arXiv subjects

T. V. Ramakrishnan

Publications and source records attributed to T. V. Ramakrishnan.

At least 19 recordsLinked to original sources

arXiv2026

Evidence of Haldane-Chain Physics in an Fe-Dehydroindigo Coordination Polymer

We investigate whether an Fe--dehydroindigo coordination polymer can support Haldane-chain physics in its intrinsic, substrate-free limit. Spin-orbit-coupled density-functional-theory calculations and magnetic energy mapping yield a nearly isotropic antiferromagnetic coupling, together with much weaker single-ion anisotropy. Using these parameters, exact diagonalization and density-matrix renormalization group calculations give a nondegenerate ground state separated from the lowest triplet-derived excitations by a gap of about $13$--$14$~meV, on the scale expected for the spin-1 Haldane gap. The anisotropy produces only a small splitting of the low-energy modes. The dynamical spin structure factor places the lowest spectral weight near the antiferromagnetic wave vector $q=π$. These results identify the isolated cis-dehydroindigo chain as a material-specific coordination-polymer platform whose microscopic interactions place it in a gapped regime consistent with Haldane-chain physics.

cond-mat.mes-hall↗

arXiv2025

Emergent Rashba spin-orbit coupling in bulk gold with buried network of nanoscale interfaces

The Rashba effect, which plays a crucial role in fundamental materials physics and potential spintronics applications, has been engineered in diverse systems, including semiconductor quantum wells, oxide heterostructures, metallic surfaces, topological insulators, ferroelectrics, etc. However, generating it in systems that preserve bulk inversion symmetry (BIS), for example, in bulk metals, has not been possible so far. We demonstrate a unique strategy to introduce and tune Rashba spin-orbit interaction (SOI) to unprecedented magnitudes in inversion-symmetric solids, by incorporating ultra-small silver nanoparticles in bulk gold. The near-identical lattice constants of Ag and Au allowed dense packing of the Ag/Au hetero-interfaces without compromising the global BIS. By varying the density of embedded nanoparticles, we generate Rashba SOI in a bulk metal with a coupling strength of ~15 meV.Angstrom, higher than any known system preserving BIS globally, and up to ~20 times increase in the spin-orbit scattering rate. We argue that the combined effect of charge-transfer at the interfaces and polaronic localization enhances the SOI.

cond-mat.mes-hall↗

arXiv2025

Intricately Entangled Spin and Charge Diffusion and the Coherence-Incoherence Crossover in the High-Dimensional Hubbard Model

Correlation-driven metal-insulator transitions and temperature-driven quantum-coherent-to-incoherent crossovers in correlated electron systems underpin the doping, temperature and frequency-resolved evolution of physical responses. Motivated by recent experimental studies that investigate the evolution of dynamical spin and charge responses, we analyze the spin and charge diffusion spectra in both half-filled and doped one-band Hubbard model using Dynamical Mean Field Theory (DMFT) combined with the Numerical Renormalization Group (NRG). We compare the relative strengths and limitations of Density Matrix NRG (DMNRG) and Full Density Matrix NRG (FDM-NRG) in capturing low-frequency spectral features and their evolution with temperature, interaction strength and band-filling. Key measures, including characteristic frequency scales, Kullback-Leibler divergence, diffusion constants, and kurtosis provide complementary but internally consistent picture for the evolution of spin and charge excitations across bandwidth as well as the band-filling driven Mott transitions and the coherent-incoherent crossover. We find that spin and charge fluctuations cross over from quantum-coherent-to-quantum-incoherent at distinct temperatures, providing a microscopic insight into the complex, two-stage, Fermi-to-non-Fermi liquid-to-bad metal crossovers seen in transport data, in particular in the $dc$ resistivity.

cond-mat.str-el↗

arXiv2025

High Temperature Superconductivity in the Cuprates: Phenomena from a Theorist's Point of View

I present a selection of experimental results on metallic cuprates, both above the superconducting transition temperature $T_c$ (often called the strange metal state) and in the superconducting state. It highlights this still poorly understood part of the physical world. After an introduction, I talk briefly about the pseudogap regime and about the unusual linear resistivity phenomenon. Several empirical correlations between observed quantities are mentioned, e.g. $T_c$ and superfluid density (Uemura), $T_c$ and next nearest neighbour hopping, slope of the linear resistivity and $T_c$. In the belief that a comprehensive explanation may need an understanding of the extremely strongly correlated metal, a few initial steps in this direction are outlined.

cond-mat.str-el↗

arXiv2024

Strong Local Bosonic Fluctuation: The Key to Understanding Strongly Correlated Metals

In this paper, we present a theoretical framework for understanding the Extremely Correlated Fermi Liquid (ECFL) phenomenon within the $U=\infty$ Hubbard model. Our approach involves deriving equations of motion for the single-particle Green's function $G$ and its associated self-energy $Σ$, which involves the product of the bosonic correlation function comprising both density ($D_N$) and spin ($D_S$) correlations with $G$. By solving these equations self-consistently, we explore the behavior of $G$, $D_N$, and $D_S$ as functions of frequency, temperature, and hole concentration. Our results reveal distinct coherent and incoherent Fermi liquid regimes characterized by the presence or absence of quasiparticle excitations. Additionally, we analyze the intrinsic dc resistivity $ρ(T)$, observing a crossover from $T^2$ to linear behavior with increasing temperature. Our findings delineate Fermi liquid, quantum incoherent, and `classical' regimes in strongly correlated systems, emphasizing the importance of quantum diffusive local charge and spin fluctuations.

cond-mat.str-el↗

arXiv2022

Superfluid Density in Conventional Superconductors: From Clean to Strongly Disordered

The highly convergent form of superfluid density in disordered conventional superconductors available in the literature and independently obtained by us following the approach of an earlier paper [Phys. Rev. B $\bm{102}$, 024514 (2020)] has been reformulated to separate out the generally used so-called `dirty-limit' term and an additional term. We use this new expression for making an extensive comparison with previously published experimental data and show that the former, generally used, term is {\em not} sufficient for analyzing these results. We point out that consequently, there is a large regime (disordered superconductors with moderate to no disorder) where theoretical predictions need to be confronted with experiment.

cond-mat.supr-con↗

arXiv2020

Microscopic free energy functional of superconductive amplitude and phase: Superfluid density in disordered superconductors

Recent experiments on disordered superconductors find that the superfluid density $n_s(T)$ decreases dramatically and characteristically with disorder, differently from what is expected for a mean order parameter field or BCS, amplitude only, picture for the superconductor. We describe here a new microscopic free energy functional which explicitly describes its dependence on both the amplitude and phase of superconducting order, in a gauge invariant manner. We use this here in an approximation of noninteracting phase fluctuations (Gaussian or harmonic approximation) to obtain $n_s(T)$ in the presence of (static) disorder. We compare our results successfully with experiment.

cond-mat.supr-con↗

arXiv2019

Marginally Self-Averaging One-Dimensional Localization in Bilayer Graphene

The combination of field tunable bandgap, topological edge states, and valleys in the band structure, makes insulating bilayer graphene a unique localized system, where the scaling laws of dimensionless conductance g remain largely unexplored. Here we show that the relative fluctuations in ln g with the varying chemical potential, in strongly insulating bilayer graphene (BLG) decay nearly logarithmically for channel length up to L/$ξ$ ${\approx}$ 20, where $ξ$ is the localization length. This 'marginal' self averaging, and the corresponding dependence of on L, suggest that transport in strongly gapped BLG occurs along strictly one-dimensional channels, where $ξ$ ${\approx}$ 0.5${\pm}$0.1 $μ$m was found to be much longer than that expected from the bulk bandgap. Our experiment reveals a nontrivial localization mechanism in gapped BLG, governed by transport along robust edge modes.

cond-mat.mes-hall↗

arXiv2017

The correlation between the Nernst effect and fluctuation diamagnetism in strongly fluctuating superconductors

We study the Nernst effect in fluctuating superconductors by calculating the transport coefficient $α_{xy}$ in a phenomenological model where relative importance of phase and amplitude fluctuations of the order parameter is tuned continuously to smoothly evolve from an effective XY model to more conventional Ginzburg-Landau description. To connect with a concrete experimental realization we choose the model parameters appropriate for cuprate superconductors and calculate $α_{xy}$ and the magnetization ${\bf M}$ over the entire range of experimentally accessible values of field, temperature and doping. We argue that $α_{xy}$ and ${\bf M}$ are both determined by the equilibrium properties of the superconducting fluctuations (and not their dynamics) despite the former being a transport quantity. Thus, the experimentally observed correlation between the Nernst signal and the magnetization arises primarily from the correlation between $α_{xy}$ and ${\bf M}$. Further, there exists a dimensionless ratio ${\bf M}/(T α_{xy})$ that quantifies this correlation. We calculate, for the first time, this ratio over the entire phase diagram of the cuprates and find it agrees with previous results obtained in specific parts of the phase diagram. We conclude that that there appears to be no sharp distinction between the regimes dominated by phase fluctuations and Gaussian fluctuations for this ratio in contrast to $α_{xy}$ and ${\bf M}$ individually. The utility of this ratio is that it can be used to determine the extent to which superconducting fluctuations contribute to the Nernst effect in different parts of the phase diagram given the measured values of magnetization.

cond-mat.supr-con↗

arXiv2016

Spin liquid like Raman signatures in hyperkagome iridate Na$_4$Ir$_3$O$_8$

Combining Raman scattering measurements with mean field calculations of the Raman response we show that Kitaev-like magnetic exchange is dominant in the hyperkagome iridate Na$_4$Ir$_3$O$_8$. In the measurements we observe a broad Raman band at $\sim$~3500 cm$^{-1}$ with a band-width $\sim 1700$~cm$^{-1}$. Calculations of the Raman response of the Kitaev-Heisenberg model on the hyperkagome lattice shows that the experimental observations are consistent with calculated Raman response where Kitaev exchange interaction ($J_K$) is much larger than the Heisenberg term J$_1$ ($J_1/J_K \sim 0.1$). A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter.

cond-mat.str-el↗

arXiv2016

Nature of single-particle states in disordered graphene

We analyze the nature of the single particle states, away from the Dirac point, in the presence of long-range charge impurities in a tight-binding model for electrons on a two-dimensional honeycomb lattice which is of direct relevance for graphene. For a disorder potential $V(\vec{r})=V_0\exp(-|\vec{r}-\vec{r}_{imp}|^2/ξ^2)$, we demonstrate that not only the Dirac state but all the single particle states remain extended for weak enough disorder. Based on our numerical calculations of inverse participation ratio, dc conductivity, diffusion coefficient and the localization length from time evolution dynamics of the wave packet, we show that the threshold $V_{th}$ required to localize a single particle state of energy $E(\vec{k})$ is minimum for the states near the band edge and is maximum for states near the band center, implying a mobility edge starting from the band edge for weak disorder and moving towards the band center as the disorder strength increases. This can be explained in terms of the low energy Hamiltonian at any point $\vec{k}$ which has the same nature as that at the Dirac point. From the nature of the eigenfunctions it follows that a weak long range impurity will cause weak anti localization effects, which can be suppressed, giving localization if the strength of impurities is sufficiently large to cause inter-valley scattering. The inter valley spacing $2|\vec{k}|$ increases as one moves in from the band edge towards the band center, which is reflected in the behavior of $V_{th}$ and the mobility edge.

cond-mat.mes-hall↗

arXiv2016

Raman Signatures of Strong Kitaev Exchange Correlations in (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ : Experiments and Theory

Inelastic light scattering studies on single crystals of (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ ($x = 0, 0.05$ and $0.15$) show a polarization independent broad band at $\sim $~2750 cm$^{-1}$ with a large band-width $\sim 1800$~cm$^{-1}$. For Na$_2$IrO$_3$ the broad band is seen for temperatures $ \leq 200$~K and persists inside the magnetically ordered state. For Li doped samples, the intensity of this mode increases, shifts to lower wave-numbers and persists to higher temperatures. Such a mode has recently been predicted (Knolle et.al.) as a signature of the Kitaev spin liquid. We assign the observation of the broad band to be a signature of strong Kitaev-exchange correlations. The fact that the broad band persists even inside the magnetically ordered state suggests that dynamically fluctuating moments survive even below $T_{N}$. This is further supported by our mean field calculations. The Raman response calculated in mean field theory shows that the broad band predicted for the spin liquid state survives in the magnetically ordered state near the zigzag-spin liquid phase boundary. A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter to be $J_K\approx 57$~meV.

cond-mat.str-el↗

arXiv2015

Doping dependence of fluctuation diamagnetism in High Tc superconductors

Using a recently proposed Ginzburg-Landau-like lattice free energy functional due to Banerjee et al. Phys. Rev. B 83, 024510 (2011) we calculate the fluctuation diamagnetism of high-Tc superconductors as a function of doping, magnetic field and temperature. We analyse the pairing fluctuations above the superconducting transition temperature in the cuprates, ranging from the strong phase fluctuation dominated underdoped limit to the more conventional amplitude fluctuation dominated overdoped regime. We show that a model where the pairing scale increases and the superfluid density decreases with underdoping produces features of the observed magnetization in the pseudogap region, in good qualitative and reasonable quantitative agreement with the experimental data. In particular, we explicitly show that even when the pseudogap has a pairing origin the magnetization actually tracks the superconducting dome instead of the pseudogap temperature, as seen in experiment. We discuss the doping dependence of the `onset' temperature for fluctuation diamagnetism and comment on the role of vortex core-energy in our model.

cond-mat.supr-con↗

arXiv2012

Ginzburg-Landau Like Theory for High Temperature Superconductivity in the Cuprates: Emergent d-wave Order

High temperature superconductivity in the cuprates remains one of the most widely investigated, constantly surprising, and poorly understood phenomena in physics. Here, we describe briefly a new phenomenological theory inspired by the celebrated description of superconductivity due to Ginzburg and Landau and believed to describe its essence. This posits a free energy functional for the superconductor in terms of a complex order parameter characterizing it. We propose, for superconducting cuprates, a similar functional of the complex, in plane, nearest neighbor spin singlet bond (or Cooper) pair amplitude psi_ij. A crucial part of it is a (short range) positive interaction between nearest neighbor bond pairs, of strength J'. Such an interaction leads to nonzero long wavelength phase stiffness or superconductive long range order, with the observed d-wave symmetry, below a temperature T_c\simzJ' where z is the number of nearest neighbours; it is thus an emergent, collective consequence. Using the functional, we calculate a large range of properties, e.g. the pseudogap transition temperature T* as a function of hole doping x, the transition curve T_c(x), the superfluid stiffness rho_s(x,T), the specific heat (without and with a magnetic field) due to the fluctuating pair degrees of freedom, and the zero temperature vortex structure. We find remarkable agreement with experiment. We also calculate the self energy of electrons hopping on the square cuprate lattice and coupled to electrons of nearly opposite momenta via inevitable long wavelength Cooper pair fluctuations formed of these electrons. The ensuing results for electron spectral density are successfully compared with recent ARPES experiments, and comprehensively explain strange features such as temperature dependent Fermi arcs above T_c and the 'bending' of the superconducting gap below T_c .

cond-mat.supr-con↗

arXiv2011

Novel effects of localization due to `intrinsic disorder' in the `two-fluid' model for manganites

We discuss the effects of a novel polaronic disorder in the recently proposed two-fluid model for manganites. Using effective field theory as well as direct numerical simulations, we show that this disorder can have dramatic effects in terms of the transition from ferromagnetic insulator to ferromagnetic metal upon hole-doping, including an Anderson localized regime where variable range hopping may be observed.

cond-mat.str-el↗

arXiv2010

Phenomenological Ginzburg-Landau-like theory for superconductivity in the cuprates

We propose a phenomenological Ginzburg-Landau-like theory of cuprate superconductivity. The free energy is expressed as a functional F of the spin-singlet pair amplitude psi_ij=psi_m=Delta_m exp(i phi_m); i and j are nearest-neighbor sites of the Cu lattice in which the superconductivity is believed to primarily reside and m labels the site at the center of the bond between i and j. The system is modeled as a weakly coupled stack of such planes. We hypothesize a simple form, F[Delta,phi]=sum_m (A Delta_m^2+ B Delta_m^4/2)+C sum_ Delta_m Delta_n cos(phi_m-phi_n), for the functional. The coefficients A, B and C are determined from comparison with experiments. We work out a number of consequences of the proposed functional for specific choices of A, B and C as functions of hole density x and temperature T. There can be a rapid crossover of from small to large values as A changes sign on lowering T and the crossover temperatures is identified with the observed pseudogap temperature. The superconducting phase-coherence transition occurs at a different temperature T_c, and describes superconductivity with d-wave symmetry for C>0. We calculate T_c(x) which has the observed parabolic shape, being strongly influenced by the coupling between Delta_m and phi_m present in F. The superfluid density, the local gap magnitude, the specific heat (with and without a magnetic field) and vortex properties are obtained using F. We compare our results successfully with experiments. We also obtain the electron spectral density as influenced by the coupling between the electrons and the pair correlation function calculated from F. Features such as temperature dependent Fermi arcs, antinodal pseudogap filling temperature, pseudogapped density of states in different momentum regions of the Fermi surface and `bending' of the energy gap versus momentum on the Fermi surface emerge from the theory.

cond-mat.supr-con↗

arXiv2009

Stochastic kinetics of ribosomes: single motor properties and collective behavior

Synthesis of protein molecules in a cell are carried out by ribosomes. A ribosome can be regarded as a molecular motor which utilizes the input chemical energy to move on a messenger RNA (mRNA) track that also serves as a template for the polymerization of the corresponding protein. The forward movement, however, is characterized by an alternating sequence of translocation and pause. Using a quantitative model, which captures the mechanochemical cycle of an individual ribosome, we derive an {\it exact} analytical expression for the distribution of its dwell times at the successive positions on the mRNA track. Inverse of the average dwell time satisfies a ``Michaelis-Menten-like'' equation and is consistent with the general formula for the average velocity of a molecular motor with an unbranched mechano-chemical cycle. Extending this formula appropriately, we also derive the exact force-velocity relation for a ribosome. Often many ribosomes simultaneously move on the same mRNA track, while each synthesizes a copy of the same protein. We extend the model of a single ribosome by incorporating steric exclusion of different individuals on the same track. We draw the phase diagram of this model of ribosome traffic in 3-dimensional spaces spanned by experimentally controllable parameters. We suggest new experimental tests of our theoretical predictions.

physics.bio-ph↗

arXiv2009

Long Range Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Correlated Oxides

Electronic, magnetic or structural inhomogeneities ranging in size from nanoscopic to mesoscopic scales seem endemic, and are possibly generic, to colossal magnetoresistance manganites and other transition metal oxides. We show here that an extension, to include long range Coulomb interactions, of a quantum two-fluid $\ell-b$ model proposed recently for manganites [Phys. Rev. Lett., {\bf 92}, 157203 (2004)] leads to an excellent description of such inhomogeneities. In the $\ell-b$ model two very different kinds of electronic states, one localized and polaronic ($\ell$), and the other extended or broad band ($b$) co-exist. For model parameters appropriate to manganites, and even within a simple dynamical mean-filed theory (DMFT) framework, it describes many of the unusual phenomena seen in manganites, including colossal magnetoresistance (CMR), qualitatively and quantitatively. However, in the absence of long ranged Coulomb interaction, a system described by such a model would actually phase separate, into macroscopic regions of $l$ and $b$ electrons respectively. As we show in this paper, in the presence of Coulomb interactions, the {\em macroscopic} phase separation gets suppressed, and instead nanometer scale regions of polarons interspersed with band electron puddles appear, constituting a new kind of quantum Coulomb glass. Our work points to an interplay of strong correlations, long range Coulomb interaction and dopant ion disorder as the origin of nanoscale inhomogeneities, rather than disorder frustrated phase competition as is generally believed. Based on this, we argue that the observed micrometer(meso)-scale inhomogeneities owe their existence to extrinsic causes, eg. strain due to cracks and defects. We suggest possible experiments to validate our speculation.

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