Searcharxiv⌕ Search

arXiv subjects

Amit Agarwal

Publications and source records attributed to Amit Agarwal.

At least 109 records · Page 6Linked to original sources
arXiv2024

Tunable moiré materials for probing Berry physics and topology

Berry curvature physics and quantum geometric effects have been instrumental in advancing topological condensed matter physics in recent decades. Although Landau level-based flat bands and conventional 3D solids have been pivotal in exploring rich topological phenomena, they are constrained by their limited ability to undergo dynamic tuning. In stark contrast, moiré systems have risen as a versatile platform for engineering bands and manipulating the distribution of Berry curvature in momentum space. These moiré systems not only harbor tunable topological bands, modifiable through a plethora of parameters, but also provide unprecedented access to large length scales and low energy scales. Furthermore, they offer unique opportunities stemming from the symmetry-breaking mechanisms and electron correlations associated with the underlying flat bands that are beyond the reach of conventional crystalline solids. A diverse array of tools, encompassing quantum electron transport in both linear and non-linear response regimes and optical excitation techniques, provide direct avenues for investigating Berry physics. This review navigates the evolving landscape of tunable moiré materials, highlighting recent experimental breakthroughs in the field of topological physics. Additionally, we delineate several challenges and offer insights into promising avenues for future research.

cond-mat.mes-hall↗
arXiv2024

Nonlinear Landau fan diagram and aperiodic magnetic oscillations in three-dimensional systems

Quantum oscillations offer a powerful probe for the geometry and topology of the Fermi surface in metals. Onsager's semiclassical quantization relation governs these periodic oscillations in 1/B, leading to a linear Landau fan diagram. However, higher-order magnetic susceptibility-induced corrections give rise to a generalized Onsager's relation, manifesting in experiments as a nonlinear Landau fan diagram and aperiodic quantum oscillations. Here, we explore the generalized Onsager's relation to three-dimensional (3D) systems to capture the B-induced corrections in the free energy and the Fermi surface. We unravel the manifestation of these corrections in the nonlinear Landau fan diagrams and aperiodic quantum oscillations by deriving the B-dependent oscillation frequency and the generalized Lifshitz-Kosevich equation, respectively. Our theory explains the necessary conditions to observe these fascinating effects and predicts the magnetic field dependence of the cyclotron mass. As a concrete example, we elucidate these effects in a 3D spin-orbit coupled system and extract zero-field magnetic response functions from analytically obtained Landau levels. Our comprehensive study deepens and advances our understanding of aperiodic quantum oscillations.

cond-mat.mes-hall↗
arXiv2024

Nonlinear Valley Hall Effect

The valley Hall effect arises from valley contrasting Berry curvature and requires inversion symmetry breaking. Here, we propose a nonlinear mechanism to generate a valley Hall current in systems with both inversion and time-reversal symmetry, where the linear and second-order charge Hall currents vanish along with the linear valley Hall current. We show that a second-order valley Hall signal emerges from the electric field correction to the Berry curvature, provided a valley-contrasting anisotropic dispersion is engineered. We demonstrate the nonlinear valley Hall effect in tilted massless Dirac fermions in strained graphene and organic semiconductors. Our work opens up the possibility of controlling the valley degree of freedom in inversion symmetric systems via nonlinear valleytronics.

cond-mat.mes-hall↗
arXiv2023

Intrinsic nonlinear conductivities induced by the quantum metric

The second-order nonlinear current originates from three physical mechanisms: extrinsic nonlinear Drude and Berry curvature dipole and intrinsic Berry connection polarizability. Here, we predict a new intrinsic contribution to the current related to the quantum metric, a quantum geometric property of the electronic wave function. This contribution manifests in systems that simultaneously break the time-reversal and the inversion symmetry. Interestingly, the new contribution is dissipative in nature and contributes to both longitudinal and the dissipative nonlinear Hall response. The quantum metric-induced NL current dominates transport in parity-time reversal symmetric systems near the band edges, something we show explicitly for topological antiferromagnets.

cond-mat.mes-hall↗
arXiv2023

Room Temperature Ferroelectricity, Ferromagnetism, and Anomalous Hall Effect in Half-metallic Monolayer CrTe

Two-dimensional materials hosting ferroelectricity and ferromagnetism are crucial for low-power and high-speed information processing technologies. However, intrinsic 2D multiferroics in the monolayer limit are rare. Here, we demonstrate that monolayer CrTe, obtained by cleaving the [002] surface, is dynamically stable multiferroic at temperatures beyond room temperature. We show that it orders ferromagnetically with significant in-plane magnetocrystalline anisotropy, and it is a half-metal featuring a large half-metal gap. Remarkably, the broken inversion symmetry and buckled geometry of monolayer CrTe make it a ferroelectric with a large spontaneous out-of-plane polarization and significant magnetoelectric coupling. In addition, we demonstrate polarization or electric field-induced tunability of the anomalous Hall effect, accompanied by substantial bandstructure modulation. Our findings establish monolayer CrTe as a room-temperature multiferroic with great potential for applications in spintronics and ferroelectric devices.

cond-mat.mes-hall↗
arXiv2023

Intrinsic nonlinear thermal Hall transport of magnons: A Quantum kinetic theory approach

We present a systematic study of the nonlinear thermal Hall responses in bosonic systems using the quantum kinetic theory framework. We demonstrate the existence of an intrinsic nonlinear boson thermal current, arising from the quantum metric which is a wavefunction dependent band geometric quantity. In contrast to the nonlinear Drude and nonlinear anomalous Hall contributions, the intrinsic nonlinear thermal conductivity is independent of the scattering timescale. We demonstrate the dominance of this intrinsic thermal Hall response in topological magnons in a two-dimensional ferromagnetic honeycomb lattice without Dzyaloshinskii-Moriya interaction. Our findings highlight the significance of band geometry induced nonlinear thermal transport and motivate experimental probe of the intrinsic nonlinear thermal Hall response with implications for quantum magnonics.

cond-mat.mes-hall↗
arXiv2023

Unconventional superconducting pairing in a B20 Kramers Weyl semimetal

Topological superconductors present an ideal platform for exploring nontrivial superconductivity and realizing Majorana boundary modes in materials. However, finding a single-phase topological material with nontrivial superconducting states is a challenge. Here, we predict nontrivial superconductivity in the pristine chiral metal RhGe with a transition temperature of 5.8 K. Chiral symmetries in RhGe enforce multifold Weyl fermions at high-symmetry momentum points and spin-polarized Fermi arc states that span the whole surface Brillouin zone. These bulk and surface chiral states support multiple type-II van Hove singularities that enhance superconductivity in RhGe. Our detailed analysis of superconducting pairing symmetries involving Chiral Fermi pockets in RhGe, indicates the presence of nontrivial superconducting pairing. Our study establishes RhGe as a promising candidate material for hosting mixed-parity pairing and topological superconductivity.

cond-mat.supr-con↗
arXiv2023

Observation of time-reversal symmetric Hall effect in graphene-WSe2 heterostructures at room temperature

In this letter, we provide experimental evidence of the time-reversal symmetric Hall effect in a mesoscopic system, namely high-mobility graphene/WSe$_2$ heterostructures. This linear, dissipative Hall effect, whose sign depends on the sign of the charge carriers, persists up to room temperature. The magnitude and the sign of the Hall signal can be tuned using an external perpendicular electric field. Our joint experimental and theoretical study establishes that the strain induced by lattice mismatch, or angle inhomogeneity, produces anisotropic bands in graphene while simultaneously breaking the inversion symmetry. The band anisotropy and reduced spatial symmetry lead to the appearance of a time-reversal symmetric Hall effect. Our study establishes graphene-transition metal dichalcogenide-based heterostructures as an excellent platform for studying the effects of broken symmetry on the physical properties of band-engineered two-dimensional systems.

cond-mat.mes-hall↗
arXiv2023

Versatility of type-II van der Waals heterostructures: a case study with SiH-CdCl2

Unlike bilayers or a few layers thick materials, heterostructures are designer materials formed by assembling different monolayers in any desired sequence. As a result, while multilayer materials come with their intrinsic properties, heterostructures can be tailor-made to suit specific applications. Taking SiH-CdCl 2 as a representative system, we show the potential of heterostructures for several applications, like piezoelectricity, photocatalytic water splitting, and tunnel field effect transistor (TFET). Our study confirms that the characteristics of the heterostructure mainly depend on the potential difference between the constituent monolayers. From the vast database of available layered materials, many such combinations with a suitable potential difference are expected to have similar properties. Our work points to a vast pool of assembled materials with multifunctionality, an excellent asset for next-generation device applications.

cond-mat.mtrl-sci↗
arXiv2023

PdSe2 based field-effect transistors

Pentagonal PdSe2 is a promising candidate for layered electronic devices, owing to its high air-stability and anisotropic transport properties. Here, we investigate the performance of p-type FET based on PdSe$_2$ mono-layer using multi-scale simulation framework combining Density functional theory and quantum transport. We find that mono-layer PdSe$_2$ devices show excellent switching characteristics ($<$ 65 mV/decade) for the source-drain direction aligned along both [010] and [100] directions. Both directions also show good on-state current and large transconductance, though these are larger along the [010] direction for a 15 nm channel device. The channel length scaling study of these p-FETs indicates that channel length can be easily scaled down to 7 nm without any significance compromise in the performance. Going below 7 nm, we find that there is a severe degradation in the sub-threshold swing for 4 nm channel length. However, this degradation can be minimized by introducing an underlap structure. The length of underlap is determined by the trade-off between on-state current and the switching performance.

cond-mat.mes-hall↗
arXiv2023

Quantum kinetic theory of nonlinear thermal current

We investigate the second-order nonlinear electronic thermal transport induced by temperature gradient. We develop the quantum kinetic theory framework to describe thermal transport in presence of a temperature gradient. Using this, we predict an intrinsic scattering time independent nonlinear thermal current in addition to the known extrinsic nonlinear Drude and Berry curvature dipole contributions. We show that the intrinsic thermal current is determined by the band geometric quantities and is non-zero only in systems where both the space inversion and time-reversal symmetries are broken. We employ the developed theory to study the thermal response in tilted massive Dirac systems. We show that besides the different scattering time dependence, the various current contributions have distinct temperature dependence in the low temperature limit. Our systematic and comprehensive theory for nonlinear thermal transport paves the way for future theoretical and experimental studies on intrinsic thermal responses.

cond-mat.mes-hall↗
arXiv2023

Intrinsic nonreciprocal bulk plasmons in noncentrosymmetric magnetic systems

Nonreciprocal plasmonics enables one-way light propagation at the nanoscale and it is an essential building block for photonics applications. Here, we explore intrinsic nonreciprocity in bulk plasmon propagation based on underlying symmetries. We demonstrate that the interband, as well as the intraband bulk plasmon modes, follow asymmetric dispersion depending on the sign of the wavevector for systems with broken inversion and time-reversal symmetry. We show that the nonreciprocity in the interband plasmon dispersion is dictated by the quantum metric connection, which is a band geometric quantity. The intrinsic nonreciprocity in bulk intraband plasmon dispersion is dictated by the quantum metric dipole and a higher-order `Drude' weight-like term. We corroborate our findings via explicit numerical calculations for the two-dimensional Qi-Wu-Zhang model and demonstrate the existence of intrinsic nonreciprocal intraband and interband plasmon modes in moire systems such as twisted bilayer graphene.

cond-mat.mes-hall↗
arXiv2023

Room temperature electron-hole liquid phase in monolayer MoSi$_2$Z$_4$ (Z = pinctogen)

Photo-excited electrons and holes in insulators, above a critical density and below a critical temperature, can condense to form an electron-hole liquid (EHL) phase. However, observing the EHL phase at room temperature is extremely challenging. Here, we introduce the monolayer MoSi$_2$Z$_4$ (Z= N, As, P) series of compounds as a promising platform for observing the EHL phase at room temperature. The higher impact of the Coulomb interactions in two dimensions helps these monolayers support the EHL phase with an increased EHL binding energy and transition temperature, along with strongly bound excitons. Our findings motivate further exploration of the MoSi$_2$Z$_4$ monolayers for realizing the EHL phase at high temperatures to harness collective phenomena for optoelectronic applications.

cond-mat.mes-hall↗
arXiv2023

Observation of highly anisotropic bulk dispersion and spin-polarized topological surface states in CoTe2

We present CoTe2 as a new type-II Dirac semimetal supporting Lorentz symmetry violating Dirac fermions in the vicinity of the Fermi energy. By combining first principle ab-initio calculations with experimental angle-resolved photo-emission spectroscopy results, we show the CoTe2 hosts a pair of type-II Dirac fermions around 90 meV above the Fermi energy. In addition to the bulk Dirac fermions, we find several topological band inversions in bulk CoTe2, which gives rise to a ladder of spin-polarized surface states over a wide range of energies. In contrast to the surface states which typically display Rashba-type in-plane spin splitting, we find that CoTe2 hosts novel out-of-plane spin polarization as well. Our work establishes CoTe2 as a potential candidate for the exploration of Dirac fermiology and applications in spintronic devices, infrared plasmonics, and ultrafast optoelectronics.

cond-mat.mtrl-sci↗
arXiv2023

Discovery of a magnetic Dirac system with large intrinsic non-linear Hall effect

Magnetic materials exhibiting topological Dirac fermions are attracting significant attention for their promising technological potential in spintronics. In these systems, the combined effect of the spin-orbit coupling and magnetic order enables the realization of novel topological phases with exotic transport properties, including the anomalous Hall effect and magneto-chiral phenomena. Herein, we report experimental signature of topological Dirac antiferromagnetism in TaCoTe2 via angle-resolved photoelectron spectroscopy (ARPES) and first-principles density functional theory (DFT) calculations. In particular, we find the existence of spin-orbit coupling-induced gaps at the Fermi level, consistent with the manifestation of a large intrinsic non-linear Hall conductivity. Remarkably, we find that the latter is extremely sensitive to the orientation of the Néel vector, suggesting TaCoTe2 a suitable candidate for the realization of non-volatile spintronic devices with an unprecedented level of intrinsic tunability.

cond-mat.str-el↗
arXiv2023

Chiral anomalies in 3D spin-orbit coupled metals: electrical, thermal, and gravitational anomaly

The discovery of a chiral anomaly in Weyl semimetals, the non-conservation of chiral charge and energy across two opposite chirality Weyl nodes, has sparked immense interest in understanding its impact on various physical phenomena. Here, we demonstrate the existence of electrical, thermal, and gravitational quantum chiral anomalies in 3D spin-orbit coupled systems. Notably, these anomalies involve chiral charge transfer across two Fermi surfaces linked to a single Weyl-like point, rather than across opposite chirality Weyl nodes as in Weyl semimetals. Our findings reveal that the Berry curvature flux piercing the Fermi surface plays a critical role in distinguishing the `chirality' of the carriers and the corresponding chiral charge and energy transfer. Importantly, we demonstrate that these quantum chiral anomalies lead to interesting thermal spin transport such as the spin Nernst effect. Our results suggest that 3D spin-orbit coupled metals offer a promising platform for investigating the interplay between quantum chiral anomalies and charge and spin transport in non-relativistic systems.

cond-mat.mes-hall↗
arXiv2022

Quantum Kinetic Theory of Nonlinear Optical Currents: Finite Fermi surface and Fermi sea contributions

The quantum kinetic framework provides a versatile method for investigating the dynamical optical and transport currents of crystalline solids. In this paper, starting from the density-matrix equations of motion, we present a general theoretical path to obtain nonlinear optical responses elegantly and transparently. We devise a kinetic theory applicable to materials with arbitrary band structures and captures intraband and interband coherence effects, finite Fermi surfaces, and disorder effects. We present a classification of nonlinear optical currents arising from the interference of interband and intraband components of the density matrix with distinct symmetry and quantum geometrical origin for each contribution. In this context, we report four findings. (i) The Fermi Golden Rule approach is insufficient to derive the correct expression for the injection current, a shortcoming that we remedy in our theory while associating the injection current with the intraband-interband contribution to the second-order density matrix. (ii) The interband-intraband contribution yields a resonant current that survives irrespective of any symmetry constraint in addition to the well-known anomalous nonlinear current (non-resonant), which requires time-reversal symmetry. (iii) Quite generally, the nonlinear current is significantly enhanced by contributions from the finite Fermi surface. (iv) The finite Fermi surface and Fermi sea additionally lead to sizable novel nonlinear effects via contributions we term double resonant and higher-order pole. As an illustration, we compute the nonlinear response of topological antiferromagnet CuMnAs and thin film tilted Weyl semimetals dominated by interband coherence effects. We find that the nonlinear response of CuMnAs is responsive to the direction of finite magnetization field and choice of the polarization angle of the beam, while Weyl semimetal only to tilt.

cond-mat.mes-hall↗
arXiv2022

Designing power efficient transistors using narrow bandwidth materials from the MA2Z4 monolayer series

The subthreshold leakage current in transistors has become a critical limiting factor for realizing ultra-low-power transistors. The leakage current is predominantly dictated by the long thermal tail of the charge carriers. We propose a solution to this problem by using narrow bandwidth semiconductors for limiting the thermionic leakage current by filtering out the high energy carriers. We specifically demonstrate this solution in transistors with laterally confined monolayer MoSi2N4 with different passivation serving as channel material. Remarkably, we find that the proposed narrow bandwidth devices can achieve a large ON/OFF current ratio with an ultra-low-power supply voltage of 0.1 V, even for devices with a 5 nm gate length. We also show that several other materials share the unique electronic properties of narrow bandwidth conduction and valance bands in the same series.

cond-mat.mes-hall↗