arXiv subjects

Yi-Ting Hsu

Publications and source records attributed to Yi-Ting Hsu.

At least 19 recordsLinked to original sources

arXiv2026

Intrinsic Nernst Effect from Berry Curvature in Superconductors

The Nernst effect in superconductors is typically linked to fluctuating Cooper pairs above $T_c$ or vortex motion below $T_c$. We show instead that Berry curvature of Bogoliubov quasiparticles can generate an intrinsic Nernst response in a clean, vortex-free superconducting state. Focusing on two-dimensional systems with Ising spin-orbit coupling, relevant to transition-metal dichalcogenides, we identify two regimes: an intervalley $s$-wave paired state where a weak magnetic field activates the effect, and an intravalley chiral $p$-wave paired state that exhibits a spontaneous charge or spin Nernst response without a field. We propose an experimental setup that circumvents screening and provide estimates of the signal magnitude. Our results establish the Nernst effect as a direct probe of Berry curvature and pairing symmetry in two-dimensional spin-orbit-coupled superconductors.

cond-mat.supr-con↗

arXiv2026

Directional Manipulation of a Staggered Charge Density Wave and Kondo Resonance in UTe2

UTe2 is a rare example of a correlated quantum material in which unconventional density wave orders, Kondo physics, spin-triplet pairing, and reentrant superconductivity coexist within the same electronic system. Its superconducting state develops out of a strongly correlated normal phase. The identification and control of competing or intertwined normal-state orders are thus central to elucidating the electronic landscape from which its superconductivity arises. Here, using scanning tunnelling microscopy (STM) in a vector magnetic field, we uncover a previously unreported staggered charge-density-wave (CDW) in high-quality UTe2 crystals and demonstrate its strong directional response to an external magnetic field: the staggered CDW is completely quenched by a modest 1.7 T field aligned with the quasi-one-dimensional uranium chain direction (a-axis), while remaining robust against fields along other crystallographic directions. This pronounced anisotropy is consistent with an orbital-driven mechanism that leads to a field-tuned quantum phase transition. Strikingly and counterintuitively, the same field orientation and strength concomitantly alter the hybridization gap and suppress the 5f Kondo resonance. Modelling indicates that this correlated evolution arises from a switch of the dominant hybridization channel from Te 5p- U 5f to U 6d- U 5f coupling, suggesting an intimate interplay between CDW and the Kondo effect. Our work establishes an effective tuning knob for the intertwined orders in UTe2 and provides evidence for orbital-selective Kondo hybridization, shedding light on its correlated normal state.

cond-mat.str-el↗

arXiv2025

Displacement-Field-Driven Transition between Superconductivity and Valley Ferromagnetism in Transition Metal Dichalcogenides

Recent experiments have observed transitions between superconductivity and correlated magnetism in twisted bilayer WSe$_2$ near van-Hove fillings, driven by the displacement field $D$. Motivated by the experiment, we theoretically propose a general mechanism for a $D$-controlled transition between superconductivity and ferromagnetism in two-dimensional (2D) spin-orbit-coupled hexagonal systems, where van Hove singularities (VHS) lie on the Fermi level. We show that such a transition can be naturally captured by a simple VHS-only model without Fermi surface details, where the inter-VHS interactions that govern the Fermi surface instabilities is controlled by $D$ through the band projection of screened Coulomb interaction. By treating this simple model with renormalization group technique beyond mean-field level, we find that a chiral $d/p$-wave superconductivity naturally dominates under a weak displacement field $D D_c$, a \textit{valley ferromagnetic phase} (vFM) takes over, which is spatially non-uniform due to valley-modulated magnetization. Finally, we discuss generic conditions for the predicted superconductivity-to-ferromagnetism transition to take place in the rich family of few-layer hexagonal van der Waals material systems. Taking twisted bilayer WSe$_2$ as a case study, we discuss experimental detections that can falsify our prediction.

cond-mat.str-el↗

arXiv2025

g-Factor Enhanced Upper Critical Field in Superconducting PdTe2 due to Quantum Confinement

The Pauli limiting field of superconductors determines the maximal possible value of magnetic field at which superconductivity remains possible. For weak-coupling superconductors, it is determined by an established relation that can be found by setting the condensation energy equal to the magnetization free energy. The latter is a function of the carrier g-factor. Here, we demonstrate in a van der Waals superconductor PdTe2, that quantum confinement can tune the effective g-factor causing the Pauli limit to become thickness dependent. We experimentally probe the in-plane upper critical field (Hc2||) of PdTe2 at intermediate thicknesses down to 20mK. Hc2|| is enhanced by more than an order of magnitude as the thickness is varied from 50nm down to 19nm. We model its temperature and thickness dependence, revealing that both orbital and spin Zeeman depairing mechanisms impact its value. While the variation of the orbital interaction is expected, our findings reveal how the Zeeman interaction impacts superconductivity in thin films. They aid in the search for mixed and odd pairing superconductivity where an enhancement of Hc2|| can be occasionally associated with those unconventional pairing symmetries.

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arXiv2025

Spectral analysis of the magneto-optical response in valley polarized Pb$_{1-x}$Sn$_x$Se

Since the last century, considerable efforts have been devoted to the study of valley-degenerate narrow gap semiconductors, such as the Pb$_{1-x}$Sn$_x$Se alloy. This material possesses band edges at the $L$-points of their Brillouin zone, yielding a valley degeneracy of four. However, in (111)-oriented films, it is still not fully understood how differences between the longitudinal valley, oriented along the growth axis, and the oblique valleys, oriented at an angle with respect to that axis, appear in infrared magneto-optical spectroscopy. In this work, we report a magneto-optical study on this family of alloys, focusing on an anomaly in the interband transition of the absorption strength ratio between longitudinal and oblique valleys under a magnetic field applied along the [111] direction. Based on the Mitchell-Wallis model, we provide a theoretical fit for the experimental transmission data, which quantitatively explains the spectral shape of the data at magnetic fields as high as 35T. In particular, we attribute this anomalous absorption strength variation to the carrier density difference between the two types of valleys as well as the field-dependent multiple-beam interference or the Fabry-Pérot interference. Our analysis also allows for the extraction of the real and imaginary parts of the dielectric function.

cond-mat.mes-hall↗

arXiv2025

Unsupervised machine learning for detecting mutual independence among eigenstate regimes in interacting quasiperiodic chains

Many-body eigenstates that are neither thermal nor many-body-localized (MBL) were numerically found in certain interacting chains with moderate quasiperiodic potentials. The energy regime consisting of these non-ergodic but extended (NEE) eigenstates has been extensively studied for being a possible many-body mobility edge between the energy-resolved MBL and thermal phases. Recently, the NEE regime was further proposed to be a prethermal phenomenon that generally occurs when different operators spread at sizably different timescales. Here, we numerically examine the mutual independence among the NEE, MBL, and thermal regimes in the lens of eigenstate entanglement spectra (ES). Given the complexity and rich information embedded in ES, we develop an unsupervised learning approach that is designed to quantify the mutual independence among general phases. Our method is first demonstrated on an illustrative toy example that uses RGB color data to represent phases, then applied to the ES of an interacting generalized Aubry Andre model from weak to strong potential strength. We find that while the MBL and thermal regimes are mutually independent, the NEE regime is dependent on the former two and smoothly appears as the potential strength decreases. We attribute our numerically finding to the fact that the ES data in the NEE regime exhibits both an MBL-like fast decay and a thermal-like long tail.

cond-mat.dis-nn↗

arXiv2025

Observation of Persistent Zero Modes and Superconducting Vortex Doublets in UTe$_2$

Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized. Recently, UTe$_2$ has emerged as a nominative spin-triplet superconductor, but its superconducting order parameter is elusive, and whether time-reversal symmetry is broken remains unsettled. Here, we visualize superconducting vortices on the (011) surface of ultra-clean UTe$_2$ single crystals ($T_c=2.1 K$) using scanning tunneling microscopy (STM). We introduce $\frac{d^2 I}{dV^2}$ imaging as an effective method for vortex visualization in superconductors with substantial residual zero-energy density of states (DOS), as in UTe$_2$. Anisotropic single-flux-quantum vortices, with coherence lengths of $\sim$ 12 nm (4 nm) parallel (perpendicular) to the a-axis, form a triangular vortex lattice (VL) under small out-of-plane magnetic fields. The vortex size decreases at higher fields, suggesting multi-band superconductivity. The robustness of vortex structures and VL against magnetic field polarity and cooling history strongly supports time-reversal invariant superconductivity. Spatially non-splitting and spectroscopically narrow zero-bias peaks (ZBPs) of differential conductance at vortex cores (VCs) persist up $8T$, well beyond the Pauli limit ($3.9T$), consistent with Majorana zero modes (MZMs) in a topological vortex line protected by a combination of mirror, time-reversal, and particle-hole symmetries. Close examination of vortex structures reveals a mirror-asymmetric doublet $--$ one with a singular core and another with an enhanced energy gap. Such unconventional vortices in a non-chiral $p$-wave superconductor could originate from the induction of a subdominant order parameter close to the VCs.

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arXiv2025

Detecting Topological Superconductivity via Berry Curvature Effects in Spectral Functions

Experimental efforts on topological superconductivity (TSC) have primarily focused on the detection of Majorana boundary modes, while the bulk properties of TSC - particularly in two dimensions (2D) - remain relatively underexplored. In this work, we theoretically propose a distinctive signature in the spectral function away from the boundaries, capable of detecting 2D chiral p-wave TSC induced in a Rashba spin-orbit-coupled (SOC) heterostructure. This signature can be probed experimentally through angle-resolved photoemission spectroscopy or momentum- and energy-resolved tunneling spectroscopy under a weak magnetic field B. We show that within the topological phase, the spectral intensity of the lowest superconducting band at small momenta k~0 brightens (darkens) linearly with increasing B, whereas it darkens (brightens) in the trivial phase when the Rashba system is electron- (hole-) doped. This sharp contrast arises from the phase-space Berry curvature (BC) of Bogoliubov quasiparticles, a novel quantum geometric property that generalizes the conventional momentum-space BC. The effect of this phase-space BC can also be detected by the differential conductance away from the boundaries. Our falsifiable prediction provides an experimental avenue for detecting Rashba-induced chiral p-wave TSC without relying on Majorana mode detection, addressing a key challenge in the realization of 2D TSC.

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arXiv2024

Probing Berry curvature in magnetic topological insulators through resonant infrared magnetic circular dichroism

Probing the quantum geometry and topology in condensed matter systems has relied heavily on static electronic transport experiments in magnetic fields. Yet, contact-free optical measurements have rarely been explored. Magnetic dichroism (MCD), the nonreciprocal absorption of circular polarized light, was theoretically linked to the quantized anomalous Hall effect in magnetic insulators and can identify the bands and momenta responsible for the underlying Berry Curvature (BC). Detecting BC through MCD faces two challenges: First, the relevant inter-band transitions usually generate MCD in the infrared (IR) range, requiring large samples with high quality. Second, while most magnetic materials are metallic, the relation between MCD and BC in metals remains unclear. Here, we report the observation of MCD in the IR range along with the anomalous Hall effect in thin film MnBi2Te4. Both phenomena emerge with a field-driven phase transition from an antiferromagnet to a canted ferromagnet. By theoretically relating the MCD to the anomalous Hall effect via BC in a metal, we show that this transition accompanies an abrupt onset of BC, signaling a topological phase transition from a topological insulator to a doped Chern insulator. Our density functional theory calculation suggests the MCD signal mainly originates from an optical transition at the Brillouin zone edge, hinting at a potential new source of BC away from the commonly considered Γ point. Our findings demonstrate a novel experimental approach for detecting BC and identifying the responsible bands and momenta, generally applicable to magnetic materials.

cond-mat.mes-hall↗

arXiv2024

Switching between superconductivity and current density waves in Bernal bilayer graphene

An out-of-plane magnetic field can always suppress superconductivity. In Bernal-stacked bilayer graphene (BBG), recently observed activation of superconductivity (SC) through either in-plane magnetic fields or proximate spin-orbit coupling (SOC) offers a rare instance of switching superconductivity on. To understand this, we must first examine the non-superconducting state. We propose an incommensurate current density wave (CrDW) driven by van Hove singularities away from the zone corners as a competing order. We note that the two switches, the in-plane field and the SOC, both break spin degeneracy. Our parquet renormalization group analysis reveals that breaking spin degeneracy shifts the balance from CrDW, favored under spin degeneracy, to SC when degeneracy is lifted. Driven by purely repulsive interactions, the pairing symmetry of the resulting SC is $p/d$-wave. The presence of CrDW accounts for the non-linear $I-V$ behavior in the normal state and suggests potential anomalous Hall effects due to time-reversal symmetry breaking. We further predict that reducing screening could enhance SC.

cond-mat.str-el↗

arXiv2024

Optical absorption signatures of superconductors driven by Van Hove singularities

Due to the diverging density of states (DOS), Van Hove singularities (VHS) near the Fermi level are known to boost the susceptibility to a wide variety of electronic instabilities, including superconductivity. We theoretically show that the number of VHS in the normal state can be qualitatively inferred from the optical absorption spectra Re $σ_{ii}(ω)$ in the superconducting state. The key feature is the absorption peak at frequency $ω=2Δ$ from the optical transition across the superconducting gap $Δ$, which is forbidden in a single-band clean superconductor when the inversion symmetry is preserved and the band is quadratic. Although VHS dispersions are mostly quadratic, we find that a divergent peak occurs when there are multiple VHS on the Fermi surface under an applied current. In contrast, we find non-diverging weak peaks when there is only a single VHS. Depending on whether this single VHS has logarithmically or power-law divergent DOS, the peaks in Re $σ_{xx}(ω)$ and Re $σ_{yy}(ω)$ are nearly isotropic and anisotropic, respectively. Therefore, we propose that experimentally measured peak magnitude and anisotropy in the optical absorption spectra of VH-driven superconductors can be used to determine the number and type of VHS on the Fermi surface. Disorders could even facilitate in distinguishing the multiple and single VHS scenarios since only the diverging peak in the latter case is expected to survive.

cond-mat.supr-con↗

arXiv2023

Higher-order topological superconductivity in a topological metal 1T$^\prime$-MoTe$_2$

One key challenge in the field of topological superconductivity (Tsc) has been the rareness of material realization. This is true not only for the first-order Tsc featuring Majorana surface modes, but also for the higher-order Tsc, which host Majorana hinge and corner modes. Here, we propose a four-step strategy that mathematically derives comprehensive guiding principles for the search and design for materials of general higher-order Tsc phases. Specifically, such recipes consist of conditions on the normal state and pairing symmetry that can lead to a given higher-order Tsc state. We demonstrate this strategy by obtaining recipes for achieving three-dimensional higher-order Tsc phases protected by the inversion symmetry. Following our recipe, we predict that the observed superconductivity in centrosymmetric MoTe$_2$ is a candidate for higher-order Tsc with corner modes. Our proposed strategy enables systematic materials search and design for higher-order Tsc, which can mobilize the experimental efforts and accelerate the material discovery for higher-order Tsc phases.

cond-mat.supr-con↗

arXiv2023

Non-Hermitian skin effects on many-body localized and thermal phases

Localization in one-dimensional interacting systems can be caused by disorder potentials or non-Hermiticity. The former phenomenon is the many-body localization (MBL), and the latter is the many-body non-Hermitian skin effect (NHSE). In this work, we numerically investigate the interplay between these two kinds of localization, where the energy-resolved MBL arises from a deterministic quasiperiodic potential in a fermionic chain. We propose a set of eigenstate properties and long-time dynamics that can collectively distinguish the two localization mechanisms in the presence of non-Hermiticity. By computing the proposed diagnostics, we show that the thermal states are vulnerable to the many-body NHSE while the MBL states remain resilient up to a strong non-Hermiticity. Finally, we discuss experimental observables that can probe the difference between the two localizations in a non-Hermitian quasiperiodic fermionic chain. Our results pave the way toward experimental observations on the interplay of interaction, quasiperiodic potential, and non-Hermiticity.

cond-mat.dis-nn↗

arXiv2022

Topological invariants beyond symmetry indicators: Boundary diagnostics for twofold rotationally symmetric superconductors

Topological crystalline superconductors are known to have possible higher-order topology, which results in Majorana modes on $d-2$ or lower-dimensional boundaries. Given the rich possibilities of boundary signatures, it is desirable to have topological invariants that can predict the type of Majorana modes from band structures. Although symmetry indicators, a type of invariant that depends only on the band data at high-symmetry points, have been proposed for certain crystalline superconductors, there exist symmetry classes in which symmetry indicators fail to distinguish superconductors with different Majorana boundaries. Here, we systematically obtain topological invariants for an example of this kind, two-dimensional time-reversal symmetric superconductors with twofold rotational symmetry $C_2$. First, we show that the nontrivial topology is independent of band data on the high-symmetry points by conducting a momentum-space classification study. Then, from the resulting K groups, we derive calculable expressions for four $\mathbb{Z}_2$ invariants defined on high-symmetry lines or general points in the Brillouin zone. Finally, together with a real-space classification study, we establish the bulk-boundary correspondence and show that the four $\mathbb{Z}_2$ invariants can predict Majorana boundary types from band structures. Our proposed invariants can fuel practical material searches for $C_2$-symmetric topological superconductors featuring Majorana edge and corner modes.

cond-mat.supr-con↗

arXiv2021

Observation of coexisting weak localization and superconducting fluctuations in strained Sn1-xInxTe thin films

Topological superconductors have attracted tremendous excitement as they are predicted to host Majorana zero modes that can be utilized for topological quantum computing. Candidate topological superconductor Sn1-xInxTe thin films (0<x<0.3) grown by molecular beam epitaxy and strained in the (111) plane are shown to host three coexisting quantum effects: localization, antilocalization and superconducting fluctuations above the critical temperature Tc. An analysis of the normal state magnetoresistance reveals these effects. Weak localization is consistently observed in superconducting samples, indicating that superconductivity originates dominantly from trivial valence band states that may be strongly spin-orbit split. A large enhancement of the conductivity is observed above Tc, indicating that quantum coherent quasiparticle effects coexist with superconducting fluctuations. Our results motivate a re-examination of the debated pairing symmetry of this material when subjected to quantum confinement and lattice strain.

cond-mat.supr-con↗

arXiv2021

Spin-valley locked instabilities in moire transition metal dichalcogenides with conventional and higher-order Van Hove singularities

Recent experiments have observed correlated insulating and possible superconducting phases in twisted homobilayer transition metal dichalcogenides (TMDs). Besides the spin-valley locked moire bands due to the intrinsic Ising spin-orbit coupling, homobilayer moire TMDs also possess either logarithmic or power-law divergent Van Hove singularities (VHS) near the Fermi surface, controllable by an external displacement field. The former and the latter are dubbed conventional and higher-order VHS, respectively. Here, we perform a perturbative renormalization group (RG) analysis to unbiasedly study the dominant instabilities in homobilayer TMDs for both the conventional and higher-order VHS cases. We find that the spin-valley locking largely alters the RG flows and leads to instabilities unexpected in the corresponding extensively-studied graphene-based moire systems, such as spin- and valley-polarized ferromagnetism and topological superconductivity with mixed parity. In particular, for the case with two higher-order VHS, we find a spin-valley-locking-driven metallic state with no symmetry breaking in the TMDs despite the diverging bare susceptibility. Our results show how the spin-valley locking significantly affects the RG analysis and demonstrate that moire TMDs are suitable platforms to realize various interaction-induced spin-valley locked phases, highlighting physics fundamentally different from the well-studied graphene-based moire systems.

cond-mat.str-el↗

arXiv2020

Higher-Order Topological Dirac Superconductors

We introduce higher-order topological Dirac superconductor (HOTDSC) as a new gapless topological phase of matter in three dimensions, which extends the notion of Dirac phase to a higher-order topological version. Topologically distinct from the traditional topological superconductors and known Dirac superconductors, a HOTDSC features helical Majorana hinge modes between adjacent surfaces, which are direct consequences of the symmetry-protected higher-order band topology manifesting in the system. Specifically, we show that rotational, spatial inversion, and time-reversal symmetries together protect the coexistence of bulk Dirac nodes and hinge Majorana modes in a seamless way. We define a set of topological indices that fully characterizes the HOTDSC. We further show that a practical way to realize the HOTDSC phase is to introduce unconventional odd-parity pairing to a three-dimensional Dirac semimetal while preserving the necessary symmetries. As a concrete demonstration of our idea, we construct a corresponding minimal lattice model for HOTDSC obeying the symmetry constraints. Our model exhibits the expected topological invariants in the bulk and the defining spectroscopic features on an open geometry, as we explicitly verify both analytically and numerically. Remarkably, the HOTDSC phase offers an example of a higher-order topological quantum critical point, which enables realizations of various higher-order topological phases under different symmetry-breaking patterns. In particular, by breaking the inversion symmetry of a HOTDSC, we arrive at a higher-order Weyl superconductor, which is yet another new gapless topological state that exhibits hybrid higher-order topology.

cond-mat.mes-hall↗

arXiv2020

Faithful derivation of symmetry indicators: A case study for topological superconductors with time-reversal and inversion symmetries

Topological crystalline superconductors have attracted rapidly rising attention due to the possibility of higher-order phases, which support Majorana modes on boundaries in $d-2$ or lower dimensions. However, although the classification and bulk topological invariants in such systems have been well studied, it is generally difficult to faithfully predict the boundary Majoranas from the band-structure information due to the lack of well-established bulk-boundary correspondence. Here we propose a protocol for deriving symmetry indicators that depend on a minimal set of necessary symmetry data of the bulk bands and can diagnose boundary features. Specifically, to obtain indicators manifesting clear bulk-boundary correspondence, we combine the topological crystal classification scheme in the real space and a twisted equivariant K group analysis in the momentum space. The key step is to disentangle the generally mixed strong and weak indicators through a systematic basis-matching procedure between our real-space and momentum-space approaches. We demonstrate our protocol using an example of two-dimensional time-reversal odd-parity superconductors, where the inversion symmetry is known to protect a higher-order phase with corner Majoranas. Symmetry indicators derived from our protocol can be readily applied to ab initio database and could fuel material predictions for strong and weak topological crystalline superconductors with various boundary features.

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