arXiv subjects

Mohit Randeria

Publications and source records attributed to Mohit Randeria.

At least 37 records · Page 2Linked to original sources

arXiv2021

Optical Spectral Weight, Phase Stiffness and Tc Bounds for Trivial and Topological Flat Band Superconductors

We present exact results that give insight into how interactions lead to transport and superconductivity in a flat band where the electrons have no kinetic energy. We obtain bounds for the optical spectral weight for flat band superconductors, that lead to upper bounds for the superfluid stiffness and the 2D $T_c$. We focus on on-site attraction $|U|$ on the Lieb lattice with trivial flat bands and on the $π$-flux model with topological flat bands. For trivial flat bands, the low-energy optical spectral weight $\widetilde{D}_\text{low} \leq \widetilde{n} |U| Ω/2$ with $\widetilde{n} = \min\left(n,2-n\right)$, where $n$ is the flat band density and $Ω$ the Marzari-Vanderbilt spread of the Wannier functions (WFs). We also obtain a lower bound involving the quantum metric. For topological flat bands, with an obstruction to localized WFs respecting all symmetries, we again obtain an upper bound for $D_{\rm low}$ linear in $|U|$. We discuss the insights obtained from our bounds by comparing them with mean-field and quantum Monte-Carlo results.

cond-mat.supr-con↗

arXiv2020

Spectral functions across an Insulator to Superconductor Transition

In a minimal 2-band model with attractive interactions between fermions, we calculate the gap to single and two-particle excitations, the band-dependent spectral functions, the superfluid density and compressibility using quantum Monte Carlo (QMC) methods. We find Fermi and Bose insulating phases with signatures of incipient pairing evident in the single-particle spectral functions, and a superconducting state with three different spectral functions: (i) both bands show "BCS" behavior in which the minimum gap locus occurs on a closed contour on the underlying Fermi surface; (ii) both bands show "BEC" behavior in which the minimum gap occurs at a point; and (iii) band selective spectral characteristics, in which one band shows "BCS" while the other shows "BEC" behavior. At large interactions, we find a Mott phase of rung bosons in which the filling is one boson for every two sites, half the typical density constraint for Mott insulators.

cond-mat.supr-con↗

arXiv2020

Strange metal behavior of the Hall angle in twisted bilayer graphene

Twisted bilayer graphene (TBG) with interlayer twist angles near the magic angle $\approx 1.08^{\circ}$ hosts flat bands and exhibits correlated states including Mott-like insulators, superconductivity and magnetism. Here we report combined temperature-dependent transport measurements of the longitudinal and Hall resistivities in close to magic-angle TBG. While the observed longitudinal resistivity follows linear temperature $T$ dependence consistent with previous reports, the Hall resistance shows an anomalous $T$ dependence with the cotangent of the Hall angle cot $Θ{_H} \propto T^2$. Boltzmann theory for quasiparticle transport predicts that both the resistivity and cot $Θ{_H}$ should have the same $T$ dependence, contradicting the observed behavior. This failure of quasiparticle-based theories is reminiscent of other correlated strange metals such as cuprates.

cond-mat.mes-hall↗

arXiv2020

Atomic scale visualization of topological spin textures in the chiral magnet MnGe

Spin polarized scanning tunneling microscopy is used to directly image topological magnetic textures in thin films of MnGe, and to correlate the magnetism with structure probed at the atomic-scale. Our images indicate helical stripe domains, each characterized by a single wavevector Q, and their associated helimagnetic domain walls, in contrast to the 3Q magnetic state seen in the bulk. Combining our surface measurements with micromagnetic modeling, we deduce the three-dimensional orientation of the helical wavevectors and gain detailed understanding of the structure of individual domain walls and their intersections. We find that three helical domains meet in two distinct ways to produce either a "target-like" or a "pi-like" topological spin texture, and correlate these with local strain on the surface. We further show that the target-like texture can be reversibly manipulated through either current/voltage pulsing or applied magnetic field, a promising step toward future applications.

cond-mat.mes-hall↗

arXiv2019

Superconductor-Insulator Transition and Fermi-Bose Crossovers

The direct transition from an insulator to a superconductor (SC) in Fermi systems is a problem of long-standing interest, which necessarily goes beyond the standard BCS paradigm of superconductivity as a Fermi surface instability. We introduce here a simple, translationally-invariant lattice fermion model that undergoes a SC-insulator transition (SIT) and elucidate its properties using analytical methods and quantum Monte Carlo simulations. We show that there is a fermionic band insulator to bosonic insulator crossover in the insulating phase and a BCS-to-BEC crossover in the SC. The SIT is always found to be from a bosonic insulator to a BEC-like SC, with an energy gap for fermions that remains finite across the SIT. The energy scales that go critical at the SIT are the gap to pair excitations in the insulator and the superfluid stiffness in the SC. In addition to giving insights into important questions about the SIT in solid state systems, our model should be experimentally realizable using ultracold fermions in optical lattices.

cond-mat.supr-con↗

arXiv2019

Band inversion and topology of the bulk electronic structure in FeSe${}_{0.45}$Te${}_{0.55}$

FeSe${}_{0.45}$Te${}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as function of photon energies ranging from 15 - 100 eV. We find that although the top of bulk valence band shows essentially no $k_z$ dispersion, its normalized intensity exhibits a periodic variation with $k_z$. We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from $Γ$ to Z. Thus we provide the first direct evidence for a topologically non-trivial bulk band structure that supports protected surface states.

cond-mat.str-el↗

arXiv2019

Upper bounds on the superfluid stiffness and superconducting $T_c$: Applications to twisted-bilayer graphene and ultra-cold Fermi gases

Understanding the material parameters that control the superconducting transition temperature $T_c$ is a problem of fundamental importance. In many novel superconductors, phase fluctuations determine $T_c$, rather than the collapse of the pairing amplitude. We derive rigorous upper bounds on the superfluid phase stiffness for multi-band systems, valid in any dimension. This in turn leads to an upper bound on $T_c$ in two dimensions (2D), which holds irrespective of pairing mechanism, interaction strength, or order-parameter symmetry. Our bound is particularly useful for the strongly correlated regime of low-density and narrow-band systems, where mean field theory fails. For a simple parabolic band in 2D with Fermi energy $E_F$, we find that $k_BT_c \leq E_F/8$, an exact result that has direct implications for the 2D BCS-BEC crossover in ultra-cold Fermi gases. Applying our multi-band bound to magic-angle twisted bilayer graphene (MA-TBG), we find that band structure results constrain the maximum $T_c$ to be close to the experimentally observed value. Finally, we discuss the question of deriving rigorous upper bounds on $T_c$ in 3D.

cond-mat.supr-con↗

arXiv2019

Topological superconductivity in Dirac honeycomb systems

We predict two topological superconducting phases in microscopic models arising from the Berry phase associated with the valley degree of freedom in gapped Dirac honeycomb systems. The first one is a topological helical spin-triplet superconductor with a nonzero center-of-mass momentum that does not break time-reversal symmetry. We also find a topological chiral-triplet superconductor with Chern number $\pm 1$ with equal-spin-pairing in one valley and opposite-spin-triplet pairing in the other valley. Our results are obtained for the Kane-Mele model in which we have explored the effect of three different interactions, onsite attraction $U$, nearest-neighbor density-density attraction $V$, and nearest-neighbor antiferromagnetic exchange $J$, within self-consistent Bogoliubov--de Gennes theory. Transition metal dichalcogenides and cold atom experiments are promising platforms to explore these phases.

cond-mat.supr-con↗

arXiv2019

Spin-Hall Topological Hall Effect in Highly Tunable Pt/Ferrimagnetic-Insulator Bilayers

Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. While magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm$_3$Fe$_5$O$_{12}$ bilayers, where the pristine Tm$_3$Fe$_5$O$_{12}$ epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm$_3$Fe$_5$O$_{12}$ is insulating, we demonstrate the detection of topological magnetic textures through a novel phenomenon: 'spin-Hall topological Hall effect' (SH-THE), where the interfacial spin-orbit torques allow spin-Hall-effect generated spins in Pt to experience the unique topology of the underlying skyrmions in Tm$_3$Fe$_5$O$_{12}$. This novel electrical detection phenomenon paves a new path for utilizing a large family of magnetic insulators in future skyrmion technologies.

cond-mat.mtrl-sci↗

arXiv2018

Topological magnons with nodal-line and triple-point degeneracies: Implications for thermal Hall effect in pyrochlore iridates

We analyze the magnon excitations in pyrochlore iridates with all-in-all-out (AIAO) antiferromagnetic order, focusing on their topological features. We identify the magnetic point group symmetries that protect the nodal-line band crossings and triple-point degeneracies that dominate the Berry curvature. We find three distinct regimes of magnon band topology, as a function of the ratio of Dzyaloshinskii-Moriya (DM) interaction to the antiferromagnetic exchange. We show how the thermal Hall response provides a unique probe of the topological magnon band structure in AIAO systems.

cond-mat.str-el↗

arXiv2018

Deformation and Stability of Surface States in Dirac semimetals

The unusual surface states of topological semimetals have attracted a lot of attention. Recently, we showed [PNAS 113, 8648 (2016)] that for a Dirac semimetal (DSM) arising from band-inversion, such as Na$_3$Bi and Cd$_3$As$_2$, the expected double Fermi arcs on the surface are not topologically protected. Quite generally, the arcs deform into states similar to those on the surface of a strong topological insulator. Here we address two questions related to deformation and stability of surface states in DSMs. First, we discuss why certain perturbations, no matter how large, are unable to destroy the double Fermi arcs. We show that this is related to certain extra (particle-hole) symmetry, which is non-generic in materials. Second, we discuss situations in which the surface states are completely destroyed without breaking any symmetry or impacting the bulk Dirac nodes. We are not aware of any experimental or density functional theory (DFT) candidates for a material which is a bulk DSM without any surface states, but our results clearly show that this is possible.

cond-mat.str-el↗

arXiv2018

Chiral Bobbers and Skyrmions in Epitaxial FeGe/Si(111) Films

We report experimental and theoretical evidence for the formation of chiral bobbers - an interfacial topological spin texture - in FeGe films grown by molecular beam epitaxy (MBE). After establishing the presence of skyrmions in FeGe/Si(111) thin film samples through Lorentz transmission electron microscopy and topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between film thickness and the slope of the susceptibility (d\c{hi}/dH). We present evidence for the evolution as a function of film thickness, L, from a skyrmion phase for L < LD/2 to a cone phase with chiral bobbers at the interface for L > LD/2, where LD ~ 70 nm is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction (DMI).

cond-mat.mtrl-sci↗

arXiv2017

Orbital and spin order in spin-orbit coupled $d^1$ and $d^2$ double perovskites

We consider strongly spin-orbit coupled double perovskites A$_2$BB'O$_6$ with B' magnetic ions in either $d^1$ or $d^2$ electronic configuration and non-magnetic B ions. We provide insights into several experimental puzzles, such as the predominance of ferromagnetism in $d^1$ versus antiferromagnetism in $d^2$ systems, the appearance of negative Curie-Weiss temperatures for ferromagnetic materials, and the size of effective magnetic moments. We develop and solve a microscopic model with both spin and orbital degrees of freedom within the Mott insulating regime at finite temperature using mean field theory. The interplay between anisotropic orbital degrees of freedom and spin-orbit coupling results in complex ground states in both $d^1$ and $d^2$ systems. We show that the ordering of orbital degrees of freedom in $d^1$ systems results in coplanar canted ferromagnetic and 4-sublattice antiferromagnetic structures. In $d^2$ systems we find additional colinear antiferromagnetic and ferromagnetic phases not appearing in $d^1$ systems. At finite temperatures, we find that orbital ordering driven by both superexchange and Coulomb interactions may occur at much higher temperatures compared to magnetic order and leads to distinct deviations from Curie-Weiss law.

cond-mat.str-el↗

arXiv2016

Effective magnetic interactions in spin-orbit coupled $d^4$ Mott insulators

Transition metal compounds with the $(t_{2g})^4$ electronic configuration are expected to be nonmagnetic atomic singlets both in the weakly interacting regime due to spin-orbit coupling, as well as in the Coulomb dominated regime with oppositely aligned $L=1$ and $S=1$ angular momenta. However, starting with the full multi-orbital electronic Hamiltonian, we show the low energy effective magnetic Hamiltonian contains isotropic superexchange spin interactions but anisotropic orbital interactions. By tuning the ratio of superexchange to spin-orbit coupling $J_\mathrm{SE}/λ$, we obtain a phase transition from nonmagnetic atomic singlets to novel magnetic phases depending on the strength of Hund's coupling, the crystal structure and the number of active orbitals. Spin-orbit coupling plays a non-trivial role in generating a triplon condensate of weakly interacting excitations at antiferromagnetic ordering vector $\vec k=\vec π$, regardless of whether the local spin interactions are ferromagnetic or antiferromagnetic. In the large $J_\mathrm{SE} / λ$ regime, the localized spin and orbital moments produce anisotropic orbital interactions that are frustrated or constrained even in the absence of geometric frustration. Orbital frustration leads to frustration in the spin channel opening up the possibility of spin-orbital liquids with both spin and orbital entanglement.

cond-mat.str-el↗

arXiv2016

Are the double Fermi arcs of Dirac semimetals topologically protected?

Motivated by recent experiments probing anomalous surface states of Dirac semimetals (DSMs) Na$_3$Bi and Cd$_3$As$_2$, we raise the question posed in the title. We find that, in marked contrast to Weyl semimetals, the gapless surface states of DSMs are not topologically protected in general, except on time-reversal-invariant planes of surface Brillouin zone. We first demonstrate this in a minimal $4$-band model with a pair of Dirac nodes at ${\bf k}=(0,0,\pm Q)$, where gapless states on the side surfaces are protected only near $k_z=0$. We then validate our conclusions about the absence of a topological invariant protecting double Fermi arcs in DSMs using a K-theory analysis for space groups of Na$_3$Bi and Cd$_3$As$_2$. Generically, the arcs deform into a Fermi pocket, similar to {the surface states of a topological insulator (TI), and this can merge into the} projection of bulk Dirac Fermi surfaces as the chemical potential is varied. We make sharp predictions for the doping-dependence of the surface states of a DSM that can be tested by ARPES and quantum oscillation experiments.

cond-mat.str-el↗

arXiv2016

Charged fermions coupled to $\mathbb{Z}_2$ gauge fields: Superfluidity, confinement and emergent Dirac fermions

We consider a 2+1 dimensional model of charged fermions coupled to a $\mathbb{Z}_2$ gauge field, and study the confinement transition in this regime. To elucidate the phase diagram of this model, we introduce a method to handle the Gauss law constraint within sign problem free determinantal quantum Monte Carlo, at any charge density. For generic charge densities, $\mathbb{Z}_2$ gauge fluctuations mediate pairing and the ground state is a gapped superfluid. Superfluidity also appears in the confined phase. This is reminiscent of the BCS-BEC crossover, except that a true zero temperature transition occurs here, with the maximum $T_c$ achieved near the transition. At half-filling also one obtains a large Fermi surface which is gapped at zero temperature. However, on increasing fermion hopping a $π$-flux phase is spontaneously generated, with emergent Dirac fermions that are stable against pairing. In contrast to a Fermi liquid of electrons, the change in Fermi surface volumes of the $\mathbb{Z}_2$ fermions occurs without the breaking of translation symmetry. Unexpectedly, the numerics indicate a single continuous transition between the deconfined Dirac phase and the confined superfluid, in contrast to the naive expectation of a split transition, where a gap to fermions precedes confinement.

cond-mat.str-el↗

arXiv2015

Skyrmions in Chiral Magnets with Rashba and Dresselhaus Spin-Orbit Coupling

Skyrmions are topological spin textures of interest for fundamental science and applications. Previous theoretical studies have focused on systems with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. We investigate here systems that break surface inversion symmetry, in addition to possible broken bulk inversion. This leads to two distinct Dzyaloshinskii-Moriya (DM) terms with strengths $D_\perp$, arising from Rashba spin-orbit coupling (SOC), and $D_\parallel$ from Dresselhaus SOC. We show that skyrmions become progressively more stable with increasing $D_\perp/D_\parallel$, extending into the regime of easy-plane anisotropy. We find that the spin texture and topological charge density of skyrmions develops nontrivial spatial structure, with quantized topological charge in a unit cell given by a Chern number. Our results give a design principle for tuning Rashba SOC and magnetic anisotropy to stabilize skyrmions in thin films, surfaces, interfaces and bulk magnetic materials that break mirror symmetry.

cond-mat.str-el↗

arXiv2015

Theory of Kerr and Faraday rotations and linear dichroism in Topological Weyl Semimetals

We consider the electromagnetic response of a topological Weyl semimetal (TWS) with a pair of Weyl nodes in the bulk and corresponding Fermi arcs in the surface Brillouin zone. We compute the frequency-dependent complex conductivities $σ_{αβ}(ω)$ and also take into account the modification of Maxwell equations by the topological $θ$-term to obtain the Kerr and Faraday rotations in a variety of geometries. For TWS films thinner than the wavelength, the Kerr and Faraday rotations, determined by the separation between Weyl nodes, are significantly larger than in topological insulators. In thicker films, the Kerr and Faraday angles can be enhanced by choice of film thickness and substrate refractive index. We show that, for radiation incident on a surface with Fermi arcs, there is no Kerr or Faraday rotation but the electric field develops a longitudinal component inside the TWS, and there is magnetic linear dichroism. Our results have implications for probing the TWS phase in various experimental systems.

cond-mat.str-el↗