Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2 (1807.01259v1)

Abstract: Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe$_2$ has bridged two of the most active fields of condensed matter physics, 2D materials and topological physics. This 2D topological crystal also displays unconventional spin-torque and gate-tunable superconductivity. While the realization of QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe$_2$, the geometrical properties of the wavefunction, such as the Berry curvature, remain unstudied. On the other hand, it has been increasingly recognized that the Berry curvature plays an important role in multiple areas of condensed matter physics including nonreciprocal electron transport, enantioselective optical responses, chiral polaritons and even unconventional superconductivity. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe$_2$. By optically exciting electrons across the inverted QSH gap, we observe an in-plane circular photogalvanic current even under normal incidence. The application of an out-of-plane displacement field further systematically controls the direction and magnitude of the photocurrent. Our observed photocurrent reveals a novel Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge. These previously unrealized Berry curvature dipole and strong electric field effect are uniquely enabled by the inverted band structure and tilted crystal lattice of monolayer WTe$_2$. Such an electrically switchable Berry curvature dipole opens the door to the observation of a wide range of quantum geometrical phenomena, such as quantum nonlinear Hall, orbital-Edelstein and chiral polaritonic effects.

• The paper demonstrates that monolayer WTe2 exhibits a novel Berry curvature dipole due to its inverted band structure.
• It employs mid-infrared optoelectronic microscopy to reveal electrically tunable CPGE currents linked to Berry curvature effects.
• The findings offer insights for engineering optoelectronic devices and advancing quantum spin Hall applications in 2D materials.

Overview of Electrically Switchable Berry Curvature Dipole in WTe$_2$

This paper presents an investigation into the electrically switchable Berry curvature dipole in monolayer WTe$_2$, a material which serves as a unique platform for the paper of quantum spin Hall (QSH) states. The authors utilized mid-infrared optoelectronic microscopy to explore Berry curvature effects manifested in the photogalvanic current, revealing a Berry curvature dipole that is both novel and previously uncharacterized.

Key Findings

• Geometric Properties of WTe$_2$: The paper successfully showcases the Berry curvature dipole in monolayer WTe$_2$, which arises due to its inverted band structure. This distinct feature diverges from typical materials which either have zero Berry curvature due to inversion symmetry or lack significant topological properties.
• Electrically Tunable Berry Curvature: The authors demonstrate that an out-of-plane displacement field can systematically manipulate the Berry curvature dipole. The direction and magnitude of the photocurrent can be electrically controlled, a property attributed to the distinct crystal structure of WTe$_2$ which responds to out-of-plane polarization with in-plane effects.
• Circular Photogalvanic Effect (CPGE): The observation of CPGE currents along a specific crystal axis in response to circularly polarized light supports a direct connection to the Berry curvature, validating theories predicting inter-band transition processes in low-dimensional systems.
• Temperature and Displacement Field Dependence: The dynamic behavior of CPGE was observed to be sensitive to both temperature and displacement fields, suggesting an interaction between the electronic states at the band edge and the external fields.

Implications

The implications of this research are profound both practically and theoretically. The ability to tune Berry curvature with an electric field suggests potential applications in the development of optoelectronic devices where non-linear optical properties are engineered through geometric phase control. Moreover, the observed CPGE influenced by Berry curvature underpins theoretical explorations on quantum geometric effects like the quantum nonlinear Hall effect and orbital Edelstein effects.

Future Directions

This paper paves the way for further exploration into the quantum geometric properties of two-dimensional materials, particularly those with tunable topological properties like Berry curvature. The integration of such materials into device architectures could potentially harness the topological field effects for innovative applications in quantum computing and energy-efficient optoelectronics. Additionally, understanding the role of topological edge states and their influence on chiral plasmons may enrich plasmonic research and the paper of polaritons.

Overall, the findings contribute significantly to our understanding of quantum geometric phenomena in condensed matter systems, broadening the horizon for new materials with exotic electronic and optical properties. The application of a Berry curvature dipole thus provides fertile ground for pioneering advancements in the field of material science and quantum technology.