arXiv subjects

G. Baskaran

Publications and source records attributed to G. Baskaran.

At least 19 recordsLinked to original sources

arXiv2024

Smart Holes: Analogue black holes with the right temperature and entropy

In analogue gravity studies, the goal is to replicate black hole phenomena, such as Hawking radiation, within controlled laboratory settings. In the realm of condensed matter systems, this may happen in 2D tilted Dirac cone materials based on honeycomb lattice. In particular, we compute the entropy of this system, and find it has the same form as black hole Bekenstein-Hawking entropy, if an analogue horizon forms. Hence, these systems can be potential analogues of quantum black holes. We show that this entropy is primarily concentrated in the region where the tilt parameter is close to one, which corresponds to the location of the analogue black hole horizon. Additionally, when nonlinear effects are taken into account, the entropy is peaked in a small pocket of the Fermi sea that forms behind the analogue event horizon, which we call the \textit{Fermi puddle}. We further refer to this new type of analogue black hole as a {\it smart hole}, since, in contrast to dumb holes, it can simulate both the correct temperature {\it and} entropy of general relativistic black holes. These results provide an opportunity to illuminate various quantum facets of black hole physics in a laboratory setting.

hep-th↗

arXiv2024

Quantum loops in the 1T transition metal dichalcogenides

Loop arrangements and their quantum superpositions describe several interesting many-particle states. We propose that they also describe bonding in a class of transition metal dichalcogenides. We present an effective quantum loop model for monolayers with 1T structure and a d$^2$ valence electron configuration: materials of the form MX$_2$ (M = Mo, W and X=S, Se, Te) and AM$'$Y$_2$ (A = Li, Na; M$'$ = V, Nb and Y = O, S, Se). Their t$_{2g}$ orbitals exhibit strongly directional overlaps between neighbouring atoms, favouring the formation of valence bonds. A transition metal atom forms two valence bonds, each with one of its neighbours. When connected, these bonds form loops that cover the triangular lattice. We construct a minimal Rokhsar-Kivelson-like model with resonance processes that cut and reconnect loops that run in proximity. The resulting dynamics is more constrained than in traditional quantum dimer models, with a `bending' constraint that arises from orbital structure. In the resulting phase diagram, we find phases that resemble distorted phases seen in materials, viz., the 1T$'$ and trimerized phases. As a testable prediction, we propose that a single d$^1$ or d$^3$ impurity will terminate a loop and give rise to a long-ranged texture. For example, a Ti/Cr defect in LiVO$_2$ will produce one or more domain walls that propagate outward from the impurity. We discuss the possibility of a loop liquid phase that can emerge in these materials.

cond-mat.str-el↗

arXiv2024

Fractional Wannier Orbitals and Tight-Binding Gauge Fields for Kitaev Honeycomb Superlattices with Flat Majorana Bands

Fractional excitations hold immense promise for both fundamental physics and quantum technologies. However, constructing lattice models for their dynamics under gauge fields remains a formidable challenge due to inherent obstructions. Here, we introduce a novel and systematic framework for deriving low-energy lattice models of fractional orbitals coupled to tight-binding gauge fields. Departing from conventional geometric approaches, our method systematically eliminates the high-energy states via virtual hopping, thereby deriving the gauge potential and quantum metric through a superexchange-like mechanism. We demonstrate the framework by constructing Wannier orbitals for Majorana states and a tight-binding $Z_2$ gauge field across various flux crystalline phases in the Kitaev spin model on a honeycomb lattice. Our study reveals a striking phase transition between two non-trivial topological phases characterized by gapless flat-band with extensive degeneracy. Furthermore, we develop a gauge-invariant mean-field theory for interacting Majorana orbitals, leading to a correlation-induced fractional Chern state. Our work establishes a general framework for gauge-mediated tight-binding models and a gauge-invariant mean-field theory for interacting fractional orbitals that can be readily extended to $U(1)$, $SU(N)$ lattice gauge theories.

cond-mat.str-el↗

arXiv2023

Broad Band Mott Localization is all you need for Hot Superconductivity: Atom Mott Insulator Theory for Cu-Pb Apatite

A hypothetical non-dimerized Cu chain in equilibrium is a spin-\half atom Mott insulator (AMI), eventhough its band width is high ~ 10 eV. This RVB reservoir has a large exchange coupling J ~ 2 eV. This idea of, \textit{broad band Mott localization} was used by us in our earlier works, including prediction of high Tc superconductivity in doped graphene, silicene and a theory for hot superconductivity reported in Ag-Au nanostructures (TP 2008). In the present work we identify possible random AMI subsystems in Cu-Pb Apatite and develop a model for reported hot superconductivity (LKK 2023). In apatite structure, network of interstitial columnar spaces run parallel to c-axis and ab-plane. They accomodate excess copper, as neutral Cu atom clusters, chains and planar segments. They are our emergent AMI's. Electron transfer from AMI's to insulating host, generates strong local superconducting correlation, via phyics of doped Mott insulator. Josephson coupling between doped AMI's, establishes hot superconductivity. A major Challenge to superconducting order in real material is competing insulating phases - valence bond solid (spin-Peirels)-lattice distortions etc. AMI theory points to ways of making the \textit{elusive superconductivity} palpable. We recommend exploration of hot superconductivity in the rich world of minerals and insulators, via metal atom inclusion.

cond-mat.supr-con↗

arXiv2023

Emergent glassiness in disorder-free Kitaev model: Density matrix renormalization group study on a one-dimensional ladder setting

The complete phase diagram of the Kitaev model with a magnetic field remains elusive, as do the experimental results in the candidate material {\alpha}-RuCl3. Here, we study the Kitaev model on a one-dimensional ladder setting within the density-matrix renormalization group method in the presence of a magnetic field at zero temperature. We find five distinct phases with increasing magnetic field, which are characterized by a homogeneous flux phase, the Z2 vortex gas, solid and emergent glass phase, and finally, a spin-polarized phase. The emergent glassiness is confirmed by calculating correlation functions showing quasi-long-range behavior and ground state fidelity, showing a plethora of energetically accessible orthogonal saddle points corresponding to different flux configurations. This glassy behavior seems to arise from the slow dynamics of the Z2 fluxes, which is a consequence of the local constraints present in the underlying Hilbert space. This phenomenon can also be explored in other spin-liquid systems where the corresponding low-energy excitations are similarly retarded due to constraints.

cond-mat.str-el↗

arXiv2020

Wohlleben Effect and Emergent Pi junctions in superconducting Boron doped Diamond thin films

Diamond is an excellent band insulator. However, boron (B) doping is known to induce superconductivity. We present two interesting effects in superconducting B doped diamond (BDD) thin films: i) Wohlleben effect (paramagnetic Meissner effect, PME) and ii) a low field spin glass like susceptibility anomaly. We have performed electrical and magnetic measurements (under pressure in one sample) at dopings (1.4 , 2.6 and 3.6) X 1021 cm-3, in a temperature range 2 - 10 K. PME, a low field anomaly in inhomogeneous superconductors could arise from flux trapping, flux compression, or for non-trivial reason such as emergent Josephson Pi junctions. Joint occurrence of PME and spin glass type anomalies points to possible emergence of Pi junctions. BDD is a disordered s-wave superconductor; and Pi junctions could be produced by spin flip scattering of spin half moments when present at weak superconducting regions (Bulaevski et al. 1978). A frustrated network of 0 and Pi junctions will result (Kusmartsev et al. 1992) in a distribution of spontaneous equilibrium supercurrents, a phase glass state. Anderson localized spin half spinons embedded in a metallic fluid (two fluid model of Bhatt et al.) could create Pi junction by spin flip scattering. Our findings are consistent with presence of Pi junctions, invoked to explain their (Bhattacharyya et al.) observation of certain resistance anomaly in BDD.

cond-mat.supr-con↗

arXiv2019

Order by singularity in Kitaev clusters

The Kitaev model is a beautiful example of frustrated interactions giving rise to deep and unexpected phenomena. In particular, its classical version has remarkable properties stemming from exponentially large ground state degeneracy. Here, we present a study of magnetic clusters with spin-$S$ moments coupled by Kitaev interactions. We focus on two cluster geometries -- the Kitaev square and the Kitaev tetrahedron -- that allow us to explicitly enumerate all classical ground states. In both cases, the classical ground state space (CGSS) is large and self-intersecting, with non-manifold character. The Kitaev square has a CGSS of four intersecting circles that can be embedded in four dimensions. The tetrahedron CGSS consists of eight spheres embedded in six dimensions. In the semi-classical large-$S$ limit, we argue for effective low energy descriptions in terms of a single particle moving on these non-manifold spaces. Remarkably, at low energies, the particle is tied down in bound states formed around singularities at self-intersection points. In the language of spins, the low energy physics is determined by a distinct set of states that lies well below other eigenstates. These correspond to `Cartesian' states, a special class of classical ground states that are constructed from dimer covers of the underlying lattice. They completely determine the low energy physics despite being a small subset of the classical ground state space. This provides an example of order by singularity, where state selection becomes stronger upon approaching the classical limit.

cond-mat.str-el↗

arXiv2019

Singlet Reservoir Theory of Ambient Tc Granular Superconductivity in Monovalent Metal Nanostructures

Monovalent metals contain half filled band (HFB) of s-electrons. Emphasizing importance of Coulomb repulsions in HFB in 2D and 1D monovalent systems we sketched a theory (2018) for ambient temperature granular superconductivity reported by Thapa and Pandey (2018) in Au-Ag nanostructures (updated by Thapa et al., 2019). Sharpening our theory, we suggest that \textit{Coulomb repulsions in certain structurally perturbed regions (atomic clusters, stacking faults, grain boundaries etc.) create nanoscale reservoirs of singlet electron pairs}. These low dimensional patches are hybridized to a background 3D jellium metal and produce observed ambient Tc granular superconductivity via proximity Josephson effect. Using repulsive Hubbard model we show presence of singlet reservoirs and physics of doped Mott insulators. Needed charge transfer arises from differing electronegativities. Our theory predicts that \textit{all elemental monovalent (alkali, Cu, Ag and Au) metals, under suitable structural perturbations, are likely to exhibit ambient temperature superconductivity}.

cond-mat.supr-con↗

arXiv2019

Photoinduced Floquet topological magnons in Kitaev magnets

We study periodically driven pure Kitaev model and ferromagnetic phase of the Kitaev-Heisenberg model on the honeycomb lattice by off-resonant linearly and circularly-polarized lights at zero magnetic field. Using a combination of linear spin wave and Floquet theories, we show that the effective time-independent Hamiltonians in the off-resonant regime map onto the corresponding anisotropic static spin model, plus a tunable photoinduced magnetic field along the $[111]$ direction, which precipitates Floquet topological magnons and chiral magnon edge modes. They are tunable by the light amplitude and polarization. Similarly, we show that the thermal Hall effect induced by the Berry curvature of the Floquet topological magnons can also be tuned by the laser field. Our results pave the way for ultrafast manipulation of topological magnons in irradiated Kitaev magnets, and could play a pivotal role in the investigation of ultrafast magnon spin current generation in Kitaev materials.

cond-mat.str-el↗

arXiv2018

Resonating valence bonds and spinon pairing in the Dicke model

Resonating valence bond (RVB) states are a class of entangled quantum many body wavefunctions with great significance in condensed matter physics. We propose a scheme to synthesize a family of RVB states using a cavity QED setup with two-level atoms (with states $\vert 0 \rangle$ and $\vert 1 \rangle$) coupled to a common photon mode. In the lossy cavity limit, starting with an initial state of $M$ atoms excited and $N$ atoms in the ground state, we show that this setup can be configured as a Stern Gerlach experiment. A measurement of photon emission collapses the wavefunction of atoms onto an RVB state composed of resonating long-ranged singlets of the form $\frac{1}{\sqrt{2}}[\vert 0 1 \rangle - \vert 1 0 \rangle]$. Each emitted photon reduces the number of singlets by unity, replacing it with a pair of lone spins or `spinons'. As spinons are formed coherently in pairs, they are analogous to Cooper pairs in a superconductor. To simulate pair fluctuations, we propose a protocol in which photons are allowed to escape the cavity undetected. This leads to a mixed quantum state with a fluctuating number of spinon pairs -- an inchoate superconductor. Remarkably, in the limit of large system sizes, this protocol reveals an underlying quantum phase transition. Upon tuning the initial spin polarization ($M-N$), the emission exhibits a continuous transition from a dark state to a bright state. This is reflected in the spinon pair number distribution which can be tuned from sub-poissonian to super-poissonian regimes. This opens an exciting route to simulate RVB states and superconductivity.

quant-ph↗

arXiv2017

Resonating Valence Bond Theory of Superconductivity: Beyond Cuprates

Resonating valence bond (RVB) theory of high Tc superconductivity, an electron correlation based mechanism, began as an insightful response by Anderson, to Bednorz and Muller's discovery of high Tc superconductivity in cuprates in late 1986. Shortly a theoretical framework for quantum spin liquids and superconductivity was developed. This theory adresses a formidable strong coupling quantum manybody problem, in modern times. It is built on certain key experimental facts: i) survival of a dynamical Mott localization in a metallic state, ii) proliferation of bond singlets and iii) absence of fermi liquid quasi particles. After summarising RVB theory I will provide an aerial view of, mostly, new superconductors where I believe that, to a large degree RVB mechanism is at work and indicate prospects for even higher Tc's.

cond-mat.supr-con↗

arXiv2017

Generating resonating valence bond states through Dicke subradiance

Dicke's original thought experiment with two spins coupled to a photon mode has recently been experimentally realized. We propose extending this experiment to N spins and show that it naturally gives rise to highly entangled states. In particular, it gives rise to dark states which have resonating valence bond (RVB) character. We first consider a system of N two level spins in a cavity with only one spin in the excited state. This initial state is a linear combination of a dark state and a bright state. We point out the dark state is a coherent superposition of singlets with resonating valence bond character. We show that the coupling to the photon mode takes the spin system into a mixed state with an entangled density matrix. We next consider an initial state with half of the spins in the excited state. We show that there is a non-zero probability for this to collapse into a dark state with RVB character. In the lossy cavity limit, if no photon is detected within several decay time periods, we may deduce that the spin system has collapsed onto the dark RVB state. We show that the probability for this scales as 2/N, making it possible to generate RVB states of 20 spins or more.

quant-ph↗

arXiv2017

Valence Bond Order and Antiferromagnetism in Silicene - ab initio Results

Silicene and Graphene are similar and have $π$-$π^*$ bands. However band width in silicene is only a third of graphene. It results in a substantial increase in the ratio of Hubbard U to band width W, U/W $\sim$ 0.5 in graphene to $\sim 1$ in silicene. This enhancement, 2 dimensionality and phenomenology suggest a Mott insulator based ground state for silicene (G. Baskaran, arXiv:1309.2242). We lend support to the above proposal by showing, in an ab-initio calculation, that unlike graphene, silicene has two instabilities: i) a valence bond (Kekule) dimerization and ii) a weak two sublattice antiferromagnetic order. Presence of these instabilities, in the absence of fermi surface nesting, point to Mott localization, \textit{within the frame work of ab-initio scheme}. Substrate dependent structural reconstructions seen experimentally in silicene are interpreted as generalized Kekule bond order.

cond-mat.mtrl-sci↗

arXiv2017

Theory of Ultra Low Tc Superconductivity in Bismuth: Tip of an Iceberg ?

Superconductivity with an ultra low Tc $\sim$ 0.5 mK was discovered recently in bismuth, a semimetal. To develop a model and scenario for Bi we begin with a cubic reference lattice, close to A7 (dimerized cubic) structure of Bi. Three valence electrons hop among 6p$_x$, 6p$_y$ and 6p$_z$ orbitals and form \textit{quasi one dimensional chains at half filling}. An interesting interplay follows: i) Mott localization tendency in the chains, ii) metallization by interchain hopping and iii) lattice dimerization by electron-phonon coupling. In our proposal, a potential high Tc superconductivity from RVB mechanism is lost in the game. However some superconducting fluctuations survive. Tiny fermi pockets seen in Bi are viewed as remnant \textit{evanescent Bogoliubov quasi particles} in an anomalous normal state. Multi band character admits possibility of PT violating \textit{chiral singlet superconductivity}. Bi has a strong spin orbit coupling; Kramers theorem protects our proposal for the bulk by replacing real spin by Kramer pair. Control of chain dimerization might resurrect high Tc superconductivity in Bi, Sb and As.

cond-mat.supr-con↗

arXiv2017

Two Mott Insulator Theory of Superconductivity in K$_3$X (X: picene, .. p-terphenyl, .. C$_{60}$)

We look for unifying aspects behind superconductivity in aromatic hydrocarbon and fullerene family K$_3$X (X: picene, .. p-terphenyl, .. C$_{60}$). Aromatic hydrocarbon molecules support RVB states. Consequent stability (aromaticity) makes them reluctant electron acceptors. We argue that X accepts only two (not all three) electrons from K$_3$ and creates charged RVB's in X$^{2-}$, and becomes a (molecular) Cooper pair box. A weak Josephson coupling between X$^{2-}$ molecules creates a Bose Mott insulator, a potential high Tc superconductor. Remaining lone electron in the complex (K$_3)^{2+}$ occupies a suitable metal orbital hybrid. They hybridize weakly through X$^{2-}$ molecular bridges, to form a half filled band of renormalized K atom orbitals, a Fermionic Mott insulator. An interplay of RVB physics and charge transfer (mutual doping) or external doping leads to superconductivity in one or both Mott insulators. In our theory there is room for room temperature superconductivity.

cond-mat.supr-con↗

arXiv2016

Silicene and Germanene as prospective playgrounds for Room Temperature Superconductivity

Combining theory and certain striking phenomenology we suggest that silicene and germanene are \textit{elemental Mott insulators} and abode of doping induced high Tc superconductivity. In our theory, a 3 fold reduction in silicene $π- π^*$ band width, in comparison to graphene, and short range coulomb interactions enable Mott localization. Recent experimental results are invoked to provide support for our Mott insulator model: i) a significant $π$-band narrowing, in silicene on ZrB$_2$ seen in ARPES, ii) a superconducting gap appearing below 35 K with a large $\frac{2Δ}{k_BTc}\sim$ 20 in silicene on Ag, iii) emergence of electron like pockets at M points, on electron doping by Na adsorbent, iv) certain coherent quantum oscillation like features exhibited by silicene transistor at room temperatures and v) absence of Landau level splitting upto 7 Tesla and vi) superstructures, not common in graphene but, ubiquitous in silicene. A synthesis of the above results using theory of Mott insulator, with and without doping, is attempted. We surmise that if competing orders are taken care of and optimal doping achieved, superconductivity in silicene and germanene could reach room temperature scales; our estimates of model parameters, t and J $\sim$ 1 eV, are encouragingly high, compared to cuprates.

cond-mat.str-el↗

arXiv2016

My Random Walks in Anderson's Garden

Anderson's Garden is a drawing presented to Philip W. Anderson on the eve of his 60th birthday celebration, in 1983. This cartoon (Fig. 1), whose author is unknown, succinctly depicts some of Anderson's pre-1983 works, as a blooming garden. As an avid reader of Anderson's papers, random walk in Anderson's garden had become a part of my routine since graduate school days. This was of immense help and prepared me for a wonderful collaboration with the gardener himself, on the resonating valence bond (RVB) theory of High Tc cuprates and quantum spin liquids, at Princeton. The result was bountiful - the first (RVB mean field) theory for i) quantum spin liquids, ii) emergent fermi surfaces in Mott insulators and iii) superconductivity in doped Mott insulators. Beyond mean field theory - i) emergent gauge fields, ii) Ginzbuerg Landau theory with RVB gauge fields, iii) prediction of superconducting dome, iv) an early identification and study of a non-fermi liquid normal state of cuprates and so on. Here I narrate this story, years of my gardening attempts and end with a brief summary of my theoretical efforts to extend RVB theory of superconductivity to encompass the recently observed very high Tc ~ 203 K superconductivity in molecular solid H2S at high pressures ~ 200 GPa.

cond-mat.str-el↗

arXiv2016

RVB States in doped Band Insulators from Coloumb forces: Theory and a case study of Superconductivity in BiS$_2$ Layers

Doped band insulators, HfNCl, WO$_3$, diamond, Bi$_2$Se$_3$, \bis2 families, STO/LAO interface, gate doped SrTiO$_3$ and MoS$_2$ etc. are unusual superconductors. With an aim to build a general theory for superconductivity in doped band insulators we focuss on \bis2 family, discovered by Mizuguchi et al. in 2012. While maximum Tc is only $\sim$ 11 K in \laofx, a number of experimental results are puzzling and anomalous; they resemble high Tc and unconventional superconductors. Using a two orbital model of Usui, Suzuki and Kuroki we show that the uniform low density free fermi sea in \laofh is \textit{unstable towards formation of next nearest neighbor Bi-S-Bi diagonal valence bond} (charge -2e Cooper pair) and their \textit{Wigner crystallization}. Instability to this novel state of matter is caused by unscreened nearest neighbor coulomb repulsions (V $\sim$ 1 eV) and a hopping pattern with sulfur mediated diagonal next nearest neighbor Bi-S-Bi hopping t' $\sim$ 0.88 eV, larger than nearest neighbor Bi-Bi hopping, t $\sim$ 0.16 eV. Wigner crystal of Cooper pairs quantum melt for doping around x = 0.5 and stabilize certain resonating valence bond states and superconductivity. We study few variational RVB states and suggest that \bis2 family members are latent high Tc superconductors, but challenged by competing orders and fragile nature of manybody states sustained by unscreened Coulomb forces. One of our superconducting state has d$_{xy}$ symmetry and a gap. We also predict 2d \textit{Bose metal or vortex liquid} normal state, as charge -2e valence bonds survive in the normal state.

cond-mat.supr-con↗