arXiv subjects

K. Levin

Publications and source records attributed to K. Levin.

At least 73 records · Page 4Linked to original sources

arXiv2011

Transport Anomalies Associated with the Pseudogap From a Preformed Pair Perspective

Transport studies seem to be one of the strongest lines of support for a preformed pair approach to the pseudogap. In this paper we provide a fresh, physically transparent look at two important quantities: the diamagnetic susceptibility and conductivity. We use a three dimensional preformed pair framework which has had some success in the cold Fermi gases and in the process we reconcile recently observed inconsistencies. Specifically, while the preformed pairs in our theory give a large contribution to the diamagnetic susceptibility, the imaginary part of the conductivity is suppressed to zero much closer to $T_c$, as is observed experimentally.

cond-mat.supr-con↗

arXiv2011

Nucleation of Spontaneous Vortices in Trapped Fermi Gases Undergoing a BCS-BEC Crossover

We study the spontaneous formation of vortices during the superfluid condensation in a trapped fermionic gas subjected to a rapid thermal quench via evaporative cooling. Our work is based on the numerical solution of the time dependent crossover Ginzburg-Landau equation coupled to the heat diffusion equation. We quantify the evolution of condensate density and vortex length as a function of a crossover phase parameter from BCS to BEC. The more interesting phenomena occur somewhat nearer to the BEC regime and should be experimentally observable; during the propagation of the cold front, the increase in condensate density leads to the formation of supercurrents towards the center of the condensate as well as possible condensate volume oscillations.

cond-mat.quant-gas↗

arXiv2011

Perfect Fluids and Bad Metals: Transport Analogies Between Ultracold Fermi Gases and High $T_c$ Superconductors

In this paper, we examine in a unified fashion dissipative transport in strongly correlated systems. We thereby demonstrate the connection between "bad metals" (such as the high temperature superconductors) and "perfect fluids" (such as the ultracold Fermi gases, near unitarity). One motivation of this work is to communicate to the high energy physics community some of the central unsolved problems in high $T_c$ superconductors. Because of interest in the nearly perfect fluidity of the cold gases and because of new tools such as the AdS/CFT correspondence, this better communication may lead to important progress in a variety of different fields. A second motivation is to draw attention to the great power of transport measurements which more directly reflect the excitation spectrum than, say, thermodynamics and thus strongly constrain microscopic theories of correlated fermionic superfluids. Our calculations show that bad metal and perfect fluid behavior is associated with the presence of a normal state excitation gap which suppresses the effective number of carriers leading to anomalously low conductivity and viscosity above the transition temperature $T_c$. Below $T_c$ we demonstrate that the condensate collective modes ("phonons") do not couple to transverse probes such as the shear viscosity. As a result, our calculated shear viscosity at low $T$ becomes arbitrarily small as observed in experiments. In both homogeneous and trap calculations we do not find the upturn in $η$ or $η/s$ (where $s$ is the entropy density) found in most theories. In the process of these studies we demonstrate compatibility with the transverse sum rule and find reasonable agreement with both viscosity and cuprate conductivity experiments.

cond-mat.supr-con↗

arXiv2010

Microscopic Approach to Shear Viscosities in Superfluid Gases: From BCS to BEC

We compute the shear viscosity, $η$, at general temperatures $T$, in a BCS-BEC crossover scheme which is demonstrably consistent with conservation laws. The study of $η$ is important because it constrains microscopic theories by revealing the excitation spectra. The onset of a normal state pairing gap and the contribution from pair degrees of freedom imply that $η$ at low $T$ becomes small, rather than exhibiting the upturn predicted by most others. Using the local density approximation, we find quite reasonable agreement with just-published experiments.

cond-mat.quant-gas↗

arXiv2010

Fermi liquid theory of ultra-cold trapped Fermi gases: Implications for Pseudogap Physics and Other Strongly Correlated Phases

We show how Fermi liquid theory can be applied to ultra-cold Fermi gases, thereby expanding their "simulation" capabilities to a class of problems of interest to multiple physics sub-disciplines. We introduce procedures for measuring and calculating position dependent Landau parameters. This lays the ground work for addressing important controversial issues: (i) the suggestion that thermodynamically, the normal state of a unitary gas is indistinguishable from a Fermi liquid (ii) that a fermionic system with strong repulsive contact interactions is associated with either ferromagnetism or localization; this relates as well to $^3$He and its p-wave superfluidity.

cond-mat.quant-gas↗

arXiv2010

Establishing the Presence of Coherence in Atomic Fermi Superfluids: Spin-Flip and Spin-Preserving Bragg Scattering at Finite Temperatures

We show how the difference between the finite temperature T structure factors, called S_-, associated with spin and density, can be used as a indication of superfluidity in ultracold Fermi gases. This observation can be exploited in two photon Bragg scattering experiments on gases which undergo BCS- Bose Einstein condensation crossover. Essential to our calculations is a proper incorporation of spin and particle number conservation laws which lead to compatibility at general T with two f-sum rules. Because it is applicable to general scattering lengths, a measurement of S- can be a useful, direct approach for establishing where superfluidity occurs.

physics.atom-ph↗

arXiv2009

Contrasting Nodal and Anti-Nodal Behavior in the Cuprates Via Multiple Gap Spectroscopies

Using a precursor superconductivity scenario for the cuprates we present a theory for the temperature dependent behavior of the spectral gaps associated with four distinct spectroscopies: angle resolved photoemission (ARPES), differential conductance $dI/dV$, quasi-particle interference spectroscopy, and the autocorrelated ARPES pattern. We find good agreement for a range of existing experiments and make predictions for others. Our theory, which incorporates the necessary (observed) contrast between the nodal and anti-nodal response, shows how different nodal gap shapes are associated with these alternative spectroscopies.

cond-mat.supr-con↗

arXiv2009

Comparative Study of BCS-BEC Crossover Theories above $T_c$: the Nature of the Pseudogap in Ultra-Cold Atomic Fermi Gases

This paper presents a comparison of two finite-temperature BCS-Bose Einstein condensation (BEC) crossover theories above the transition temperature: Nozieres Schmitt-Rink (NSR) theory and finite $T$-extended BCS-Leggett theory. The comparison is cast in the form of numerical studies of the behavior of the fermionic spectral function both theoretically and as constrained by (primarily) radio frequency (RF) experiments. Both theories include pair fluctuations and exhibit pseudogap effects, although the nature of this pseudogap is very different. The pseudogap in finite $T$-extended BCS-Leggett theory is found to follow a BCS-like dispersion which, in turn, is associated with a broadened BCS-like self energy, rather more similar to what is observed in high temperature superconductors (albeit, for a d-wave case). The fermionic quasi-particle dispersion is different in NSR theory and the damping is considerably larger. We argue that the two theories are appropriate in different temperature regimes with the BCS-Leggett approach more suitable nearer to condensation. There should, in effect, be little difference at higher $T$ as the pseudogap becomes weaker and where the simplifying approximations used in the BCS-Leggett approach break down. On the basis of momentum-integrated radio frequency studies of unpolarized gases, it would be difficult to distinguish which theory is the better. A full comparison for polarized gases is not possible since there is claimed to be inconsistencies in the NSR approach (not found in the BCS-Leggett scheme). Future experiments along the lines of momentum resolved experiments look to be very promising in distinguishing the two theories.

cond-mat.quant-gas↗

arXiv2009

Quasiparticle Interference in STM As a Function of Temperature: Signatures of Coherence

In this paper we explore the behavior of the quasi-particle interference pattern (QPI) of scanning tunneling microscopy as a function of temperature, $T$. After insuring a minimal consistency with photoemission, we find that the QPI pattern is profoundly sensitive to quasi-particle coherence and that it manifests two energy gap scales. The nearly dispersionless QPI pattern above $T_c$ is consistent with data on moderately underdoped cuprates. To illustrate the important two energy scale physics we present predictions of the QPI--inferred energy gaps as a function of $T$ for future experiments on moderately underdoped cuprates.

cond-mat.supr-con↗

arXiv2009

A Two Energy Gap Preformed-Pair Scenario For the Cuprates: Implications for Angle-Resolved Photoemission Spectroscopy

We show how, within a preformed pair scenario for the cuprate pseudogap, the nodal and antinodal responses in angle resolved photoemission spectroscopy necessarily have very different temperature $T$ dependences. We examine the behavior and the contrasting $T$ dependences for a range of temperatures both below and above $T_c$. Our calculations are based on a fully microscopic $T$-matrix approach for addressing pairing correlations in a regime where the attraction is stronger than BCS and the coherence length is anomalously short. Previously, the distinct nodal and anti-nodal responses have provided strong support for the "two-gap scenario" of the cuprates in which the pseudogap competes with superconductivity. Instead, our theory supports a picture in which the pseudogap derives from pairing correlations, identifying the two gap components with non-condensed and condensed pairs. It leads to reasonably good agreement with a range of different experiments in the moderately underdoped regime and we emphasize that here there is no explicit curve fitting. Ours is a microscopic rather than a phenomenological theory. We briefly address the more heavily underdoped regime in which the behavior is more complex.

cond-mat.supr-con↗

arXiv2008

Finite-Temperature Behavior of an Inter-species Fermionic Superfluid with Population Imbalance

We determine the superfluid transition temperature $T_c$ and related finite temperature phase diagrams for the entire BCS-Bose Einstein condensation crossover in a homogeneous mixture of $^{6}$Li and $^{40}$K atoms with population imbalance. Our work is motivated by the recent observation of an inter-species Feshbach resonance. Pairing fluctuation effects, which significantly reduce $T_c$ from the onset temperature for pairing ($T^*$), provide reasonable estimates of $T_c$ and indicate that the inter-species superfluid phase should be accessible in future experiments. Although a generalized-Sarma phase is not stable in the ground state near unitarity, our phase diagrams show that it appears as an intermediate-temperature superfluid.

cond-mat.other↗

arXiv2008

Comparison of Different Pairing Fluctuation Approaches to BCS-BEC Crossover

The subject of BCS - Bose Einstein condensation (BEC) crossover is particularly exciting because of its realization in ultracold Fermi gases and its possible relevance to high temperature superconductors. In the paper we review that body of theoretical work on this subject which represents a natural extension of the seminal papers by Leggett and by Nozieres and Schmitt-Rink (NSR). The former addressed only the ground state, now known as the "BCS-Leggett" wave-function and the key contributions of the latter pertain to calculations of the superfluid transition temperature $T_c$. These two papers have given rise to two main and, importantly, distinct, theoretical schools in the BCS-BEC crossover literature. The first of these extends the BCS-Leggett ground state to finite temperature and the second extends the NSR scheme away from $T_c$ both in the superfluid and normal phases. It is now rather widely accepted that these extensions of NSR produce a different ground state than that first introduced by Leggett. Our analysis shows how the NSR-based approach views the bosonic contributions more completely but it treats the fermions as "quasi-free". By contrast, the BCS-Leggett based approach treats the fermionic contributions more completely but it treats the bosons as "quasi-free". The NSR based schemes approach the crossover between BCS and BEC by starting from the BEC limit and the BCS-Leggett based scheme approaches this crossover by starting from the BCS limit. Ultimately, one would like to combine these two schemes. In this paper we review the strengths and weaknesses of both approaches. To reach a full understanding, it is important in the future to invest effort in investigating in more detail the T=0 aspects of NSR-based theory and the $T \neq 0$ aspects of BCS-Leggett theory.

cond-mat.other↗

arXiv2008

Theory of Radio Frequency Spectroscopy Experiments in Ultracold Fermi Gases and Their Relation to Photoemission Experiments in the Cuprates

In this paper we present an overview of radio frequency (RF) spectroscopy in the atomic Fermi superfluids. An ultimate goal is to suggest new directions in the cold gas research agenda from the condensed matter perspective.Our focus is on the experimental and theoretical literature of cold gases and photoemission spectroscopy of the cuprates particularly as it pertains to areas of overlap. This paper contains a systematic overview of the theory of RF spectroscopy, both momentum integrated and momentum resolved. We discuss the effects of traps, population imbalance, final state interactions over the entire range of temperatures and compare theory and experiment. We show that this broad range of phenomena can be accomodated within the BCS-Leggett description of BCS-BEC crossover and that this scheme also captures some of the central observations in photoemission experiments in the cuprates. In this last context, we note that the key themes which have emerged in cuprate photoemission studies involve characterization of the fermionic self energy, of the pseudogap and of the effects of superconducting coherence (in passing from above to below the superfluid transition temperature, $T_c$).These issues have a counterpart in the cold Fermi gases and it would be most useful in future to use these atomic systems to address these and the more sweeping question of how to describe that anomalous superfluid phase which forms in the presence of a normal state excitation gap.

cond-mat.supr-con↗

arXiv2008

Fermions with attractive interactions on optical lattices and implications for correlated systems

In this paper we address the behavior of the superfluid transition temperature $T_c$ in the attractive Hubbard model. We study systematically the effects of pairing fluctuations and address all filling fractions over the entire range of attractive interaction strength. While the attractive Hubbard model can be regarded as the generalization of BCS to Bose Einstein condensation (BEC) crossover to a lattice, we find that the BEC limit of this Hubbard model is very different from that of jellium, owing to the strong inter-site repulsion between pairs, which becomes important near half filling when the on-site attraction is strong. A central conclusion of our work is that in a lattice, around half filling, the smooth evolution from the BCS to the BEC limits is interrupted. For the attractive Hubbard model, $T_c$ vanishes when the system approaches the bosonic regime with increasing interaction strength. We suggest that the vanishing of $T_c$ at strong coupling strength may signal a quantum critical transition to another form of superfluid not continuously connected to a BCS-like phase. We present a simple variational ansatz for the ground state in this more strongly coupled superfluid. We further generalize the (s-wave) Hubbard model to d-wave pairing and address issues of potential relevance to high temperature superconductors. For the d-wave case, we present a phase diagram and show that here too, one observes a vanishing of $T_c$ when the pairing onset temperature $T^*$ becomes sufficiently large. We suggest that future experiments on ultracold fermions in optical lattices should not be exclusively limited to the repulsive Hubbard model, but should address the attractive model in order to elucidate features of high temperature superconductivity.

cond-mat.other↗

arXiv2008

Understanding the Protected Nodes and Collapse of the Fermi Arcs in Underdoped Cuprate Superconductors

We show how recent angle resolved photoemission measurements addressing the Fermi arcs in the cuprates reveal a very natural phenomenological description of the complex superfluid phase. Importantly, this phenomenology is consistent with a previously presented microscopic theory. By distinguishing the order parameter and the excitation gap, we are able to demonstrate how the collapse of the arcs below $T_c$ into well defined nodes is associated with the \emph{smooth} emergence of superconducting coherence. Comparison of this theory with experiment shows good semi-quantitative agreement.

cond-mat.supr-con↗

arXiv2008

Temperature and final state effects in radio frequency spectroscopy experiments on atomic Fermi gases

We present a systematic characterization of the radio frequency (RF) spectra of homogeneous, paired atomic Fermi gases at finite temperatures, $T$, in the presence of final state interactions. The spectra, consisting of possible bound states and positive as well as negative detuning ($ν$) continua, satisfy exactly the zeroth- and first-moment sum rules at all $T$. We show how to detect the $ν< 0$ continuum arising from thermally excited quasiparticles, which has not yet been seen experimentally. We explain semi-quantitatively recent RF experiments on "bound-bound" transitions and, thereby, predict the associated effects of varying temperature.

cond-mat.supr-con↗

arXiv2008

Probing the Spectral Function Using Momentum Resolved Radio Frequency Spectroscopy in Trapped Fermi Gases

We address recent momentum resolved radio frequency (RF) experiments on ultracold trapped Fermi gases of $^{40}$K. We show that momentum resolved RF probes provide measurements of the centrally important fermionic spectral function. They also serve to remove ambiguity plaguing the interpretation of momentum integrated RF experiments by establishing a clear dispersion signature of pairing. We find that the temperature dependence of the spectral function is dramatic at unitarity, and, importantly, smooth from above to below $T_c$ throughout BCS-BEC crossover. This should be tested experimentally, given widespread predictions of first order behavior.

cond-mat.str-el↗

arXiv2007

Superfuid-insulator transitions at non-integer filling in optical lattices of fermionic atoms

We determine the superfluid transition temperatures $T_c$ and the ground states of the attractive Hubbard model and find new insulating phases associated with non-integer filling at sufficiently strong pairing attraction $|U|$. These states, distinct from band and Mott insulating phases, derive from pair localization; pair hopping at large $|U|$ and high densities is impeded by inter-site, inter-pair repulsive interactions. The best way to detect the breakdown of superfluidity is using fermionic optical lattices which should reveal new forms of ``bosonic'' order, reflecting ground state pairing without condensation.

cond-mat.other↗