arXiv subjects

Leo Radzihovsky

Publications and source records attributed to Leo Radzihovsky.

At least 91 records · Page 5Linked to original sources

arXiv2007

Superfluidity and phase transitions in a resonant Bose gas

The atomic Bose gas is studied across a Feshbach resonance, mapping out its phase diagram, and computing its thermodynamics and excitation spectra. It is shown that such a degenerate gas admits two distinct atomic and molecular superfluid phases, with the latter distinguished by the absence of atomic off-diagonal long-range order, gapped atomic excitations, and deconfined atomic pi-vortices. The properties of the molecular superfluid are explored, and it is shown that across a Feshbach resonance it undergoes a quantum Ising transition to the atomic superfluid, where both atoms and molecules are condensed. In addition to its distinct thermodynamic signatures and deconfined half-vortices, in a trap a molecular superfluid should be identifiable by the absence of an atomic condensate peak and the presence of a molecular one.

cond-mat.supr-con↗

arXiv2007

Nonlinear Elasticity, Fluctuations and Heterogeneity of Nematic Elastomers

Liquid crystal elastomers realize a fascinating new form of soft matter that is a composite of a conventional crosslinked polymer gel (rubber) and a liquid crystal. These {\em solid} liquid crystal amalgams, quite similarly to their (conventional, fluid) liquid crystal counterparts, can spontaneously partially break translational and/or orientational symmetries, accompanied by novel soft Goldstone modes. As a consequence, these materials can exhibit unconventional elasticity characterized by symmetry-enforced vanishing of some elastic moduli. Thus, a proper description of such solids requires an essential modification of the classical elasticity theory. In this work, we develop a {\em rotationally invariant}, {\em nonlinear} theory of elasticity for the nematic phase of ideal liquid crystal elastomers. We show that it is characterized by soft modes, corresponding to a combination of long wavelength shear deformations of the solid network and rotations of the nematic director field. We study thermal fluctuations of these soft modes in the presence of network heterogeneities and show that they lead to a large variety of anomalous elastic properties, such as singular length-scale dependent shear elastic moduli, a divergent elastic constant for splay distortion of the nematic director, long-scale incompressibility, universal Poisson ratios and a nonlinear stress-strain relation fo arbitrary small strains. These long-scale elastic properties are {\em universal}, controlled by a nontrivial zero-temperature fixed point and constitute a qualitative breakdown of the classical elasticity theory in nematic elastomers. Thus, nematic elastomers realize a stable ``critical phase'', characterized by universal power-law correlations, akin to a critical point of a continuous phase transition, but extending over an entire phase.

cond-mat.soft↗

arXiv2007

Comment on "Superfluid stability in the BEC-BCS crossover"

We point out an error in recent work by Pao, Wu, and Yip [Phys. Rev.B {\bf 73}, 132506 (2006)], that stems from their use of a necessary but not sufficient condition [positive compressibility (magnetic susceptibility) and superfluid stiffness] for the stability of the ground state of a polarized Fermi gas. As a result, for a range of detunings their proposed ground-state solution is a local maximum rather than a minimum of the ground state energy, which thereby invalidates their proposed phase diagram for resonantly interacting fermions under an imposed population difference.

cond-mat.supr-con↗

arXiv2007

BEC-BCS crossover, phase transitions and phase separation in polarized resonantly-paired superfluids

We study resonantly-paired s-wave superfluidity in a degenerate gas of two species (hyperfine states labeled by $\uparrow,\downarrow$) of fermionic atoms when the numbers $N_{\uparrow}$ and $N_{\downarrow}$ of the two species are {\it unequal}, i.e., the system is "polarized". We find that the continuous crossover from the Bose-Einstein condensate (BEC) limit of tightly-bound diatomic molecules to the Bardeen-Cooper-Schrieffer (BCS) limit of weakly correlated Cooper pairs, studied extensively at equal populations, is interrupted by a variety of distinct phenomena under an imposed population difference $ΔN \equiv N_\uparrow - N_\downarrow$. Our findings are summarized by a `` polarization\rq\rq ($ΔN$) versus Feshbach-resonance detuning ($δ$) zero-temperature phase diagram, which exhibits regions of phase separation, a periodic FFLO superfluid, a polarized normal Fermi gas and a polarized molecular superfluid consisting of a molecular condensate and a fully polarized Fermi gas. We describe numerous experimental signatures of such phases and the transitions between them, in particular focusing on their spatial structure in the inhomogeneous environment of an atomic trap.

cond-mat.supr-con↗

arXiv2006

Thermal Fluctuations and Rubber Elasticity

The effects of thermal elastic fluctuations in rubber materials are examined. It is shown that, due to an interplay with the incompressibility constraint, these fluctuations qualitatively modify the large-deformation stress-strain relation, compared to that of classical rubber elasticity. To leading order, this mechanism provides a simple and generic explanation for the peak structure of Mooney-Rivlin stress-strain relation, and shows a good agreement with experiments. It also leads to the prediction of a phonon correlation function that depends on the external deformation.

cond-mat.soft↗

arXiv2006

Superfluid transition in a rotating resonantly-interacting Fermi gas

We study a rotating atomic Fermi gas near a narrow s-wave Feshbach resonance in a uniaxial harmonic trap with frequencies $Ω_\perp$, $Ω_z$. Our primary prediction is the upper-critical angular velocity, $ω_{c2} (δ,T)$, as a function of temperature $T$ and resonance detuning $δ$, ranging across the BEC-BCS crossover. The rotation-driven suppression of superfluidity at $ω_{c2}$ is quite distinct in the BCS and BEC regimes, with the former controlled by Cooper-pair depairing and the latter by the dilution of bosonic molecules. At low $T$ and $Ω_z\llΩ_\perp$, in the BCS and crossover regimes of $0 \lesssim δ\lesssim δ_c$, $ω_{c2}$ is implicitly given by $\hbar \sqrt{ω_{c2}^2 +Ω_\perp^2}\approx 2Δ\sqrt{\hbar Ω_\perp/ε_F}$, vanishing as $ω_{c2} \simΩ_\perp(1-δ/δ_c)^{1/2}$ near $δ_c\approx 2ε_{F} + \fracγ2ε_{F} \ln(ε_F/\hbarΩ_\perp)$ (with $Δ$ the BCS gap and $γ$ resonance width), and extending bulk result $\hbarω_{c2} \approx 2Δ^2/ε_{F}$ to a finite number of atoms in a trap. In the BEC regime of $δ< 0$ we find $ω_{c2} \toΩ^-_\perp$, where molecular superfluidity can only be destroyed by large quantum fluctuations associated with comparable boson and vortex densities.

cond-mat.supr-con↗

arXiv2006

Thermal depinning and transverse-field tilting transitions in a planar vortex array pinned by a columnar defect

We study a thermal and a transverse magnetic field response of a vortex line array confined to a plane with a single columnar pinning defect. By integrating out ``bulk'' degrees of freedom away from the columnar defect we reduce this two-dimensional problem to a one-dimensional one, localized on the defect and exhibiting a long-range elasticity along the defect. We show that as a function of temperature, for a magnetic field aligned with the defect this system exhibits a one-dimensional analog of a roughening transition, with a low-temperature ``smooth'' phase corresponding to a vortex array pinned by the defect, and a high-temperature ``rough'' phase in which at long scales thermal fluctuations effectively average away the pinning by the defect. We also find that in the low-temperature pinned phase, the vortex lattice tilt response to a transverse magnetic field proceeds via a soliton proliferation transition, governed by an integrable sine-Hilbert equation and analogous to the well-known commensurate-incommensurate transition in sine-Gordon systems. The distinguishing feature here is the long-range nature of the one-dimensional elasticity, leading to a logarithmic soliton energy and interaction. We predict the transverse-field -- temperature phase diagram and discuss extension of our results to a bulk vortex array in the presence of a distribution of columnar defects.

cond-mat.stat-mech↗

arXiv2006

BEC-BCS crossover in "magnetized" Feshbach-resonantly paired superfluids

We map out the detuning-magnetization phase diagram for a ``magnetized'' (unequal number of atoms in two pairing hyperfine states) gas of fermionic atoms interacting via an s-wave Feshbach resonance (FR). For large positive FR detuning a normal magnetized Fermi gas is stable above an exponentially small value of the population difference. Below this critical value the phase diagram is dominated by coexistence of a magnetized normal gas and a singlet paired superfluid with the latter exhibiting a BCS-Bose Einstein condensate crossover with reduced detuning. On the BCS side of strongly overlapping Cooper pairs, a sliver of finite-momentum paired Fulde-Ferrell-Larkin-Ovchinnikov magnetized phase intervenes between the phase separated and normal states. In contrast, for large negative detuning a uniform, polarized superfluid, that is a coherent mixture of singlet Bose-Einstein-condensed molecules and fully magnetized single-species Fermi-sea, is a stable ground state.

cond-mat.supr-con↗

arXiv2005

Quantum decoupling transition in a one-dimensional Feshbach-resonant superfluid

We study a one-dimensional gas of fermionic atoms interacting via an s-wave molecular Feshbach resonance. At low energies the system is characterized by two Josephson-coupled Luttinger liquids, corresponding to paired atomic and molecular superfluids. We show that, in contrast to higher dimensions, the system exhibits a quantum phase transition from a phase in which the two superfluids are locked together to one in which, at low energies, quantum fluctuations suppress the Feshbach resonance (Josephson) coupling, effectively decoupling the molecular and atomic superfluids. Experimental signatures of this quantum transition include the appearance of an out-of-phase gapless mode (in addition to the standard gapless in-phase mode) in the spectrum of the decoupled superfluid phase and a discontinuous change in the molecular momentum distribution function.

cond-mat.stat-mech↗

arXiv2004

Vortex Lattice Inhomogeneity in Spatially Inhomogeneous Superfluids

A trapped degenerate Bose gas exhibits superfluidity with spatially nonuniform superfluid density. We show that the vortex distribution in such a highly inhomogeneous rotating superfluid is nevertheless nearly uniform. The inhomogeneity in vortex density, which diminishes in the rapid-rotation limit, is driven by the discrete way vortices impart angular momentum to the superfluid. This effect favors highest vortex density in regions where the superfluid density is most uniform (e.g., the center of a harmonically trapped gas). A striking consequence of this is that the boson velocity deviates from a rigid-body form exhibiting a radial-shear flow past the vortex lattice.

cond-mat.stat-mech↗

arXiv2004

Phases and Transitions in Phantom Nematic Elastomer Membranes

Motivated by recently discovered unusual properties of bulk nematic elastomers, we study a phase diagram of liquid-crystalline polymerized phantom membranes, focusing on in-plane nematic order. We predict that such membranes should enerically exhibit five phases, distinguished by their conformational and in-plane orientational properties, namely isotropic-crumpled, nematic-crumpled, isotropic-flat, nematic-flat and nematic-tubule phases. In the nematic-tubule phase, the membrane is extended along the direction of {\em spontaneous} nematic order and is crumpled in the other. The associated spontaneous symmetries breaking guarantees that the nematic-tubule is characterized by a conformational-orientational soft (Goldstone) mode and the concomitant vanishing of the in-plane shear modulus. We show that long-range orientational order of the nematic-tubule is maintained even in the presence of harmonic thermal luctuations. However, it is likely that tubule's elastic properties are ualitatively modified by these fluctuations, that can be studied using a nonlinear elastic theory for the nematic tubule phase that we derive at the end of this paper.

cond-mat.soft↗

arXiv2004

Vortices in Spatially Inhomogeneous Superfluids

We study vortices in a radially inhomogeneous superfluid, as realized by a trapped degenerate Bose gas in a uniaxially symmetric potential. We show that, in contrast to a homogeneous superfluid, an off-axis vortex corresponds to an anisotropic superflow whose profile strongly depends on the distance to the trap axis. One consequence of this superflow anisotropy is vortex precession about the trap axis in the absence of an imposed rotation. In the complementary regime of a finite prescribed rotation, we compute the minimum-energy vortex density, showing that in the rapid-rotation limit it is extremely uniform, despite a strongly inhomogeneous (nearly) Thomas-Fermi condensate density $ρ_s(r)$. The weak radially-dependent contribution ($\propto \nabla^2\lnρ_s(r)$) to the vortex distribution, that vanishes with the number of vortices $N_v$ as $\frac{1}{N_v}$, arises from the interplay between vortex quantum discretness (namely their inability to faithfully support the imposed rigid-body rotation) and the inhomogeneous superfluid density. This leads to an enhancement of the vortex density at the center of a typical concave trap, a prediction that is in quantitative agreement with recent experiments (cond-mat/0405240). One striking consequence of the inhomogeneous vortex distribution is an azimuthally-directed, radially-shearing superflow.

cond-mat.stat-mech↗

arXiv2003

Anisotropic and Heterogeneous Polymerized Membranes

In these lectures I discuss long-scale properties of fluctuating polymerized membranes in the presence of network anisotropy and random heterogeneities. Amazingly, even infinitesimal amount of these seemingly innocuous but physically important ingredients in the membrane's internal structure leads to a wealth of striking qualitatively new phenomena. Anisotropy leads to a ``tubule'' phase that is intermediate in its properties and location on the phase diagram between previously discussed ``crumpled'' and ``flat'' phases. At low temperature, network heterogeneity generates conformationally glassy phases, with membrane normals exhibiting glass order analogous to spin-glasses. The common thread to these distinct membrane phases is that they exhibit universal anomalous elasticity, (singularly length-scale dependent elastic moduli, universal Poisson ratio, etc.), driven by thermal fluctuations and/or disorder and controlled by a nontrivial low-temperature fixed point.

cond-mat.soft↗

arXiv2003

Superfluid transitions in bosonic atom-molecule mixtures near Feshbach resonance

We study bosonic atoms near a Feshbach resonance, and predict that in addition to a standard normal and atomic superfluid phases, this system generically exhibits a distinct phase of matter: a molecular superfluid, where molecules are superfluid while atoms are not. We explore zero- and finite-temperature properties of the molecular superfluid (a bosonic, strong-coupling analog of a BCS superconductor), and study quantum and classical phase transitions between the normal, molecular superfluid and atomic superfluid states.

cond-mat.supr-con↗

arXiv2003

Fluctuating Nematic Elastomer Membranes: a New Universality Class

We study the flat phase of nematic elastomer membranes with rotational symmetry spontaneously broken by in-plane nematic order. Such state is characterized by a vanishing elastic modulus for simple shear and soft transverse phonons. At harmonic level, in-plane orientational (nematic) order is stable to thermal fluctuations, that lead to short-range in-plane translational (phonon) correlations. To treat thermal fluctuations and relevant elastic nonlinearities, we introduce two generalizations of two-dimensional membranes in a three dimensional space to arbitrary D-dimensional membranes embedded in a d-dimensional space, and analyze their anomalous elasticities in an expansion about D=4. We find a new stable fixed point, that controls long-scale properties of nematic elastomer membranes. It is characterized by singular in-plane elastic moduli that vanish as a power-law eta_lambda=4-D of a relevant inverse length scale (e.g., wavevector) and a finite bending rigidity. Our predictions are asymptotically exact near 4 dimensions.

cond-mat.soft↗

arXiv2002

Comment on "Freezing by heating in a driven mesoscopic system"

We point out that the phenomenon ``heating by freezing'' discovered in nonequilibrium simulations by Helbing, et al. (PRL 84, 1240 (2000)) extends to equilibrium systems as well. We argue that such reentrant fluctuation-driven freezing can, for example, be realized in two-dimensional colloidal systems subjected to a one-dimensional periodic potential.

cond-mat.soft↗

arXiv2002

Thermal fluctuations and anomalous elasticity of homogeneous nematic elastomers

We present a unified formulation of a rotationally invariant nonlinear elasticity for a variety of spontaneously anisotropic phases, and use it to study thermal fluctuations in nematic elastomers and spontaneously anisotropic gels. We find that in a thermodynamic limit homogeneous nematic elastomers are universally incompressible, are characterized by a universal ratio of shear moduli, and exhibit an anomalous elasticity controlled by a nontrivial low temperature fixed point perturbative in D=3-epsilon dimensions. In three dimensions, we make predictions that are asymptotically exact.

cond-mat.soft↗

arXiv2002

Universal Elasticity and Fluctuations of Nematic Gels

We study elasticity of spontaneously orientationally-ordered amorphous solids, characterized by a vanishing transverse shear modulus, as realized for example by nematic elastomers and gels. We show that local heterogeneities and elastic nonlinearities conspire to lead to anomalous nonlocal universal elasticity controlled by a nontrivial infared fixed point. Namely, at long scales, such solids are characterized by universal shear and bending moduli that, respectively, vanish and diverge at long scales, are universally incompressible and exhibit a universal negative Poisson ratio and a non-Hookean elasticity down to arbitrarily low strains. Based on expansion about five dimensions, we argue that the nematic order is stable to thermal fluctuation and local hetergeneities down to d_lc < 3.

cond-mat.soft↗