arXiv subjects

Chandan Setty

Publications and source records attributed to Chandan Setty.

At least 55 records · Page 3Linked to original sources

arXiv2020

Extracting correlation effects from Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS): Synergistic origin of the dispersion kink in Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+x}$

We employ Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS) on Bi2.1Sr1.9CaCu2O8+x to resolve the issue of the kink feature in the electron dispersion widely observed in the cuprates. To this end, we utilize the GW approximation to relate the density response function measured in in M-EELS to the self-energy, isolating contributions from phonons, electrons, and the momentum dependence of the effective interaction to the decay rates. The phononic contributions, present in the M-EELS spectra due to electron-phonon coupling, lead to kink features in the corresponding single-particle spectra at energies between 40 meV and 80 meV, independent of the doping level. We find that a repulsive interaction constant in momentum space is able to yield the kink attributed to phonons in ARPES. Hence, our analysis of the M-EELS spectra points to local repulsive interactions as a factor that enhances the spectroscopic signatures of electron-phonon coupling in cuprates. We conclude that the strength of the kink feature in cuprates is determined by the combined action of electron-phonon coupling and electron-electron interactions.

cond-mat.supr-con↗

arXiv2020

Bogoliubov Fermi Surfaces in Spin-1/2 Systems: Model Hamiltonians and Experimental Consequences

Bogoliubov Fermi surfaces (BFSs) are topologically protected regions of zero energy excitations in a superconductor whose dimension equals that of the underlying normal state Fermi surface. Examples of Hamiltonians exhibiting this "ultranodal" phase are known to preserve charge-conjugation ($C$) and parity ($P$) but break time-reversal ($T$). In this work, we provide examples of model Hamiltonians that do not necessarily preserve this symmetry pattern but have well-defined sign-changing Pfaffians yielding BFSs. While their topological character has not been recognized previously, some of the models we present have been extensively studied in prior literature. We further examine thermodynamic and electronic properties arising from the ultranodal state. In particular, we study the effect of a weak Zeeman field close to the topological transition and propose distinguishing features of BFSs using residual specific heat and tunneling conductance. Our calculation of the superfluid density in a toy multi-band model indicates a window of interband pairing strength where BFSs are stable with a positive superfluid density. We also present additional signatures of BFSs in spin-polarized spectral weight and total magnetization measurements.

cond-mat.supr-con↗

arXiv2020

Effective Theory of Superconductivity in Strongly-Coupled Amorphous Materials

A theory of phonon-mediated superconductivity in strong-coupling amorphous materials is developed based on an effective description of structural disorder and its effect on the vibrational spectrum. The theory accounts for the diffusive-like transport of vibrational excitations due to disorder-induced scattering within the Eliashberg theory of strong-coupling superconductivity. The theory provides a good analytical description of the Eliashberg function $α^{2}F(ω)$ in comparison with experiments, and allows one to disentangle the effects of transverse and longitudinal excitations on the Eliashberg function. In particular, it shows that the transverse excitations play a crucial role in driving an increase or excess in the Eliashberg function at low energy, which is related to the boson peak phenomenon in vibrational spectra of glasses. This low-energy excess, on one hand drives an enhancement of the electron-phonon coupling but at the same time reduces the characteristic energy scale $ω_{log}$ in the Allen-Dynes formula. As a consequence, the non-monotonicity of $T_{c}$ as a function of alloying (disorder) in $\text{Pb}$-based systems can be rationalized. The case of $\text{Al}$-based systems, where disorder increases $T_{c}$ from the start, is also analyzed. General material-design principles for enhancing $T_{c}$ in amorphous superconductors are presented.

cond-mat.supr-con↗

arXiv2020

Pairing instability on a Luttinger surface: A non-Fermi liquid to superconductor transition and its Sachdev-Ye-Kitaev dual

Superconductivity results from an instability of the Fermi surface -- contour of \textit{poles} of the single particle propagator -- to an infinitesimally small attraction between electrons. Here, we instead discuss the analogous problem on a model \textit{Luttinger} surface, or contour of \textit{zeros} of the Green function. At zero temperature ($β\rightarrow \infty$) and a critical interaction strength ($u_{c\infty}$) characterized by the residue of self-energy pole, we find that the pair susceptibility diverges leading to a superconducting instability. We evaluate the pair fluctuation partition function and find that the spectral density in the normal state has an interaction-driven, power-law $\frac{1}{\sqrtω}$ type, van-Hove singularity (vHS) indicating non-Fermi liquid (NFL) physics. Crucially, in the strong coupling limit ($βu \gg 1$), the leading order fluctuation free energy terms in the normal state of this NFL-SC transition resemble the equivalent $\left(O(1)\right)$ terms of the Sachdev-Ye-Kitaev (SYK) model. This free energy contribution takes a simple form $-βF = βu_{c\infty} - γ~\text{ln}\left(βu_{c \infty}\right)$ where $γ$ is a constant equal to $\frac{1}{2}$. Weak impurity scattering ($τ\gg β^{-1}$) leaves the low-energy spectral density unaffected, but leads to an interaction-driven enhancement of superconductivity. Our results shed light on the role played by order-parameter fluctuations in providing the key missing link between Mott physics and strongly coupled toy-models exhibiting gravity duals.

cond-mat.str-el↗

arXiv2020

Topological Ultranodal pair states in iron-based superconductors

Bogoliubov Fermi surfaces are contours of zero-energy excitations that are protected in the superconducting state. Here we show that multiband superconductors with dominant spin singlet, intraband pairing of spin-1/2 electrons can undergo a transition to a state with Bogoliubov Fermi surfaces if spin-orbit coupling, interband pairing and time reversal symmetry breaking are also present. These latter effects may be small, but drive the transition to the topological state for appropriate nodal structure of the intra-band pair. Such a state should display nonzero zero-bias density of states and corresponding residual Sommerfeld coefficient as for a disordered nodal superconductor, but occurring even in the pure case. We present a model appropriate for iron-based superconductors where the topological transition associated with creation of a Bogoliubov Fermi surface can be studied. The model gives results that strongly resemble experiments on FeSe$_{1-x}$S$_x$ across the nematic transition, where this ultranodal behavior may already have been observed.

cond-mat.supr-con↗

arXiv2019

Detecting isotropic density and nematic fluctuations using ultrafast coherent phonon spectroscopy

We propose a theoretical framework for the detection of order parameter fluctuations in three dimensions using ultrafast coherent phonon spectroscopy. We focus our attention on long wavelength charge density fluctuations (plasmons), and charged nematic fluctuations where the direction of the propagation vector is fixed perpendicular to the plane of anisotropy. By treating phonons and light classically and decoupling interactions to integrate out the fermionic degrees of freedom, we arrive at an effective theory of order parameter fluctuations about the spatially uniform saddle-point solution. We find that, due to the $(k_x^2-k_y^2) (B_{1g})$ symmetry of the form factor appearing in the vertex, nematic fluctuations couple to light only at fourth order, unlike isotropic density fluctuations which couple at second order. Hence, to lowest order, the interaction between electrons and the electromagnetic field contributes a driving force for plasmon oscillations while it provides a frequency shift for nematic fluctuations. From the resulting coupled harmonic oscillator equations of motion, we argue that ultrafast coherent phonon spectroscopy could be a useful tool to extract and analyze various electronic properties of interest such as the frequency of the collective mode and the coupling between electrons and phonons. Specific experiments are proposed on the normal state of FeSe to observe the frequency shift predicted here resulting directly from orbital ordering (nematic) fluctuations. Our paper presents a new mechanism for generating coherent phonons from long-range interactions (coherent long-range interaction induced phonons) that does not require the existence of multiple bands to act as intermediary states for quasiparticles.

cond-mat.str-el↗

arXiv2019

Glass-induced enhancement of superconducting $T_c$: Pairing via dissipative mediators

With substantial evidence of glassy behavior in the phase diagram of high $T_c$ superconductors and its co-existence with superconductivity, we attempt to answer the question: what are the properties of a superconducting state where the force driving cooper pairing becomes dissipative? We find that when the bosonic mediator is local, dissipation acts to reduce the superconducting critical temperature ($T_c$). On the other hand, contrary to naïve expectations, $T_c$ behaves non-monotonically with dissipation for a non-local mediator -- weakly dissipative bosons at different energy scales act coherently to give rise to an increase in $T_c$ and eventually destroy superconductivity when the dissipation exceeds a critical value. The critical value occurs when dissipative effects become comparable to the energy scale associated with the spatial stiffness of the mediator, at which point, $T_c$ acquires a maximum. We outline consequences of our results to recent proton irradiation experiments (M. Leroux et al.,~\cite{Welp2018}) on the cuprate superconductor La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) which observe a disorder induced increase in $T_c$ even when the transition temperature of the proximate charge density wave (CDW) is unaffected by the presence of irradiation. Our mechanism is a novel way to raise $T_c$ that does not require a `tug-of-war' -type scenario between two competing phases.

cond-mat.supr-con↗

arXiv2018

Scale-invariance as the cause of the superconducting dome in the cuprates

Recent photoemission spectroscopy measurements (T. J. Reber et al., arXiv:1509.01611) of cuprate superconductors have inferred that the self-energy exhibits critical scaling over an extended doping regime, thereby calling into question the conventional wisdom that critical scaling exists only at isolated points. In particular, this new state of matter, dubbed a power-law liquid, has a self-energy whose imaginary part scales as $Σ^{\prime\prime}\sim(ω^{2}+π^{2}T^{2})^α$, with $α=1$ in the overdoped Fermi-liquid state and $α\leq0.5$ in the optimal to underdoped regime. Previously, we showed that this self-energy can arise from interactions between electrons and unparticles, a scale-invariant sector that naturally emerges from strong correlations. Here, taking the self-energy as a given, we first reconstruct the real part of the self-energy. We find that the resultant quasiparticle weight vanishes for any doping level less than optimal, implying an absence of particle-like excitations in the underdoped regime. Consequently, the Fermi velocity vanishes and the effective mass diverges for $α\leq\frac{1}{2}$, in agreement with earlier experimental observations. We then use the self-energy to reconstruct the spectral function and compute the superconducting $T_c$ within the BCS formalism. We find that the $T_c$ has a dome-like structure, implying that broad scale invariance manifested in the form of a power-law liquid is the likely cause of the superconducting dome in the cuprates.

cond-mat.str-el↗

arXiv2018

Low energy inelastic response in the superconducting phases of $PrOs_4Sb_{12}$

Recent AC susceptibility and polar Kerr effect measurements in the skutterudite superconductor PrOs4Sb12 (POS) uncovered the nature of the superconducting double transition from a high temperature, high field, time reversal symmetric phase (or the A phase) to a low temperature, low field, time reversal symmetry broken phase (or the B phase). Starting from a microscopic model, we derive a Ginzburg-Landau expansion relevant to POS that describes this entrance into the time reversal symmetry broken phase along the temperature axis. We also provide a study of the low energy inelastic (Raman) response in both the A and B phases of POS, and seek additional signatures which could help reveal the exact form of the gap functions previously proposed in these phases. By appropriately manipulating the incoming and scattered light geometries, along with additional subtraction procedures and suitable assumptions, we show that one can access the various irreducible representations contained in the point group describing POS. We demonstrate how to use this technique on example order parameters proposed in POS. Depending on whether there exist nodes along the c-axis, we find additional low energy spectral weight within the superconducting gap in the Eg geometry, a feature that could pin point the location of nodes on the Fermi surface.

cond-mat.supr-con↗

arXiv2018

Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization not Mott Insulation

We devise a model to explain why twisted bi-layer graphene (TBLG) exhibits insulating behavior when $ν=2,3$ charges occupy a unit moiré cell, a feature attributed to Mottness, but not for $ν=1$, clearly inconsistent with Mott insulation. We compute $r_s=E_U/E_K$, where $E_U$ and $E_K$ are the potential and kinetic energies, respectively, and show that (i) the Mott criterion lies at a density $10^4$ higher than in the experiments and (ii) a transition to a series of Wigner crystalline states exists as a function of $ν$. We find, for $ν=1$, $r_s$ fails to cross the threshold ($r_s = 37$) for the triangular lattice and metallic transport ensues. However, for $ν=2$ and $ν=3$, the thresholds, $r_s=22$, and $r_s=17$, respectively are satisfied for a transition to Wigner crystals (WCs) with a honeycomb ($ν=2$) and kagome ($ν=3$) structure. We believe, such crystalline states form the correct starting point for analyzing superconductivity.

cond-mat.str-el↗

arXiv2018

Conjecture on the Butterfly Velocity across a Quantum Phase Transition

We study an anisotropic holographic bottom-up model displaying a quantum phase transition (QPT) between a topologically trivial insulator and a non-trivial Weyl semimetal phase. We analyze the properties of quantum chaos in the quantum critical region. We do not find any universal property of the Butterfly velocity across the QPT. In particular it turns out to be either maximized or minimized at the quantum critical point depending on the direction of propagation. We observe that instead of the butterfly velocity, it is the dimensionless information screening length that is always maximized at a quantum critical point. We argue that the null-energy condition (NEC) is the underlying reason for the upper bound, which now is just a simple combination of the number of spatial dimensions and the anisotropic scaling parameter.

hep-th↗

arXiv2018

Inequivalence of the zero-momentum Limits of Transverse and Longitudinal Dielectric Response in the Cuprates

We address the question of the mismatch between the zero momentum limits of the transverse and longitudinal dielectric functions for a fixed direction of the driving field observed in the cuprates. This question translates to whether or not the order in which the longitudinal and transverse momentum transfers are taken to zero commute. While the two limits commute for both isotropic and anisotropic Drude metals, we argue that a scaleless vertex interaction that depends solely on the angle between scattered electron momenta is sufficient to achieve non-commutativity of the two limits even for a system that is inherently isotropic. We demonstrate this claim for a simple case of the Drude conductivity modified by electron-boson interactions through appropriate vertex corrections, and outline possible consequences of our result to optical and electron energy loss spectroscopy (EELS) measurements close to zero momentum transfer

cond-mat.str-el↗

arXiv2018

Log-rise of the Resistivity in the Holographic Kondo Model

We study a single-channel Kondo effect using a recently developed holographic large-$N$ technique. In order to obtain resistivity of this model, we introduce a probe field. The gravity dual of a localized fermionic impurity in 1+1-dimensional host matter is constructed by embedding a localized 2-dimensional Anti-de Sitter (\ads{2})-brane in the bulk of \ads{3}. This helps us construct an impurity charge density which acts as a source to the bulk equation of motion of the probe gauge field. The functional form of the charge density is obtained independently by solving the equations of motion for the fields confined to the \ads{2}-brane. The asymptotic solution of the probe field is dictated by the impurity charge density, which in turn, affects the current-current correlation functions, and hence the resistivity. Our choice of parameters tunes the near-boundary impurity current to be marginal, resulting in a $\log T$ behavior in the UV resistivity, as is expected for the Kondo problem. The resistivity at the IR fixed point turns out to be zero, signaling a complete screening of the impurity.

hep-th↗

arXiv2017

Absence of a Charge Diffusion Pole at Finite Energies in an Exactly Solvable Interacting Flat Band Model in d-dimensions

Motivated by recent bounds for charge diffusion in critical matter, we investigate the question: What sets the scale for charge diffusion in a scale-invariant system? To make our statements precise, we analyze the diffusion pole in an exactly solvable model for a Mott transition in the presence of a long-range interaction term. To achieve scale invariance, we limit our discussion to the flat-band regime. We find in this limit that the diffusion pole which would normally obtain at finite energy is pushed to zero energy resulting in a vanishing of the diffusion constant. This occurs even in the presence of interactions in certain limits, indicating the robustness of this result to the inclusion of a scale in the problem. Consequently, scale-invariance precludes any reasonable definition of the diffusion constant. Nonetheless, we do find that a scale can be defined, all be it, irrelevant to diffusion, which is the product of the squared band velocity and the density of states.

cond-mat.str-el↗

arXiv2017

Power-law liquid in cuprate superconductors from fermionic unparticles

Recent photoemission spectroscopy measurements [arXiv:1509.01611] on cuprate superconductors have inferred that over a wide range of doping, the imaginary part of the electron self-energy scales as $Σ^{\prime\prime}\sim(ω^2+π^2T^2)^a$ with $a=1$ in the overdoped Fermi-liquid state and $a<0.5$ in the optimal to underdoped regime. We show that this non-Fermi-liquid scaling behavior can naturally be explained by the presence of a scale-invariant state of matter known as unparticles. We evaluate analytically the electron self-energy due to interactions with fermionic unparticles. We find that, in agreement with experiments, the imaginary part of the self-energy scales with respect to temperature and energy as $Σ^{\prime\prime}\sim T^{2+2α}$ and $ω^{2+2α}$, where $α$ is the anomalous dimension of the unparticle propagator. In addition, the calculated occupancy and susceptibility of fermionic unparticles, unlike those of normal fermions, have significant spectral weights even at high energies. This unconventional behavior is attributed to the branch cut in the unparticle propagator which broadens the unparticle spectral function over a wide energy range and non-trivially alters the scattering phase space by enhancing (suppressing) the intrinsic susceptibility at low energies for negative (positive) $α$. Our work presents new evidence suggesting that unparticles might be important low-energy degrees of freedom in strongly coupled systems such as the cuprate superconductors.

cond-mat.str-el↗

arXiv2017

Quasiparticle interference and resonant states in normal and superconducting line nodal semimetals

We study impurity scattering in the normal and d-wave superconducting states of line nodal semimetals and show that, due to additional scattering phase space available for impurities on the surface, the quasiparticle interference pattern acquires an extended character instead of a discrete collection of delta function peaks. Moreover, using the T-matrix formalism, we demonstrate that the conventional behavior of a scalar impurity in a d-wave superconductor breaks down on the surface of a line nodal semimetal in the quasi flat band limit.

cond-mat.supr-con↗

arXiv2016

Absence of nematic ordering transition in a diamond lattice: Application to $FeSc_2S_4$

Recent neutron scattering observations by Plumb et al. [1] reveal that the ground state of $FeSc_2S_4$ is magnetic with two distinct Fe environments, instead of a quantum spin liquid as had been previously thought. Starting with the relevant O(N)-symmetric vector model of $FeSc_2S_4$, we study how the discrete ($Z_2$) and continuous rotational symmetries are successively broken, yielding nematic and ordered phases. At high temperatures, we find that the nematic order parameter falls as $T^{-γ}$ ($γ>0$), and therefore, $FeSc_2S_4$ lacks any distinct nematic ordering temperature. This feature indicates that the three-dimensional diamond lattice of $FeSc_2S_4$ is highly susceptible to the breaking of Ising symmetries, and explains the two distinct Fe environments that is present even at high temperatures, as seen by Mössbauer and far infrared optical spectroscopy.

cond-mat.str-el↗

arXiv2016

Realizing infrared power-law liquids in the cuprates from unparticle interactions

Recent photoemission experiments \cite{dessau} reveal that the excitations along the nodal region in the strange metal of the cuprates, rather than corresponding to poles in the single-particle Green function, exhibit power-law scaling as a function of frequency and temperature. Because such power-law scaling is indicative of a scale-invariant sector, as a first step, we perturbatively evaluate the electron self-energy due to interactions with scale-invariant unparticles. We focus on a $G_0W$ type diagram with an interaction $W$ mediated by a bosonic scalar unparticle. We find that, in the high-temperature limit, the imaginary part of the self-energy $\mathrm{Im}Σ$ is linear in temperature. In the low-temperature limit, $\mathrm{Im}Σ$ exhibits the same power law in both temperature and frequency, with an exponent that depends on the scaling dimension of an unparticle operator. Such behavior is qualitatively consistent with the experimental observations. We then expand the unparticle propagator into coherent and incoherent contributions, and study how the incoherent part violates the density of states (DOS) and density-density correlation function sum rules (f-sum rule). Such violations can, in principle, be observed experimentally. Our work indicates that the physical mechanism for the origin of the power-law scaling is the incoherent background, which is generated from the Mott-scale physics.

cond-mat.str-el↗