arXiv subjects

Qijin Chen

Publications and source records attributed to Qijin Chen.

At least 55 records · Page 3Linked to original sources

arXiv2017

Cuprate diamagnetism in the presence of a pseudogap: Beyond the standard fluctuation formalism

It is often claimed that among the strongest evidence for preformed-pair physics in the cuprates are the experimentally observed large values for the diamagnetic susceptibility and Nernst coefficient. These findings are most apparent in the underdoped regime, where a pseudogap is also evident. While the conventional (Gaussian) fluctuation picture has been applied to address these results, this preformed-pair approach omits the crucial effects of a pseudogap. In this paper we remedy this omission by computing the diamagnetic susceptibility and Nernst coefficient in the presence of a normal state gap. We find a large diamagnetic response for a range of temperatures much higher than the transition temperature. In particular, we report semi-quantitative agreement with the measured diamagnetic susceptibility onset temperatures, over the entire range of hole dopings. Notable is the fact that at the lower critical doping of the superconducting dome, where the transition temperature vanishes and the pseudogap onset temperature remains large, the onset temperature for both diamagnetic and transverse thermoelectric transport coefficients tends to zero. Due to the importance attributed to the cuprate diamagnetic susceptibility and Nernst coefficient, this work helps to clarify the extent to which pairing fluctuations are a component of the cuprate pseudogap.

cond-mat.str-el↗

arXiv2017

Exotic superfluidity and pairing phenomena in atomic Fermi gases in mixed dimensions

Atomic Fermi gases have been an ideal platform for simulating conventional and engineering exotic physical systems owing to their multiple tunable control parameters. Here we investigate the effects of mixed dimensionality on the superfluid and pairing phenomena of a two-component ultracold atomic Fermi gas with a short-range pairing interaction, while one component is confined on a one-dimensional (1D) optical lattice whereas the other is in a homogeneous 3D continuum. We study the phase diagram and the pseudogap phenomena throughout the entire BCS-BEC crossover, using a pairing fluctuation theory. We find that the effective dimensionality of the non-interacting lattice component can evolve from quasi-3D to quasi-1D, leading to strong Fermi surface mismatch. Upon pairing, the system becomes effectively quasi-two dimensional in the BEC regime. The behavior of $T_c$ bears similarity to that of a regular 3D population imbalanced Fermi gas, but with a more drastic departure from the regular 3D balanced case, featuring both intermediate temperature superfluidity and possible pair density wave ground state. Unlike a simple 1D optical lattice case, $T_c$ in the mixed dimensions has a constant BEC asymptote.

cond-mat.quant-gas↗

arXiv2017

Instability of Fulde-Ferrell-Larkin-Ovchinnikov states in three and two dimensions

The exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states have been actively searched for experimentally since the mean-field based FFLO theories were put forward half a century ago. Here we investigate the stability of FFLO states against unavoidable pairing fluctuations, and conclude that FFLO superfluids cannot exist due to their intrinsic instability in three and two dimensions. This explains their absence in experimental observations in both condensed matter systems and the most recent, more promising ultracold atomic Fermi gases with a population imbalance.

cond-mat.supr-con↗

arXiv2016

Enhancement effect of mass imbalance on Fulde-Ferrell-Larkin-Ovchinnikov type of pairing in Fermi-Fermi mixtures of ultracold quantum gases

Ultracold two-component Fermi gases with a tunable population imbalance have provided an excellent opportunity for studying the exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, which have been of great interest in condensed matter physics. However, the FFLO states have not been observed experimentally in Fermi gases in three dimensions (3D), possibly due to their small phase space volume and extremely low temperature required for an equal-mass Fermi gas. Here we explore possible effects of mass imbalance, mainly in a $^6$Li--$^{40}$K mixture, on the one-plane-wave FFLO phases for a 3D homogeneous case at the mean-field level. We present various phase diagrams related to the FFLO states at both zero and finite temperatures, throughout the BCS-BEC crossover, and show that a large mass ratio may enhance substantially FFLO type of pairing.

cond-mat.quant-gas↗

arXiv2016

Impurity effects on BCS-BEC crossover in ultracold atomic Fermi gases

We present a systematic investigation of the effects of "nonmagnetic" impurities on the $s$-wave BCS-BEC crossover in atomic Fermi gases within a pairing fluctuation theory. Both pairing and impurity scattering $T$-matrices are treated self-consistently at the same time. While the system is less sensitive to impurity scattering in the Born limit, for strong impurity scatterers, both the frequency and the gap function are highly renormalized, leading to significant suppression of the superfluid $T_c$, the order parameter and the superfluid density. We also find the formation of impurity bands and smearing of coherence peak in the fermion density of states, leading to a spectrum weight transfer and finite lifetime of Bogoliubov quasiparticles. In the BCS regime, the superfluidity may be readily destroyed by the impurity of high density. In comparison, the superfluidity in unitary and BEC regimes is relatively more robust.

cond-mat.quant-gas↗

arXiv2016

Reentrant Superfluidity and Pair Density Wave in Single Component Dipolar Fermi Gases

We study the superfluidity of single component dipolar Fermi gases in three dimensions within a pairing fluctuation theory. The transition temperature $T_{c}$ for the dominant $p_z$ wave superfluidity exhibits a remarkable re-entrant behavior as a function of the pairing strength induced by the dipole-dipole interaction (DDI), which leads to an anisotropic pair dispersion. The anisotropy and the long range nature of the DDI cause $T_c$ to vanish for a narrow range of intermediate interaction strengths, where a pair density wave state emerges as the ground state. The superfluid density and thermodynamics below $T_{c}$, along with the density profiles in a harmonic trap, are investigated as well, throughout the BCS-BEC crossover. Implications for experiments are discussed.

cond-mat.quant-gas↗

arXiv2014

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high $T_c$ superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum simulator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high $T_c$ superconductivity. One obstacle to the ultimate understanding of high $T_c$ superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics community, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fluctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high $T_c$ superconductivity.

cond-mat.quant-gas↗

arXiv2012

Phase diagrams of Fermi gases in a trap with mass and population imbalances at finite temperature

The pairing and superfluid phenomena in a two-component Fermi gas can be strongly affected by the population and mass imbalances. Here we present phase diagrams of atomic Fermi gases as they undergo BCS--Bose-Einstein condensation (BEC) crossover with population and mass imbalances, using a pairing fluctuation theory. We focus on the finite temperature and trap effects, with an emphasis on the mixture of $^{6}$Li and $^{40}$K atoms. We show that there exist exotic types of phase separation in the BEC regime as well as sandwich-like shell structures at low temperature with superfluid or pseudogapped normal state in the central shell in the BCS and unitary regimes, especially when the light species is the majority. Such a sandwich-like shell structure appear when the mass imbalance increases beyond certain threshold. Our result is relevant to future experiments on the $^6$Li--$^{40}$K mixture and possibly other Fermi-Fermi mixtures.

cond-mat.quant-gas↗

arXiv2012

Effect of the particle-hole channel on BCS--Bose-Einstein condensation crossover in atomic Fermi gases

BCS--Bose-Einstein condensation (BEC) crossover is effected by increasing pairing strength between fermions from weak to strong in the particle-particle channel. Here we study the effect of the particle-hole channel on the zero $T$ gap $Δ(0)$, superfluid transition temperature $T_{\text{c}}$ and the pseudogap at $T_{\text{c}}$, as well as the mean-field ratio $2Δ(0)/T_{\text{c}}^{\text{MF}}$, from BCS through BEC regimes, in the framework of a pairing fluctuation theory which includes self-consistently the contributions of finite-momentum pairs. These pairs necessarily lead to a pseudogap in single particle excitation spectrum above and below $T_{\text{c}}$. We sum over the infinite particle-hole ladder diagrams so that the particle-particle and particle-hole $T$-matrices are entangled with each other. We find that the particle-hole susceptibility has a complex dynamical structure, with strong momentum and frequency dependencies, and is sensitive to temperature, gap size and interaction strength. We conclude that neglecting the self-energy feedback causes a serious over-estimate of the particle-hole susceptibility. In the BCS limit, the particle-hole channel effect may be approximated by the same reduction in the overall pairing strength so that the ratio $2Δ(0)/T_{\text{c}}$ is unaffected, in agreement with Gor'kov \textit{et al.} to the leading order. However, the effect becomes more complex and pronounced in the crossover regime, where the particle-hole susceptibility is reduced by both a smaller Fermi surface and a big (pseudo)gap. Deep in the BEC regime, the particle-hole channel contributions drop to zero. We propose that precision measurements of the magnetic field for Feshbach resonance at low temperatures as a function of density can be used to quantify the particle-hole susceptibility and test different theories.

cond-mat.quant-gas↗

arXiv2012

Zero density limit extrapolation of the superfluid transition temperature in a unitary atomic Fermi gas on a lattice

The superfluid transition temperature $T_c$ of a unitary Fermi gas on a three-dimensional isotropic lattice with an attractive on-site interaction is investigated as a function of density $n$, from half filling down to $5.0\times 10^{-7}$ per unit cell, using a pairing fluctuation theory. We show that except at very low densities ($n^{1/3} <0.2$), where $T_c/E_F$ is linear in $n^{1/3}$, $T_c/E_F$ exhibits significant higher order nonlinear dependence on $n^{1/3}$. Therefore, linear extrapolation using results at intermediate densities such as in typical quantum Monte Carlo simulations leads to a significant underestimate of the zero density limit of $T_c/E_F$. Our result, $T_c/E_F=0.256$, at $n=0$ is subject to reduction from particle-hole fluctuations and incoherent single particle self energy corrections.

cond-mat.quant-gas↗

arXiv2011

Probing the homogeneous spectral function of a strongly interacting superfluid atomic Fermi gas in a trap using phase separation and momentum resolved rf spectroscopy

It is of central importance to probe the \emph{local} spectral function $A(\mathbf{k},ω)$ of a strongly interacting Fermi gas in a trap. Momentum resolved rf spectroscopy has been demonstrated to be able to probe the trap averaged $A(\mathbf{k},ω)$. However, the usefulness of this technique was limited by the trap inhomogeneity. Independent of a specific theory, here we propose that by studying the momentum resolved rf spectra of the minority fermions of a phase separated, population imbalanced Fermi gas at low temperature, one can effectively extract $A(\mathbf{k},ω)$ of a homogeneous superfluid Fermi gas (at the trap center). In support, we present calculated spectral functions and spectral intensity maps for various cases from BCS through BEC regimes using different theories.

cond-mat.quant-gas↗

arXiv2011

Superfluidity in atomic Fermi gases

In a trapped atomic Fermi gas, one can tune continuously via a Feshbach resonance the effective pairing interaction between fermionic atoms from very weak to very strong. As a consequence, the low temperature superfluidity evolves continuously from the BCS type in the weak interaction limit to that of Bose-Einstein condensation in the strong pairing limit, exhibiting a BCS-BEC crossover. In this paper, we review recent experimental progress in atomic Fermi gases which elucidates the nature of the superfluid phase as the interaction is continuously tuned. Of particular interest is the intermediate or crossover regime where the $s$-wave scattering length diverges. We will present an intuitive pairing fluctuation theory, and show that this theory is in quantitative agreement with existing experiments in cold atomic Fermi gases.

cond-mat.quant-gas↗

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↗