arXiv subjects

Hridis K. Pal

Publications and source records attributed to Hridis K. Pal.

At least 19 recordsLinked to original sources

arXiv2026

Switchable Surface Linear Photogalvanic Effect in the Magnetic Weyl Semimetal Co3Sn2S2

We investigate the linear photogalvanic effect (LPGE) on the surface of the magnetic Weyl semimetal Co3Sn2S2 using a Green's-function and diagrammatic formalism. While the LPGE vanishes in the centrosymmetric bulk, it is symmetry-allowed on the surface where inversion symmetry is broken. We show that unitary crystal symmetries on the surface produce characteristic sign reversals of the total photocurrent at certain polarization angles upon flipping the magnetization. We further find that the intrinsic contribution to the LPGE is strongly constrained by an antiunitary mirror symmetry, which forces several nonlinear response tensor elements to vanish. In contrast, the extrinsic contribution is not subject to these constraints and displays a large magnitude which, we argue, is due to the enhanced density of states associated with Fermi-arc surface states. The current exhibits an approximately linear temperature dependence and a low-frequency power-law scaling, |jy| proportional to omega^-2.2, with weak temperature dependence of the scaling exponent. Our results identify Co3Sn2S2 as a promising platform for experimentally accessing symmetry-controlled nonlinear transport in realistic systems and for applications in magnetically controlled optoelectronic devices.

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arXiv2025

Anomalous Chiral Anomaly in Spin-1 Fermionic Systems

Chiral anomaly is a key feature of Lorentz-invariant quantum field theories: in presence of parallel external electric and magnetic fields, the number of massless Weyl fermions of a given chirality is not conserved. In condensed matter, emergent chiral fermions in Weyl semimetals exhibit the same anomaly, directly tied to the topological charge of the Weyl node, ensuring a quantized anomaly coefficient. However, many condensed matter systems break Lorentz symmetry while retaining topological nodes, raising the question of how chiral anomaly manifests in such settings. In this work, we investigate this question in spin-1 fermionic systems and show that the conventional anomaly equation is modified by an additional nontopological contribution, leading to a nonquantized anomaly coefficient. This surprising result arises because spin-1 fermions can be decomposed into 2-flavor Weyl fermions coupled to a Lorentz-breaking, momentum-dependent non-Abelian background potential. The interplay between this potential and external electromagnetic fields generates the extra term in the anomaly equation. Our framework naturally generalizes to other Lorentz-breaking systems beyond the spin-1 case.

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arXiv2024

Effect of nonlocal interlayer hopping on wave function in twisted bilayer graphene

The conventional low-energy theory employed to describe twisted bilayer graphene (TBG) relies on a local interlayer Hamiltonian. According to this theory, TBG has the same linear-in-momentum dispersion and spinor wave function at the Dirac point as single-layer graphene (SLG), albeit with a renormalized velocity that decreases as the rotation angle between the layers decreases, eventually reaching zero at the magic angle. In this work, I expand upon this low-energy theory by including nonlocal terms in the interlayer part of the Hamiltonian, and explore the consequences at the Dirac point. It is found that the nonlocality predominantly influences the wave function rather than the energy spectrum: despite the persistence of the linear-in-momentum dispersion with a renormalized velocity, the wave functions no longer mirror those of SLG. Instead, an additional contribution to the phase difference between the sublattice components of the spinor emerges. This gives rise to interesting effects in scattering which are demonstrated with a simple example.

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arXiv2023

Interacting ground states of moir\'e ladders

Moir\'e materials have emerged as a rich platform for exploring strong correlation effects in low dimensions, with twisted bilayer graphene (TBG) as a paradigmatic example. To distill the essential ingredients driving moir\'e-induced phases, a simplified one-dimensional analog -- a two-leg ladder with spatially modulated interleg hopping and a uniform magnetic flux -- was recently introduced. This model, which we refer to as the moir\'e ladder, features a nearly flat lowest-energy band in a suitable parameter regime, capturing the band-flattening mechanism of TBG. We investigate the ground-state phase diagram of the moir\'e ladder using a combination of bosonization and density matrix renormalization group (DMRG) techniques, and systematically disentangle the respective roles of the flux and the hopping modulation. At half filling, previous numerical work identified a metal-insulator transition at finite interaction strength and an unexpected ferromagnetic ground state. Revisiting this, we show that the metal-insulator transition can be understood perturbatively within bosonization, governed by the number of Fermi points. In contrast, the ferromagnetic correlations are nonperturbative and require both flux and spatial modulation -- neither alone is sufficient. We extend our analysis to other fillings: one-quarter, three-quarters, slightly above half filling (half filling plus two electrons), and slightly below half filling (half filling minus two electrons). At moderate interactions, we observe ferromagnetism below half filling and antiferromagnetism above; at stronger interactions, ferromagnetism dominates across all studied fillings. Crucially, the analysis demonstrates that periodic interleg hopping alone does not engender new correlated phases; the magnetic flux is essential for the observed unconventional behavior.

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arXiv2023

Universal route towards field-free electrically polarity-reversible Josephson diode

The realisation of superconducting diodes that operate without external magnetic fields and allow electrical control of polarity is a key goal for the integration of nonreciprocal elements into cryogenic and quantum technologies. Here, we demonstrate a universal and scalable approach to achieving such field-free and electrically reconfigurable Josephson diode functionality. Our method relies on long Josephson junctions with ferromagnetic barriers and asymmetric current injection - a configuration that inherently breaks both time-reversal and inversion symmetries. We show that the diode polarity is set by an applied bias current and can be reversed using short current pulses, without the need for magnetic fields or thermal cycling. The effect is robust, material-agnostic, and compatible with standard established fabrication processes. Our results provide a practical platform for integrating low-dissipation, programmable diodes into superconducting and quantum electronic circuits

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arXiv2023

Effect of many-body interaction on de Haas-van Alphen oscillations in insulators

De Haas-van Alphen (dHvA) oscillations are oscillations in the magnetization as a function of the inverse magnetic field. These oscillations are usually considered to be a property of the Fermi surface and, hence, a metallic property. Recently, however, such oscillations have been shown to arise, both experimentally and theoretically, in certain insulators which have a narrow gap and an inverted band structure. In this work, we develop a theory to study the effect of many-body interaction on these unconventional oscillations. We consider weak interaction, focusing on the effect of renormalization of the quasiparticle spectrum on these oscillations. We find that interaction has an unusual effect: unlike in metals, in a certain regime the amplitude of oscillations may be enhanced substantially, both at zero and nonzero temperatures, even when the interaction is perturbatively weak.

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arXiv2022

Anomalous Surface Conductivity of Weyl Semimetals

We calculate the surface dc conductivity of Weyl semimetals and show that it contains an anomalous contribution in addition to a Drude contribution from the Fermi arc. The anomalous part is independent of the surface scattering time, and appears at nonzero temperature and doping (away from the Weyl nodes), increasing quadratically with both with a universal ratio of coefficients. Microscopically, it results from the contribution of the gapless bulk to the surface conductivity. We argue that this can be interpreted as the conductivity of an effective interacting surface fluid that coexists with the Fermi arc metal. In a certain regime of low temperatures, the temperature dependence of the surface conductivity is dominated by the anomalous response, which can be probed experimentally to unravel the unusual behavior.

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arXiv2022

Formation of tungsten carbide by focused ion beam process: A route to high magnetic field resilient patterned superconducting nanostructures

A scale for magnetic field resilience of a superconductor is set by the paramagnetic limit. Comparing the condensation energy of the Bardeen-Cooper-Schrieffer (BCS) singlet ground state with the paramagnetically polarised state suggests that for an applied field ${μ_0}H > 1.8~T_c$ (in SI), singlet pairing is not energetically favourable. Materials exceeding or approaching this limit are interesting from fundamental and technological perspectives. This may be a potential indicator of triplet superconductivity, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing and other mechanisms involving topological aspects of surface states, and also allow Cooper pair injection at high magnetic fields. We have analysed the microscopic composition of such a material arising from an unexpected source. A microjet of an organo-metallic gas, $\rm {W[(CO)_6]}$ can be decomposed by gallium ion-beam, leaving behind a track of complex residue of gallium, tungsten and carbon with remarkable superconducting properties, like an upper critical field, $H_{c2} > 10~{\rm T} $, above its paramagnetic limit. We carried out Atomic probe tomography to establish the formation of nano-crystalline tungsten carbide (WC) in the tracks and the absence of free tungsten. Supporting calculations show for Ga distributed on the surface of WC, its s,p-orbitals enhance the density of states near the Fermi energy. The observed variation of $H_{c2}(T)$ does not show features typical of enhancement of critical field due to granularity. Our observations may be significant in the context of some recent theoretical calculation of the band structure of WC and experimental observation of superconductivity in WC-metal interface.

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arXiv2022

Quantum contribution to magnetotransport in weak magnetic fields and negative longitudinal magnetoresistance

Longitudinal magnetoresistance (LMR) refers to the change in resistance due to a magnetic field when the current and the magnetic field are parallel to each other. For this to be nonzero in weak magnetic fields, kinetic theory stipulates that the electronic dispersion must satisfy certain conditions: it should either be sufficiently anisotropic or have topological features. The former results in a positive LMR, while the latter results in a negative LMR. Here, I propose a different mechanism that leads to LMR in any dispersion without a need to satisfy the above requirements. The mechanism is quantum in origin but is applicable in the said regime. It arises due to the change in the density of states with the magnetic field and is not kinetic in origin. Remarkably, LMR is found to be negative even if the dispersion is nontopological, provided it is nonparabolic. An analytical expression is derived for this contribution to LMR. It is found to depend on the orbital magnetic susceptibility. The analytical findings are confirmed by numerical calculations.

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arXiv2021

Topological Anomalous Skin Effect in Weyl Superconductors

We show that a Weyl superconductor can absorb light via a novel surface-to-bulk mechanism, which we dub the topological anomalous skin effect. This occurs even in the absence of disorder for a single-band superconductor, and is facilitated by the topological splitting of the Hilbert space into bulk and chiral surface Majorana states. In the clean limit, the effect manifests as a characteristic absorption peak due to surface-bulk transitions. We also consider the effects of bulk disorder, using the Keldysh response theory. For weak disorder, the bulk response is reminiscent of the Mattis-Bardeen result for $s$-wave superconductors, with strongly suppressed spectral weight below twice the pairing energy, despite the presence of gapless Weyl points. For stronger disorder, the bulk response becomes more Drude-like and the $p$-wave features disappear. We show that the surface-bulk signal survives when combined with the bulk in the presence of weak disorder. The topological anomalous skin effect can therefore serve as a fingerprint for Weyl superconductivity. We also compute the Meissner response in the slab geometry, incorporating the effect of the surface states.

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arXiv2020

Quasi-flat-band physics in a two-leg ladder model and its relation to magic-angle twisted bilayer graphene

We study the single- and many-particle properties of a two-leg ladder model threaded by a flux with the legs coupled by a spatially varying term. Although a priori unrelated to twisted bilayer graphene (TBG), the model is found to have striking similarities: a quasi-flat low-energy band emerges with characteristics similar to that of magic angle TBG. We study the effect of interparticle interaction in our model using the density matrix renormalization group and find that when the band is quasi-flat, the ground state is a ferromagnetic Mott insulator. As the band becomes more dispersive, the system undergoes a ferromagnetic to antiferromagnetic transition. We discuss how our model is relevant not only to magic-angle physics in TBG, but also in the larger context of one-dimensional correlations and magnetism.

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arXiv2020

Power-law Temperature Dependence of the Penetration Depth in a Topological Superconductor due to Surface States

We study the temperature dependence of the magnetic penetration depth in a 3D topological superconductor (TSC), incorporating the paramagnetic current due to the surface states. A TSC is predicted to host a gapless 2D surface Majorana fluid. In addition to the bulk-dominated London response, we identify a $T^3$ power-law-in-temperature contribution from the surface, valid in the low-temperature limit. Our system is fully gapped in the bulk, and should be compared to bulk nodal superconductivity, which also exhibits power-law behavior. Power-law temperature dependence of the penetration depth can be one indicator of topological superconductivity.

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arXiv2019

Anomalies in a slightly doped insulator with strong particle-hole asymmetry and narrow gap---the case for SmB$_6$?

SmB$_6$, known to be a Kondo insulator, has received intense scrutiny in recent years due to its paradoxical experimental signatures: while some quantities show an insulating behavior, others point to a metallic state. This has led to the conjecture that SmB$_6$ hosts nontrivial excitations within its bulk gap, and has spawned several theories to that effect. In principle, there exists an alternative possibility: the system is a metal but unusually with both metal- and insulator-like properties. Inspired by this possibility, I consider a minimal model of a Kondo insulator---a flat band hybridized with a parabolic band---that is slightly electron doped, i.e., the chemical potential is in the conduction band but close to the band edge. By calculating the dc conductivity, ac conductivity, specific heat, and quantum oscillations at the phenomenological level, I show that these quantities exhibit unusual behaviors that are, surprisingly, qualitatively consistent with those observed experimentally in SmB$_6$. The rapid change of band curvature around the chemical potential arising from the strong particle-hole asymmetry and the narrow gap in the model, a feature not usually encountered in the textbook cases of metals or insulators, is at the heart of the unusual behaviors.

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arXiv2018

Prediction of novel `magic' angles and correlations for twisted bilayer graphene in a perpendicular electric field

At certain angles of rotation called `magic angles' twisted bilayer graphene features almost flat bands. The resulting strong correlations drive the system to novel phases which have been observed in experiments recently. A complete understanding of the `magic' angle physics---both at the single-particle as well as the many-particle level---is still missing, and the search is ongoing. Here, we identify a new set of `magic' angles, where locally flat bands with a variety of possible many-body instabilities arise, but where the single-particle problem admits an exact solution. This occurs in the presence of an external perpendicular electric field at multicritical Lifshitz points. At these angles, which can be quantified exactly, the central band features a monkey saddle, resulting in strong electronic correlations.

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arXiv2018

On magic angles and band flattening in twisted bilayer graphene

When two graphene layers are rotated from AA or AB configuration by a small angle, the band structure changes dramatically. Numerical calculations have shown that, at certain discrete angles called magic angles, the low energy bands become flat leading to localization of electrons. The origin of this strange behavior, however, is not well understood. Here, I propose a theory that offers an understanding of the phenomenon, focusing on the first magic angle. It is shown that coupling between the layers, in addition to renormalizing the Dirac velocity, introduces higher order momentum terms in the energy dispersion that are not all of the same sign. Partial cancellation among these terms leads to the flatness of the low energy bands. Also, while there is modulation of electron density in real space, there is no localization---the modulation arises due to the superposition of plane wave states with different momenta in the two layers. In addition, it is conjectured that there is an underlying geometric reason for the appearance of more than one magic angle which can be exploited to predict higher magic angles approximately without computing the band structure.

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arXiv2018

Emergent geometric frustration and flat band in moiré bilayer graphene

So far the physics of moiré graphene bilayers at large, incommensurate rotation angles has been considered uninteresting. It has been held that the interlayer coupling in such structures is weak and the system can be thought of as a pair of decoupled single graphene sheets to a good approximation. Here, we demonstrate that for large rotation angles near commensurate ones, the interlayer coupling, far from being weak, is able to completely localize electrons to within a large scale, geometrically frustrated network of topologically protected modes. The emergent geometric frustration of the system gives rise to completely flat bands, with strong correlation physics as a result. All of this arises although in the lattice structure no large scale pattern appears to the unguided eye. Sufficiently close to commensuration the low-energy physics of this remarkable system has an exact analytical solution.

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arXiv2017

Unusual Frequency of Quantum Oscillations in Strongly Particle-Hole Asymmetric Insulators

Quantum oscillations, conventionally thought to be a metallic property, have recently been shown to arise in certain kinds of insulators, with properties very different from those in metals. All departures from the canonical behavior found so far arise only in the amplitude and the phase but not in the frequency. Here I show that such robustness in the behavior of the frequency is only valid for a particle-hole symmetric insulator; in a strongly particle-hole asymmetric insulator, de Haas-van Alphen oscillations (oscillations in magnetization and susceptibility) and Shubnikov-de Haas oscillations (oscillations in the density of states) exhibit different frequencies, with the frequency of the latter changing with temperature. I demonstrate these effects with numerical calculations on a lattice model, and provide a theory to account for the unusual behavior.

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arXiv2017

Quantum Oscillations from Fermi Sea

Quantum oscillations are conventionally understood to arise from the Fermi level; hence, they are considered to be a proof of the existence of an underlying Fermi surface. In this article, we show that in certain situations quantum oscillations can also arise from inside the Fermi sea. We establish this analytically, supporting it with numerical calculations. Possible scenarios where such unusual behavior can occur are pointed out. In particular, in strongly particle-hole asymmetric insulators, models of which have been recently used in the context of the topological Kondo insulator SmB$_6$, we show that the oscillations arise from inside the filled band, and are not related to the gap.

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