arXiv subjects

Bumseop Kim

Publications and source records attributed to Bumseop Kim.

7 recordsLinked to original sources

arXiv2026

Unified First-Principles Theory of Finite-Wave-Vector Photocurrents in Crystals

Spatially structured light carries independent phase and envelope variations in addition to its polarization and frequency. We develop a first-principles theory of second-order dc photocurrents (especially shift and injection) that resolves how these optical degrees of freedom drive distinct dc responses in crystals. The second-order dc response couples two optical field components at the same frequency but with generally different wave vectors. Expanding the response to first order in these wave vectors identifies two rank-four material tensors: $ξ$, which couples to the optical phase gradient and contains the plane-wave photon-drag limit, and $ζ$, which couples to the intensity gradient and describes a spatially varying dc current. Wannier-interpolated calculations for monolayer GeS show symmetry-allowed finite-wave-vector responses that survive at a centrosymmetric structure where conventional bulk photovoltaic effect vanishes. For a Gaussian beam with a uniform phase gradient, the phase-gradient contribution drives current in the same direction on both sides of the beam center, whereas the intensity-gradient contribution drives currents in opposite directions, showing that distinct photocurrent patterns can be selected through the wavefront and envelope of light. A multipolar analysis further resolves the optical corrections into electric-quadrupole, orbital magnetic-dipole, and band-dispersion contributions. Together, these results show how the spatial structure of light can serve as an independent control variable for nonlinear photocurrent generation and provide a first-principles route to predict this response in real materials.

cond-mat.mes-hall↗

arXiv2026

Spatially dispersive photogalvanic effects as a probe of quantum geometric tensor

Berry curvature and quantum metric of Bloch states govern a wide range of phenomena, yet unified experimental access to both remains limited. Here we show that light spatial-dispersion alters the selection rules of second-order photocurrents and responses forbidden by parity become allowed in centrosymmetric crystals without any external fields, magnetic order, interfaces, strain, or engineered symmetry breaking. The resulting response is intrinsic and its weight is set by the quantum geometry of the Bloch states. Conventional photogalvanic effects are constrained by crystal symmetry and entangle the quantum metric with shift vector contributions. The spatially-dispersive response instead isolates the quantum metric and Berry curvature in distinct polarization channels. Implementing this approach in 1T'-MoTe2 across its temperature-driven transition to Td-Weyl phase, we resolve helicity-even and helicity-odd photocurrents corresponding to quantum metric and Berry curvature contributions, respectively, from the same device. The metric-dominated response persists across both phases and exhibits a robust spectral structure reproduced by first-principles calculations and linked to momentum-resolved quantum metric hotspots. In contrast, the curvature-driven channel emerges only when inversion symmetry is broken and shows strong sensitivity to carrier doping through competing momentum-space contributions. Our results establish photogalvanic effects with spatially varying optical fields as a general route to accessing quantum geometry in materials where photon energy and electronic filling probe different parts of the excitation manifold.

cond-mat.mes-hall↗

arXiv2026

Defect-Controlled Multiferroicity via Stacking Control in Nonmagnetic van der Waals Bilayers

We present a general paradigm that directly couples vacancy-localized magnetism to interfacial sliding ferroelectricity in nonmagnetic van der Waals (vdWs) bilayers. Utilizing bilayer hexagonal boron nitride (hBN) as a prototypical vdWs host system, we use first-principles calculations to show that a single vacancy acts as a local registry sensor, lifting the degeneracy between polar sliding partners via a defect-centered polarization offset. For interlayer vacancy pairs, we discover a defect selectivity where the hosting sublattice fully dictates the interlayer exchange, stabilizing either ferrimagnetic or antiferromagnetic configurations. Applying an out-of-plane electric field selects the polar registry and drives an amplitude modulation of the compensated Néel order parameter. These findings establish a robust, sublattice-dependent engineering of multiferroic functionality via stacking control across a wide class of nonmagnetic 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Optical Signature of Moiré Superlattices Formed by Twisted SrTiO$_3$ Membranes

Moiré superlattices formed at the interfaces of mismatched lattices have attracted significant interest over the past decade due to their large tunability of band parameters and interactions among electrons, spins, and lattices. Superlattices made from twisted perovskite oxides may have strong structure and potential modulation, but evidence of such modulation over macroscopic areas, particularly at large twisting angles, has not been clearly demonstrated so far. Here, we fabricated millimeter-scale twisted oxide bilayers at $36^\circ$ angle, close to the simple coincidence site lattice condition $\Sigma5$, from freestanding SrTiO$_3$ membranes. We discovered new low-frequency vibrational modes whose Raman activity, according to molecular dynamics simulations, is greatly enhanced by an asymmetric, twisted interface between the SrO and TiO$_2$ layers. Such an interface is energetically favorable from first-principles calculations and is corroborated by the observation of strong second harmonic generation from the interface comparable to that from the SrTiO$_3$ surface throughout the bilayer region. The results are consistent with interlayer coupling enhanced by high-temperature annealing and confirmed by cross-sectional scanning transmission electron microscopy imaging. Our work sheds light on the structural behavior of twisted oxides and provides directions for tuning their phononic and nonlinear optical properties in future studies.

cond-mat.mtrl-sci↗

arXiv2026

Field-unmasked quantum geometry in a symmetry-forbidden photocurrent

Frequency- and polarization-resolved photocurrents provide a sensitive probe of hidden symmetry and band geometry in quantum materials. Here we study a chiral cubic sillenite whose global crystal symmetry forbids a longitudinal odd-in-B magneto-photocurrent in the Voigt geometry. Nevertheless, we observe a pronounced longitudinal response across the visible range that is predominantly linear in magnetic field, persists below the band gap, and exhibits strong helicity selectivity, with the circular channel exceeding the linear one and reversing sign upon switching light helicity. We resolve this apparent contradiction by identifying defect-enabled, field-selected spin ordering as the mechanism that lowers the effective magnetic symmetry without altering the global crystal structure. First-principles calculations show that O vacancies generate in-gap bound states and localized magnetic moments on neighboring Bi-O units, stabilized by strong SOC. Although symmetry-related vacancy configurations remain energetically degenerate and preserve the macroscopic T symmetry at zero field, an applied magnetic field selects a time-reversal-broken sector of the defect ensemble and reduces the effective magnetic symmetry to the subgroup that leaves B invariant, thereby lifting the longitudinal selection rule. Importantly, this field-selected symmetry reduction does more than activate a nominally forbidden photocurrent: it unmasks latent quantum-geometric responses encoded in the electronic structure. Momentum-resolved calculations show that the dominant circular and linear magneto-photocurrent channels spatially correlate with Berry-curvature-rich and quantum-metric-rich regions of the Brillouin zone, respectively. Our results establish field-selected defect symmetry lowering as a route to revealing hidden quantum geometry and activating forbidden nonlinear photocurrents in chiral quantum materials.

cond-mat.mes-hall↗

arXiv2025

Relativistic Spin-Lattice Interaction Compatible with Discrete Translation Symmetry in Solids

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.

cond-mat.mes-hall↗

arXiv2023

Optoelectronic manifestation of orbital angular momentum driven by chiral hopping in helical Se chains

Chiral materials have garnered significant attention in the field of condensed matter physics. Nevertheless, the magnetic moment induced by the chiral spatial motion of electrons in helical materials, such as elemental Te and Se, remains inadequately understood. In this work, we investigate the development of quantum angular momentum enforced by chirality using static and time-dependent density functional theory calculations for an elemental Se chain. Our findings reveal the emergence of an unconventional orbital texture driven by the chiral geometry, giving rise to a non-vanishing current-induced orbital moment. By incorporating spin-orbit coupling, we demonstrate that a current-induced spin accumulation arises in the chiral chain, which fundamentally differs from the conventional Edelstein effect. Furthermore, we demonstrate the optoelectronic detection of the orbital angular momentum in the chiral Se chain, providing a conceptually novel alternative to the interband Berry curvature, which is ill-defined in low dimensions.

cond-mat.mes-hall↗