arXiv subjects

Chih-Chun Chien

Publications and source records attributed to Chih-Chun Chien.

At least 109 records · Page 6Linked to original sources

arXiv2012

Gauge Invariant Linear Response Theory of Relativistic BCS superfluids

We develop a gauge-invariant linear response theory for relativistic Bardeen-Cooper-Schrieffer (BCS) superfluids based on a consistent-fluctuation-of-the order-parameter (CFOP) approach. The response functions from the CFOP approach satisfy important generalized Ward identities. The gauge invariance of the CFOP theory is a consequence of treating the gauge transformation and the fluctuations of the order parameter on equal footing so collective-mode effects are properly included. We demonstrate that the pole of the response functions is associated with the massless Goldstone boson. Important physical quantities such as the compressibility and superfluid density of relativistic BCS superfluids can also be inferred from our approach. We argue that the contribution from the massless Goldstone boson is crucial in obtaining a consistent expression for the compressibility.

math-ph↗

arXiv2012

Bosonic and fermionic transport phenomena of ultra-cold atoms in 1D optical lattices

Using the micro-canonical picture of transport -- a framework ideally suited to describe the dynamics of closed quantum systems such as ultra-cold atom experiments -- we show that the exact dynamics of non-interacting fermions and bosons exhibit very different transport properties when the system is set out of equilibrium by removing the particles from half of the lattice. We find that fermions rapidly develop a finite quasi steady-state current reminiscent of electronic transport in nanoscale systems. This result is robust -- it occurs with or without a harmonic confining potential and at zero or finite temperature. The zero-temperature bosonic current instead exhibits strong oscillatory behavior that decays into a steady-state of zero current only in the thermodynamic limit. These differences appear most strikingly in the different particle number fluctuations on half of the lattice as a consequence of the spin statistics. These predictions can be readily verified experimentally.

cond-mat.quant-gas↗

arXiv2012

Composite-Field Goldstone States and Higgs Mechanism in Dilute Bose Gases

We show that a composite-field (diatom) Goldstone state is expected in a dilute Bose gas for temperatures between the Bose gas critical temperature where the atom Bose-Einstein condensate appears and the temperature where superfluidity sets in. The presence of superfluidity is tied to the existence of a U(1) charge-two diatom condensate in the system. By promoting the global U(1) symmetry of the theory to a gauge symmetry, we find that the mass of the gauge particle generated through the Anderson-Higgs mechanism is related to the superfluid density via the Meissner effect and the superfluid density is related to the square of the anomalous density in the Bose system.

cond-mat.quant-gas↗

arXiv2011

Spatially varying interactions induced in atomic gases by optical Feshbach resonance

Optical Feshbach resonance is capable of inducing spatially varying interactions in ultra-cold atoms. Its applications to pancake-shaped clouds of bosons and fermions enable one to study several fresh phenomena. We examine possibilities of inducing counter-intuitive structures such as creating a superfluid enclave inside a Mott insulator for bosons and a normal-gas core enclosed by a superfluid shell for fermions. We discuss feasible experimental setups and signatures of those interesting structures, which can be very different from common structures observed in experiments so far. While a superfluid enclave in a Mott insulator can be useful for constructing atomic devices for atomtronics, superconducting islands observed in scanning-tunneling microscopy of heavily underdoped high-temperature superconductors may be studied with cold Fermi gases with spatially varying attractions.

cond-mat.quant-gas↗

arXiv2011

Conductivity in Pseudogapped Superconductors: The Role of the Fermi Arcs

We calculate the dc conductivity $σ$ in a pseudogapped high $T_c$ superconductor within a theory which is consistent with gauge invariance. Our results contain additional terms beyond those identified previously. Although it has been thought that lifetime effects dominate the $T$ dependence of transport, here we show (consistent with growing experimental support) that the temperature dependence of the effective carrier number $(n/m(T))_{\rm{eff}}$ plays a critical role and thereby leads to the contrasting behavior between over and under-doped regimes. The role of Fermi arcs is secondary.

cond-mat.supr-con↗

arXiv2011

Analytical limits for cold atom Bose gases with tunable interactions

We discuss the equilibrium properties of dilute Bose gases using a non-perturbative formalism based on auxiliary fields related to the normal and anomalous densities. We show analytically that for a dilute Bose gas of weakly-interacting particles at zero temperature, the leading-order auxiliary field (LOAF) approximation leads to well-known analytical results. Close to the critical point the LOAF predictions are the same as those obtained using an effective field theory in the large-N approximation. We also report analytical approximations for the LOAF results in the unitarity limit, which compare favorably with our numerical results. LOAF predicts that the equation of state for the Bose gas in the unitarity limit is E / (p V) = 1, unlike the case of the Fermi gas when E / (p V) = 3/2.

cond-mat.quant-gas↗

arXiv2011

Auxiliary field approach to dilute Bose gases with tunable interactions

We rewrite the Lagrangian for a dilute Bose gas in terms of auxiliary fields related to the normal and anomalous condensate densities. We derive the loop expansion of the effective action in the composite-field propagators. The lowest-order auxiliary field (LOAF) theory is a conserving mean-field approximation consistent with the Goldstone theorem without some of the difficulties plaguing approximations such as the Hartree and Popov approximations. LOAF predicts a second-order phase transition. We give a set of Feynman rules for improving results to any order in the loop expansion in terms of composite-field propagators. We compare results of the LOAF approximation with those derived using the Popov approximation. LOAF allows us to explore the critical regime for all values of the coupling constant and we determine various parameters in the unitarity limit.

cond-mat.quant-gas↗

arXiv2011

Auxiliary field formalism for dilute fermionic atom gases with tunable interactions

We develop the auxiliary field formalism corresponding to a dilute system of spin-1/2 fermions. This theory represents the Fermi counterpart of the BEC theory developed recently by F. Cooper et al. [Phys. Rev. Lett. 105, 240402 (2010)] to describe a dilute gas of Bose particles. Assuming tunable interactions, this formalism is appropriate for the study of the crossover from the regime of Bardeen-Cooper-Schriffer (BCS) pairing to the regime of Bose-Einstein condensation (BEC) in ultracold fermionic atom gases. We show that when applied to the Fermi case at zero temperature, the leading-order auxiliary field (LOAF) approximation gives the same equations as those obtained in the standard BCS variational picture. At finite temperature, LOAF leads to the theory discussed by by Sa de Melo, Randeria, and Engelbrecht [Phys. Rev. Lett. 71, 3202(1993); Phys. Rev. B 55, 15153(1997)]. As such, LOAF provides a unified framework to study the interacting Fermi gas. The mean-field results discussed here can be systematically improved upon by calculating the one-particle irreducible (1-PI) action corrections, order by order.

cond-mat.quant-gas↗

arXiv2011

Driving denaturation: Nanoscale thermal transport as a probe of DNA melting

DNA denaturation has long been a subject of intense study due to its relationship to DNA transcription and its fundamental importance as a nonlinear, structural transition. Many aspects of this phenomenon, however, remain poorly understood. Existing models fit quite well with experimental results on the fraction of unbound base pairs versus temperature. Yet, these same models give incorrect results for other essential quantities. For example, the predicted base pair fluctuation timescales - relevant to transcription - are orders of magnitude different from those observed experimentally. Here, we demonstrate that nanoscale thermal transport can serve as a sensitive probe of the underlying microscopic mechanisms responsible for the dynamics of DNA denaturation. Specifically, we show that the heat transport properties of DNA are altered significantly and abruptly as it denaturates, and this alteration encodes detailed information on the dynamics of thermal fluctuations and their interaction along the chain. This finding allows for the unambiguous discrimination between models of DNA denaturation. Measuring the thermal conductance will thus shed new light on the nature of this important molecule.

cond-mat.mes-hall↗

arXiv2011

Spin Transport in Cold Fermi gases: A Pseudogap Interpretation of Spin Diffusion Experiments at Unitarity

We address recent spin transport experiments in ultracold unitary Fermi gases. We provide a theoretical understanding for how the measured temperature dependence of the spin diffusivity at low $T$ can disagree with the expected behavior of a Fermi liquid (FL) while the spin susceptiblity(following the experimental protocols) is consistent with a Fermi liquid picture. We show that the experimental protocols for extracting $χ_s$ are based on a FL presumption; relaxing this leads to consistency within (but not proof of) a pseudogap-based approach. Our tranport calculations yield insight into the measured strong suppression of the spin diffusion constant at lower $T$.

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

Dynamics of a quantum quench in an ultra-cold atomic BCS superfluid

We study dynamics of an ultra-cold atomic BCS superfluid driven towards the BCS superfluid-Fermi liquid quantum critical point by a gradual decrease of the pairing interaction. We analyze how the BCS superfluid falls out of equilibrium and show that the non-equilibrium gap and Cooper pair size reflect critical properties of the transition. We observe three stages of evolution: adiabatic where the Cooper pair size is inversely proportional to the equilibrium gap, weakly non-equilibrium where it is inversely proportional to the non-equilibrium gap, and strongly non-equilibrium where it decouples from both equilibrium and non-equilibrium gap. These phenomena should stimulate future experimental characterization of non-equilibrium ultra-cold atomic BCS superfluids.

cond-mat.quant-gas↗

arXiv2010

Non-perturbative predictions for cold atom Bose gases with tunable interactions

We derive a theoretical description for dilute Bose gases as a loop expansion in terms of composite-field propagators by rewriting the Lagrangian in terms of auxiliary fields related to the normal and anomalous densities. We demonstrate that already in leading order this non-perturbative approach describes a large interval of coupling-constant values, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second-order, and is consistent with the critical temperature predicted in the weak-coupling limit by the next-to-leading order large-N expansion.

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↗