has recently been generalized from transport of ... more The celebrated phenomenon of quantum Hall effect has recently been generalized from transport of conserved charges to that of other approximately conserved state variables, including spin and valley, which are characterized by spin- or valley-polarized boundary states with different chiralities. Here, we report a new class of quantum Hall effect in ABA-stacked graphene trilayers (TLG), the quantum parity Hall (QPH) effect, in which boundary channels are distinguished by even or odd parity under the systems mirror reflection symmetry. At the charge neutrality point and a small perpendicular magnetic field $B_{\perp}$, the longitudinal conductance $\sigma_{xx}$ is first quantized to $4e^2/h$, establishing the presence of four edge channels. As $B_{\perp}$ increases, $\sigma_{xx}$ first decreases to $2e^2/h$, indicating spin-polarized counter-propagating edge states, and then to approximately $0$. These behaviors arise from level crossings between even and odd parity bulk Landau levels...
2) field effect transistors (FET) devices and develop an effective gas annealing technique that s... more 2) field effect transistors (FET) devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3-4 orders of magnitude. Temperature dependence measurements reveal two transport mechanisms: electron-phonon scattering at high temperatures and thermal activation over a gatetunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates. Moreover, this suspended MoS 2 device structure provides double surface access for ionic liquid gating. We are able to extract the dielectric constant of the ionic liquid, and the latest experimental results will be presented.
La these a porte sur l'etude par spectroscopie magneto-optique de boites quantiques magnetiqu... more La these a porte sur l'etude par spectroscopie magneto-optique de boites quantiques magnetiques et de nanofils semiconducteurs. Ces nanostructures a base de semiconducteurs magnetiques dilues sont elaborees et etudiees dans l'equipe depuis une quinzaine d'annee. L'interet de ces structures repose sur la possibilite de controler et d'etudier tres finement le couplage d'un petit nombre de spins localises avec des porteurs libres. La premiere partie de la presente these a porte sur l'etude des conditions de formation de polarons magnetiques (orientation d'un petit ensemble de spins par l'intermediaire d'un porteur ou d'une paire electron-trou) dans des boites quantiques CdMnTe. La formation de polarons magnetiques encore assez mal comprise avec les systemes 0D tels que les boites quantiques auto-assemblees. Une serie d'echantillons originaux de boites quantiques magnetiques auto-assemblees (concentration en manganese intermediaires) a ete...
The coexistence of superconducting and correlated insulating states in magic-angle twisted bilaye... more The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene 1-11 prompts fascinating questions about their relationship. Independent control of the microscopic mechanisms that govern these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing the separation distance between the graphene and a metallic screening layer 12,13. We observe quenching of correlated insulators in devices with screening layer separations that are smaller than the typical Wannier orbital size of 15 nanometres and with twist angles that deviate slightly from the magic angle of 1.10 ± 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 tesla, giving rise to quantized Hall states with a Chern number of 2. Our study suggests re-examination of the often-assumed 'parent-and-child' relation between the insulating and superconducting phases in moiré graphene, and suggests a way of directly probing the microscopic mechanisms of superconductivity in strongly correlated systems.
Rashba spin-orbit coupling in atomically thin InSe is varied layer by layer and by tuning out-of-... more Rashba spin-orbit coupling in atomically thin InSe is varied layer by layer and by tuning out-of-plane electric field.
In transition metal dichalcogenides layers of atomic scale thickness, the electron-hole Coulomb i... more In transition metal dichalcogenides layers of atomic scale thickness, the electron-hole Coulomb interaction potential is strongly influenced by the sharp discontinuity of the dielectric function across the layer plane. This feature results in peculiar non-hydrogenic excitonic states, in which exciton-mediated optical nonlinearities are predicted to be enhanced as compared to their hydrogenic counterpart. To demonstrate this enhancement, we performed optical transmission spectroscopy of a MoSe2 monolayer placed in the strong coupling regime with the mode of an optical microcavity, and analyzed the results quantitatively with a nonlinear input-output theory. We find an enhancement of both the exciton-exciton interaction and of the excitonic fermionic saturation with respect to realistic values expected in the hydrogenic picture. Such results demonstrate that unconventional excitons in MoSe2 are highly favourable for the implementation of large exciton-mediated optical nonlinearities, potentially working up to room temperature.
The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of supe... more The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of superconducting, magnetic and topologically nontrivial phases. The existence of all these phases in one material, and their tunability, has opened new pathways for the creation of unusual gate tunable junctions. However, the required conditions for their creation – gate induced transitions between phases in zero magnetic field – have so far not been achieved. Here, we report on the first experimental demonstration of a device that is both a zero-field Chern insulator and a superconductor. The Chern insulator occurs near moiré cell filling factor = +1 in a hBN non-aligned MATBG device and manifests itself via an anomalous Hall effect. The insulator has Chern number C= ±1 and a relatively high Curie temperature of Tc ≈ 4.5 K. Gate tuning away from this state exposes strong superconducting phases with critical temperatures of up to Tc ≈ 3.5 K. In a perpendicular magnetic field above B > 0.5 ...
Superconductivity often occurs close to broken-symmetry parent states and is especially common in... more Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators 1. When twisted close to a magic relative orientation angle near !°, bilayer graphene has flat moiré superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics 2-5 , notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moiré band filling factors # = %, ±!, ±(, ±), and superconductivity below critical temperatures as high as ~ 3 K close to-2 filling. We also observe three new superconducting domes at much lower temperatures close to the # = % and # = ±! insulating states. Interestingly, at # = ±! we find states with non-zero Chern numbers. For # = −! the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moiré flat bands, including near charge neutrality.
Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with it... more Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum 1. Likewise, in the optical domain, a synthetic spin-orbit coupling is accessible, for instance, by engineering optical anisotropies in photonic materials 2. Both, akin, yield the possibility to create devices directly harnessing spin-and polarization as information carriers 3. Atomically thin layers of transition metal dichalcogenides provide a new material platform which promises intrinsic spin-valley Hall features both for free carriers, two-particle excitations (excitons), as well as for photons 4. In such materials, the spin of an exciton is closely linked to the high-symmetry point in reciprocal space it emerges from (K and K' valleys) 5,6. Here, we demonstrate, that spin, and hence valley selective propagation is accessible in an atomically thin layer of MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a wire-like device, where we can clearly trace the flow, and the helicity of exciton-polaritons expanding along a channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley selective expansion of the polariton cloud without neither any applied external magnetic fields nor coherent Rayleigh scattering. Unlike 17-02-00383.
Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-... more Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-state platform for quantum optics. Within the nanowire section, the emitter position simultaneously determines the strength of the light-matter interaction, as well as the coupling to potential decoherence channels. Therefore, to quantitatively understand device performance and guide future optimization, it is highly desirable to map the emitter position with an accuracy much smaller than the waveguide diameter, on the order of a few hundreds of nanometers. We introduce here a nondestructive, all-optical mapping technique that exploits the QD emission into two guided modes with different transverse profiles. These two modes are fed by the same emitter and thus interfere. The resulting intensity pattern, which is highly sensitive to the emitter position, is resolved in the far-field using Fourier microscopy. We demonstrate this technique on a standard microphotoluminescence setup and map the position of individual QDs in a nanowire antenna with a spatial resolution of ±10 nm. This work opens important perspectives for the future development of light-matter interfaces based on nanowire antennas. Beyond single-QD devices, it will also provide a valuable tool for the investigation of collective effects that imply several emitters coupled to an optical waveguide.
Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics... more Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 µm-long, insulating channel only when it is biased into the ν=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of ferromagnetic and antiferromagnetic spintronics. An important goal of spintronics research is to identify mechanisms that minimize dissipation in devices that seek to exploit the action of spin currents. In magnetic insulators, spin-currents can be carried dissipatively by magnon quasiparticles 1-3. In the case of systems with easy plane magnetic order, they can also be carried collectively in the form of dissipationless spin supercurrents 4-9. While magnon transport is much less efficient in an ideal antiferromagnetic insulators (AFMI) than that in ferromagnetic insulators in the absence of a thermal gradient, spin superfluidity is theoretically expected to be a possibility in both cases. Although the potential of AFM materials 5,6,10 as active spintronic components that can be
Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination... more Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination. In this paper, an interesting and surprising phenomenon in YAG transparent ceramics is reported. UV light leads to significant changes in the microstructure of open volume defects and nano clusters as well as in the optical properties. Lightinduced lattice relaxation is suggested as the mechanism behind this intriguing behavior. The complex F-type color center with broad absorption bands is caused by the aliovalent sintering additives (Ca 2 ∕Mg 2) and Fe ion impurities. Two individual peaks in the thermoluminescence spectra illustrate both shallow and deep level traps. From positron annihilation lifetime data, vacancy clusters and nanovoids are detected and characterized, although these free-volume defects could not be observed by high-resolution transmission electron microscopy. The solarization induced by UV irradiation is associated with a change in the structure and size of defect clusters due to lattice relaxation. Therefore, this work shows how UV irradiation leads not only to a change in the charge state of defects, but also to a permanent change in defect structure and size. It significantly affects the optical properties of YAG ceramics and their performance in lasers and other optical applications. These results are crucial for advancing transparent ceramics technology.
As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide i... more As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI and demonstrate that the main degradation pathway of CrI is the photocatalytic substitution of iodine by water. While simple encapsulation by AlO, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.
Exciton polaritons constitute a unique realization of a quantum fluid interacting with its enviro... more Exciton polaritons constitute a unique realization of a quantum fluid interacting with its environment. Using selenide-based microcavities, we exploit this feature to warm up a polariton condensate in a controlled way and monitor its spatial coherence. We determine directly the amount of heat picked up by the condensate by measuring the phonon-polariton scattering rate and comparing it with the loss rate. We find that, upon increasing the heating rate, the spatial coherence length decreases markedly, while localized phase structures vanish, in good agreement with a stochastic mean-field theory. From the thermodynamical point of view, this regime is unique, as it involves a nonequilibrium quantum fluid with no well-defined temperature but which is nevertheless able to pick up heat with dramatic effects on the order parameter.
As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy... more As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulating few-layer BP transistors with mobility up to 55,000 cm 2 /Vs, we extract LL gaps over an exceptionally wide range of B for QH statesat filling factors ν =-1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν ~ -4/3 and an additional feature at - 0.56 ± 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.
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Papers by Petr Stepanov