Noejung Park

Quantum geometric quantities, such as Berry curvature (BC) and quantum metric, are known to govern the physical and topological properties of materials, making access to these quantities critically important for the comprehensive understanding of the materials. Chiral crystals offer a natural platform in which structural handedness can control quantum geometry and nonlinear transport, but direct experimental access to the quantum geometric quantities has remained elusive. Here, we show that trigonal tellurium (Te) hosts an enantiomer-dependent Berry curvature dipole (BCD) that can be accessed through the measurement of the cross-axial nonlinear Hall effect (NHE), unlike in previously studied in-plane configurations, where the BCD contribution vanishes. Angle-resolved measurements on left- and right-handed Te devices reveal a second-order transverse response whose sign reverses with crystal chirality. The extracted cross-axial nonlinear susceptibility, χyxz, exceeds the in-plane susceptibility, χxxx, by more than two-fold, indicating a strong BCD-driven contribution. First-principles calculations attribute this behavior to a topologically nontrivial hedgehog-like BC texture near the H points and predict a BCD that reverses sign between opposite enantiomers, consistent with experiment. These results establish cross-axial nonlinear Hall transport as a symmetry-selective probe of chirality-controlled quantum geometry in tellurium and related chiral materials.

Quantized charge pumping in one-dimensional chiral wires has been widely studied in the context of topological physics in (1 + 1)-dimensional synthetic space, yet the role of orbital and spin degrees of freedom remains largely unexplored. Here, we show that topological charge pumping in insulating chiral systems intrinsically generates orbital and spin polarization, providing a new perspective on spin-selective transport in chiral materials, often associated with chirality-induced spin selectivity. Using time-dependent Schrödinger dynamics of multiorbital tight-binding models driven by circularly polarized light, we identify two key results. First, the screw-like geometry enables a single-parameter topological charge pumping. Second, while the energy gap remains open throughout the pumping cycle, Berry phase driven dynamics induces nonequilibrium orbital polarization. Through spin–orbit coupling, this orbital response is partially converted into spin polarization. By analogy between synthetic (1 + 1)- and two-dimensional topological insulators, we suggest that nontrivial spin–orbital dynamics may accompany anomalous quantum charge Hall states.

The spin dynamics of electrons in chiral molecular systems remains a topic of intense interest, particularly regarding whether geometric chirality inherently induces spin polarization in current-carrying electrons. In this work, we employ ab initio real-time time-dependent density functional theory (rt-TDDFT) to directly simulate the interplay among charge current, spin, and orbital. This real-time tracking extends beyond perturbative treatments, and we analyze how nonequilibrium currents effectively lift the symmetry constraints of screw rotation and time-reversal symmetry. We find that the emergence of spin and orbital angular momenta is dynamically correlated with a concomitant loss of translational (linear) momentum, which we interpret as an intrinsic consequence of current-driven symmetry lowering. The implications of this mechanism for chirality-induced spin selectivity and spintronics device design are discussed.

Quantized charge pumping in one-dimensional chiral wires has been widely studied in the context of topological physics in a (1+1)-dimensional synthetic space, yet the role of orbital and spin degrees of freedom in such topological pumps remains largely unexplored. Here, we examine how the topologically quantized charge pump in insulators generates spin polarizations, and assess whether this mechanism may offer distinct insight into the widely known spin-selective transport in chiral wires-commonly referred to as chirality-induced spin selectivity. We performed time-dependent Schrodinger equations of multi-orbital tight-binding Hamiltonians driven by a circularly polarized electric field. Our main findings are twofold. First, the intrinsic screw-like geometry of the system generates a distinctive winding structure governed by a single control parameter, in contrast to conventional adiabatic pumping mechanisms that require at least two independently modulated parameters, thereby providing a clear interpretation of one-dimensional pumping in terms of the topological structure in a (1+1)-dimensional Brillouin zone. Second, while the energy gap remains open throughout the pumping cycle, the Berry-phase driven real-time dynamics of the charge flow induces a nonequilibrium orbital polarization. Through spin-orbit coupling, this orbital response is partially converted into spin polarization whose direction is determined by the current and chirality. On the analogy between the synthetic (1+1)- and 2-dimensional topological insulators, we suggest that non-trivial spin-orbital dynamics may accompany the anomalous quantum charge Hall states of even-dimensional real materials.

The interaction between electron spin and molecular chirality plays a fundamental role in quantum phenomena with significant implications for spintronics and quantum computing. The chirality-induced spin selectivity (CISS) effect, where chiral materials preferentially transmit electrons of a particular spin, has sparked intense interest and debate regarding its underlying mechanism. Despite extensive research, the spatial distribution of spin polarization in chiral systems, the key evidence to reveal the spin scattering mechanism in CISS, has remained experimentally elusive, particularly due to complications arising from spin-orbit coupling in metal electrodes typically used in such studies. Here we show, through reflective magnetic circular dichroism measurements on chiral tellurium nanowires with graphene electrodes, that current-induced spin polarization exhibits identical signs in both the nanowire and the electrodes, distinct from the presumed spin filter scenario. The observed spin polarization scales linearly with current amplitude, aligns parallel to the current direction, reverses with chirality or current flow, and demonstrates spin relaxation lengths of several micrometers into graphene. Our findings provide direct visualization of spatial spin distribution in chiral devices. This work demonstrates an approach for investigating spin-dependent phenomena in chiral materials and explores their potential impact for developing chirality-based spintronic and quantum devices.

The interaction between electron spin and molecular chirality plays a fundamental role in quantum phenomena, with significant implications for spintronics and quantum computing. The chirality-induced spin selectivity (CISS) effect, where chiral materials preferentially transmit electrons of a particular spin, has sparked intense interest and debate regarding its underlying mechanism. Despite extensive research, the spatial distribution of spin polarization in chiral systems, the key evidence to reveal the spin scattering mechanism in CISS, has remained experimentally elusive particularly due to complications arising from spin-orbit coupling in metal electrodes typically used in such studies. Here we show, through reflective magnetic circular dichroism measurements on chiral tellurium nanowires with graphene electrodes, that current-induced spin polarization exhibits identical signs in both the nanowire and electrodes, distinct from the presumed spin filter scenario. The observed spin polarization scales linearly with current amplitude, aligns parallel to the current direction, reverses with chirality or current flow, and demonstrates spin relaxation lengths of several micrometers into graphene. Our findings provide the first direct visualization of spatial spin distribution in chiral devices. This work establishes a new paradigm for investigating spin-dependent phenomena in chiral materials and opens avenues for developing chirality-based spintronic and quantum devices.

Recent interest in orbital angular momentum has led to a rapid expansion of research on spin-orbit coupling effects in solids, while also highlighting significant technical challenges. The breaking of rotational symmetry renders the orbital angular momentum operator ill defined, causing conceptual and computational issues in describing orbital motion. To address these issues, here we propose an alternative framework. Based on the Bloch representation of the full relativistic interaction, we derive a field that directly couples to electron spins while preserving discrete translational symmetry, thereby eliminating the need for the position operator. Our approach is fully compatible with existing first-principles computational frameworks for both static and time-dependent density functional theory. We demonstrate that this method offers a more effective description of the Edelstein and spin Hall effects compared to conventional orbital angular momentum formalisms.

The recent rise in interest in altermagnetism─a magnetic state distinct from ferromagnetism and antiferromagnetism─nessitates the development of a reliable probing method to distinguish it from other types of compensated magnetism. Here, we investigate nonrelativistic spin groups and relativistic magnetic groups to identify and categorize the permissible spin and charge photocurrents of compensated collinear magnets. Our results indicate that, in altermagnets, the charge and spin photocurrents exhibit a similar temperature profile due to their shared dependence on carrier relaxation effects, whereas in antiferromagnets, these currents behave in stark contrast. As a real-material example, we performed first-principles calculations of the altermagnetic phase of MnO and analyzed it using a simplified four-band model. We demonstrate that the normal injection current directly reflects the magnetic band geometry of altermagnets: the direction and magnitude of the injection current are delicately correlated with the interband Berry curvature and the alternating spin splitting of energy bands. We suggest that nonlinear photocurrent could serve as a promising probing tool to reveal the particular magnetic phase and band geometry of altermagnets.

Ultrafast optical control of ferroelectricity based on short and intense light can be utilized to achieve accurate manipulations of ferroelectric materials, which may pave a basis for future breakthrough in nonvolatile memories. Here, we demonstrate that phase manipulation of electric field in the strong field sub-cycle regime induces a nonlinear injection current, efficiently coupling with the topology of band structure and enabling dynamic reversal of both current and polarization. Our time-dependent first-principles calculations reveal that tuning the phase of linearly or circularly polarized light through time-varying chirp, or constant carrier envelop phases within sub-laser-cycle dynamics effectively breaks the time-reversal symmetry, allowing the control over current and electronic polarization reversal over multi-ferroelectric states. Our time- and momentum-resolved transverse current analysis reveal the significance of Berry curvature higher order poles in the apparent association between the odd (even) orders of Berry curvature multipoles to odd (even) pseudo-harmonics in driving polarization dynamics reversal. We suggest that these phase manipulations of short pulse waveform may lead to unprecedented accurate control of nonlinear photocurrents and polarization states, which facilitate the development of precise ultrafast opto-ferroelectric devices.