Showing papers by "Jian-Hao Chen published in 2021"

Electrically switchable van der Waals magnon valves.

Abstract: Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology. A major challenge in magnon based approaches to information processing lies in developing valves to allow or supress the magnon signal. Here, Chen et al demonstrate a van der Waals magnet based magnon valve which can be tuned electrically over an exceptionally wide range.

Circular photogalvanic effect from third-order nonlinear effect in 1T’-MoTe2

Abstract: The two-dimensional layered material MoTe2 has aroused extensive research interests in its rich optoelectronic properties in various phases. One property of particular interest is the circular photogalvanic effect (CPGE): a conventional second order nonlinear optical effect that is related to the chirality of materials. It has been demonstrated in Td-MoTe2, a type-II topological Weyl semimetal candidate, while it has been unclear so far whether it exists in the semimetallic 1T’ phase, another interesting phase that hosts a quantum spin hall state. In this article, we report a clear experimental observation of in-plane CPGE in 1T’-MoTe2. The observation is confirmed under various experimental designs with excitation by normally incident mid-infrared laser, and we find it to be related to an in-plane internal DC electric field. We attribute the circular photogalvanic response to a third-order nonlinear optical effect involving this DC electric field, which is consistent with the crystal symmetry of the lattices and present in both the 1T’ and Td phases of the material.

Electron-electron interactions and weak antilocalization in few-layer Zr Te 5 devices

Abstract: Much effort has been devoted to the electronic properties of relatively thick $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, focusing on their three-dimensional topological effects. Thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, on the other hand, were much less explored experimentally. Here we present detailed magnetotransport studies of few-layer $\mathrm{Zr}{\mathrm{Te}}_{5}$ devices, in which electron-electron interactions and weak antilocalization are observed. The coexistence of the two effects manifests itself in corroborating evidence presented in the temperature and magnetic field dependence of the resistance. Notably, the temperature-dependent phase coherence length extracted from weak antilocalization agrees with strong electron-electron scattering in the sample. Meanwhile, universal conductance fluctuations have temperature and gate voltage dependence that is similar to that of the phase coherence length. Last, all the transport properties in thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals show strong two-dimensional characteristics. Our results provide insight into the highly intricate properties of topological material $\mathrm{Zr}{\mathrm{Te}}_{5}$.

Two superconductor-insulator phase transitions in the spinel oxide Li 1 ± x Ti 2 O 4 − δ induced by ionic liquid gating

Abstract: The associations between emergent physical phenomena (e.g., superconductivity) and orbital, charge, and spin degrees of freedom of $3d$ electrons are intriguing in transition metal compounds. Here, we successfully manipulate the superconductivity of spinel oxide ${\mathrm{Li}}_{1\ifmmode\pm\else\textpm\fi{}x}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ (LTO) by ionic liquid gating. A dome-shaped superconducting phase diagram is established, where two insulating phases are disclosed both in heavily electron-doping and hole-doping regions. The superconductor-insulator transition (SIT) in the hole-doping region can be attributed to the loss of Ti valence electrons. In the electron-doping region, LTO exhibits an unexpected SIT instead of a metallic behavior despite an increase in carrier density. Furthermore, a thermal hysteresis is observed in the normal state resistance curve, suggesting a first-order phase transition. We speculate that the SIT and the thermal hysteresis stem from the enhanced $3d$ electron correlations and the formation of orbital ordering by comparing the transport and structural results of LTO with the other spinel oxide superconductor ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ (MTO), as well as analyzing the electronic structure by first-principles calculations. Further comprehension of the detailed interplay between superconductivity and orbital ordering would contribute to the revealing of unconventional superconducting pairing mechanism.

Crossover behavior in the magnetoresistance of thin flakes of the topological material Zr Te 5

Abstract: $\mathrm{Zr}{\mathrm{Te}}_{5}$ is a layered material that exhibits intricate topological effects. Intensive theoretical and experimental efforts have been devoted to try to understand the physics in this material. In this paper the temperature dependent magnetotransport properties of $\mathrm{Zr}{\mathrm{Te}}_{5}$ thin flakes are investigated. A characteristic temperature ${T}^{*}$ is observed in the temperature dependence of three different types of magnetoresistance simultaneously, which are the saturated Hall anomaly, the chiral anomaly, and the longitudinal magnetoresistance. Furthermore, the value of ${T}^{*}$ decreases monotonically from 200 to 160 K with increasing thickness of the $\mathrm{Zr}{\mathrm{Te}}_{5}$ thin flakes from 42 to 89 nm. Temperature induced topological phase transitions are attributed to the cause of such anomaly in the three types of magnetoresistance at ${T}^{*}$. Our findings provide a multiparameter indicator for the emergence of topological phase transition in $\mathrm{Zr}{\mathrm{Te}}_{5}$ and could be extended to the study of other topological materials. The temperature dependence of the three types of magnetoresistance also sheds light on the role of anomalous Hall effect in the transport properties of $\mathrm{Zr}{\mathrm{Te}}_{5}$.

Enhancement of spin-orbit coupling and magnetic scattering in hydrogenated graphene

Abstract: Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum environment. Transition from weak localization to weak antilocalization with increasing hydrogen adatom density is observed, indicating enhancing Bychkov-Rashba-type SOC in a mirror symmetry broken system. From the low-temperature saturation of phase breaking scattering rate, the existence of spin-flip scattering is identified, which corroborates the existence of magnetic moments in hydrogenated graphene.

Realization of graphene logics in an exciton-enhanced insulating phase

Abstract: For two decades, two-dimensional carbon species, including graphene, have been the core of research in pursuing next-generation logic applications beyond the silicon technology. Yet the opening of a gap in a controllable range of doping, whilst keeping high conductance outside of this gapped state, has remained a grand challenge in them thus far. Here we show that, by bringing Bernal-stacked bilayer graphene in contact with an anti-ferromagnetic insulator CrOCl, a strong insulating behavior is observed in a wide range of positive total electron doping $n_\mathrm{tot}$ and effective displacement field $D_\mathrm{eff}$ at low temperatures. Transport measurements further prove that such an insulating phase can be well described by the picture of an inter-layer excitonic state in bilayer graphene owing to electron-hole interactions. The consequential over 1 $\mathrm{G\Omega}$ excitonic insulator can be readily killed by tuning $D_\mathrm{eff}$ and/or $n_\mathrm{tot}$, and the system recovers to a high mobility graphene with a sheet resistance of less than 100 $\mathrm{\Omega}$. It thus yields transistors with "ON-OFF" ratios reaching 10$^{7}$, and a CMOS-like graphene logic inverter is demonstrated. Our findings of the robust insulating phase in bilayer graphene may be a leap forward to fertilize the future carbon computing.

Electrically Switchable van der Waals Magnon Valves.

Abstract: Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology.

Enhancement of spin-orbit coupling and magnetic scattering in hydrogenated graphene.

Abstract: Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum environment. Transition from weak localization to weak antilocalization with increasing hydrogen adatom density is observed, indicating enhancing Bychkov-Rashba-type SOC in a mirror symmetry broken system. From the low-temperature saturation of phase breaking scattering rate, the existence of spin-flip scattering is identified, which corroborates the existence of magnetic moments in hydrogenated graphene.