arXiv subjects

J. A. Sauls

Publications and source records attributed to J. A. Sauls.

At least 55 records · Page 3Linked to original sources

arXiv2018

Vortex lattices and broken time reversal symmetry in the topological superconductor UPt3

The topological superconductor UPt3, has three distinct vortex phases, a strong indication of its unconventional character. Using small-angle neutron scattering we have probed the vortex lattice in the UPt3 B phase with the magnetic field along the crystal c-axis. We find a difference in the vortex lattice configuration depending on the sign of the magnetic field relative to the field direction established upon entering the B phase at low temperature in a field sweep, showing that the vortices in this material posses an internal degree of freedom. This observation is facilitated by the discovery of a field driven non-monotonic vortex lattice rotation, driven by competing effects of the superconducting gap distortion and the vortex-core structure. From our bulk measurements we infer that the superconducting order parameter in the UPt3 B phase breaks time reversal symmetry and exhibits chiral symmetry with respect to the c-axis.

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arXiv2018

New Phases of Superfluid $^3$He Confined in Aerogels

Liquid $^3$He confined in low-density, highly porous random solids such as silica aerogel provides tuneable systems to study the effects of disorder and confinement on the properties of a quantum liquid. New superfluid phases result from the interplay between disorder, confinement and complex symmetry-breaking. An extended bibliography is appended. An edited and abbreviated version of this article appeared in Physics Today 71, 11, 30 (2018).

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arXiv2018

Strong Coupling Theory of Superconductivity

The electronic properties of correlated metals with a strong electron-phonon coupling may be understood in terms of a combination of Landau's Fermi-liquid theory and the strong-coupling theory of Migdal and Eliashberg. In these lecture notes we discuss the microscopic foundations of this phenomenological Fermi-liquid model of correlated, strong-coupling metals. We formulate the basic equations of the model, which are quasiclassical transport equations that describe both equilibrium and non-equilibrium phenomena for the normal and superconducting states of a metal. Our emphasis is on superconductors close to equilibrium, for which we derive the general linear response theory. As an application we calculate the dynamical conductivity of strong-coupling superconductors.

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arXiv2018

The Effect of Inhomogeneous Surface Disorder on the Superheating Field of Superconducting RF Cavities

Recent advances in surface treatments of Niobium superconducting radio frequency (SRF) cavities have led to substantially increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for such performance enhancements. We report theoretical results for the effects of inhomogeneous surface disorder on the superheating field --- the surface magnetic field above which the Meissner state is globally unstable. We find that inhomogeneous disorder, such as that introduced by infusion of Nitrogen into the surface layers of Niobium SRF cavities, can increase the superheating field above the maximum for superconductors in the clean limit or with homogeneously distributed disorder. Homogeneous disorder increases the penetration of screening current, but also suppresses the maximum supercurrent. Inhomogeneous disorder in the form of an impurity diffusion layer biases this trade-off by increasing the penetration of the screening currents into cleaner regions with larger critical currents, thus limiting the suppression of the screening current to a thin dirty region close to the surface. Our results suggest that the impurity diffusion layers play a role in enhancing the maximum accelerating gradient of Nitrogen treated Niobium SRF cavities.

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arXiv2018

Spontaneous helical order of a chiral p-wave superfluid confined in nano-scale channels

Strong interactions that favor chiral p-wave pairing, combined with strong pair breaking by confining boundaries, are shown to lead to new equilibrium states with different broken symmetries. Based on a strong-coupling Ginzburg-Landau (GL) theory that accurately accounts for the thermodynamics and phase diagram of the bulk phases of superfluid $^3$He, we predict new phases of superfluid $^3$He for confined geometries that spontaneously break rotational and translational symmetry in combination with parity and time-reversal symmetry. One of the newly predicted phases exhibits a unique combination of chiral and helical order that is energetically stable in cylindrical channels of radius approaching the Cooper pair coherence length, e.g. $R\sim 100\,\mbox{nm}$. Precise numerical mimimization of the GL free energy yields a broad region of stability of the helical phase as a function of pressure and temperature, in addition to three translationally invariant phases with distinct broken spin- and orbital rotation symmetries. The helical phase is stable at both high and low pressures and favored by boundaries with strong pair-breaking. We present calculations of transverse NMR frequency shifts as functions of rf pulse tipping angle, magnetic field orientation, and temperature as signatures of these broken symmetry phases.

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arXiv2018

Half-Quantum Vortices in Superfluid Helium

This is the manuscript version of the Physics ViewPoint article introducing the paper "Observation of half-quantum vortices in topological superfluid $^3$He", by S. Autti et al., which appeared in Physical Review Letters, December 14, 2016.

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arXiv2018

Andreev Bound States and Their Signatures

Many of the properties of superconductors related to quantum coherence are revealed when the superconducting state is forced to vary in space - in response to an external magnetic field, a proximity contact, an interface to a ferromagnet, or to impurities embedded in the superconductor. Among the earliest examples is Andreev reflection of an electron into a retro-reflected hole at a normal-superconducting interface. In regions of strong inhomogeneity multiple Andreev reflection leads to the formation of sub-gap states, Andreev bound states, with excitation energies below the superconducting gap. These states play a central role in our understanding of inhomogeneous superconductors. The discoveries of unconventional superconductivity in many classes of materials, advances in fabrication of superconducting/ferromagnetic hybrids and nano-structures for confining superfluid $^3$He, combined with theoretical developments in topological quantum matter have dramatically expanded the significance of branch conversion scattering and Andreev bound state formation. This collection of articles highlights developments in inhomogeneous superconductivity, unconventional superconductivity and topological phases of superfluid $^3$He, in which Andreev scattering and bound states underpin much of the physics of these systems. This article provides an introduction to the basic physics of Andreev scattering, bound-state formation and their signatures. The goal is both an introduction for interested readers who are not already experts in the field, as well as to highlight several examples in which branch conversion scattering and Andreev bound states provide unique signatures in the transport properties of inhomogeneous, topological and unconventional superconductors.

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arXiv2018

Bosonic Surface States and Acoustic Spectroscopy of Confined Superfluid $^3$He-B

Using an effective field theory we study the low-lying bosonic excitations and their couplings to phonons at ultrasonic frequencies in superfluid $^3$He-B under strong confinement. We show that confinement induces a rich spectrum of low-lying bosons, including surface-bound bosonic states, as well as fine structure of long-lived massive bosons. Under sufficiently strong confinement we find a dynamical instability of the $^3$He-B film: the frequency of the surface-bound boson softens at finite wavevector, then develops a pole in the upper half of the complex frequency plane, signalling a dynamical instability of the translationally invariant superfluid vacuum towards pair-density-wave "crystallization". We discuss the signatures and observability of the low-lying bosonic spectrum based on analysis of the ultrasound attenuation from resonant excitation of the bosonic modes. We also note that surface-bound bosonic modes are not unique to $^3$He-B, but are expected to be common to unconventional superconductors with a multi-component order parameter.

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arXiv2017

On Nambu's Fermion-Boson Relations for Superfluid $^3$He-B

Superfluid $^3$He is a spin-triplet ($S=1$), p-wave ($L=1$) BCS condensate of Cooper pairs with total angular momentum $J=0$ in the ground state. In addition to the breaking of $U(1)$ gauge symmetry, separate spin or orbital rotation symmetry is broken to the maximal sub-group, $SO(3)_S\times SO(3)_L\rightarrow SO(3)_J$. The Fermions acquire mass, $m_F\equivΔ$, where $Δ$ is the BCS gap. There are also 18 Bosonic excitations - 4 Nambu-Goldstone (NG) modes and 14 massive amplitude Higgs (AH) modes. The Bosonic modes are labelled by the total angular momentum, $J\in\{0,1,2\}$, and parity under particle-hole symmetry, $C=\pm 1$. For each pair of angular momentum quantum numbers, $J,J_z$, there are two Bosonic partners with $C=\pm 1$. Based this spectrum Nambu proposed a sum rule connecting the Fermion and Boson masses for BCS type theories, which for $^3$He-B is $M_{J^+}^2 + M_{J^-}^2 = 4m_F^2$ for each family of Bosonic modes labelled by $J$, where $M_{J^C}$ is the mass of the Bosonic mode with quantum numbers $(J,C)$. Nambu's sum rule (NSR) has recently been discussed in the context of Nambu-Jona-Lasinio models for physics beyond the standard model to speculate on possible partners to the recently discovered Higgs Boson at higher energies. Here we point out that Nambu's Fermion-Boson mass relations are not exact. Corrections to the Bosonic masses from (i) leading order strong-coupling corrections to BCS theory, and (ii) polarization of the parent Fermionic vacuum lead to violations of the sum-rule. Results for these mass corrections are given in both the $T\rightarrow 0$ and $T\rightarrow T_c$ limits. We also discuss experimental results, and theoretical analysis, for the masses of the $J^{C}=2^{\pm}$ Higgs modes and the magnitude of the violation of the NSR.

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arXiv2016

Electron bubbles and Weyl Fermions in chiral superfluid $^3$He-A

Electrons embedded in liquid $^3$He form mesoscopic bubbles with radii large compared to the interatomic distance between $^3$He atoms, voids of $N_{bubble}\approx 200$ $^3$He atoms, generating a negative ion with a large effective mass that scatters thermal excitations. We develop scattering theory of Bogoliubov quasiparticles by negative ions embedded in $^3$He-A that incorporates the broken symmetries of $^3$He-A, particularly time-reversal and mirror symmetry in a plane containing the chiral axis $\hat{\bf l}$. Multiple scattering by the ion potential, combined with Andreev scattering by the chiral order parameter, leads to a spectrum of Weyl Fermions bound to the ion that support a mass current circulating the electron bubble - the mesoscopic realization of chiral edge currents in superfluid $^3$He-A films. A consequence is that electron bubbles embedded in $^3$He-A acquire angular momentum, ${\bf L}\approx -(N_{bubble}/2)\hbar\,\hat{\bf l}$, inherited from the chiral ground state. We extend the scattering theory to calculate the forces on a moving electron bubble, both the Stokes drag and a transverse force, ${\bf F}_{W} = \frac{e}{c}{\bf v}\times{\bf B}_{W}$, defined by an effective magnetic field, ${\bf B}_{W}\propto\hat{\bf l}$, generated by the scattering of thermal quasiparticles off the spectrum of Weyl Fermions bound to the moving ion. The transverse force is responsible for the anomalous Hall effect for electron bubbles driven by an electric field reported by the RIKEN group. Our results for the scattering cross section, drag and transverse forces on moving ions are compared with experiments, and shown to provide a quantitative understanding of the temperature dependence of the mobility and anomalous Hall angle for electron bubbles in normal and superfluid $^3$He-A. We also discuss our results in relation to earlier theoretical work on negative ions in superfluid $^3$He.

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arXiv2016

Electron Bubbles in Superfluid $^3$He-A: Exploring the Quasiparticle-Ion Interaction

When an electron is forced into liquid $^3$He it forms an "electron bubble", a heavy ion with radius, $R\simeq 1.5$ nm, and mass, $M\simeq 100\,m_3$, where $m_3$ is the mass of a $^3$He atom. These negative ions have proven to be powerful local probes of the physical properties of the host quantum fluid, especially the excitation spectra of the superfluid phases. We recently developed a theory for Bogoliubov quasiparticles scattering off electron bubbles embedded in a chiral superfluid that provides a detailed understanding of the spectrum of Weyl Fermions bound to the negative ion, as well as a theory for the forces on moving electron bubbles in superfluid $^3$He-A (Shevtsov et al. in arXiv:1606.06240). This theory is shown to provide quantitative agreement with measurements reported by the RIKEN group [Ikegami et al., Science 341:59, 2013] for the drag force and anomalous Hall effect of moving electron bubbles in superfluid $^3$He-A. In this report, we discuss the sensitivity of the forces on the moving ion to the effective interaction between normal-state quasiparticles and the ion. We consider models for the quasiparticle-ion (QP-ion) interaction, including the hard-sphere potential, constrained random-phase-shifts, and interactions with short-range repulsion and intermediate range attraction. Our results show that the transverse force responsible for the anomalous Hall effect is particularly sensitive to the structure of the QP-ion potential, and that strong short-range repulsion, captured by the hard-sphere potential, provides an accurate model for computing the forces acting on the moving electron bubble in superfluid $^3$He-A.

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arXiv2016

Strong-coupling and the Stripe phase of $^3$He

Thin films of superfluid $^3$He were predicted, based on weak-coupling BCS theory, to have a stable phase which spontaneously breaks translational symmetry in the plane of the film. This crystalline superfluid, or "stripe" phase, develops as a one dimensional periodic array of domain walls separating degenerate B phase domains. We report calculations of the phases and phase diagram for superfluid $^3$He in thin films using a strong-coupling Ginzburg-Landau theory that accurately reproduces the bulk $^3$He superfluid phase diagram. We find that the stability of the Stripe phase is diminished relative to the A phase, but the Stripe phase is stable in a large range of temperatures, pressures, confinement, and surface conditions.

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arXiv2015

Superfluid phases of $^3$He in nano-scale channels

Confinement of superfluid $^3$He on length scales comparable to the radial size of the p-wave Cooper pairs can greatly alter the phase diagram by stabilizing broken symmetry phases not observed in bulk $^3$He. We consider superfluid $^3$He confined within long cylindrical channels of radius $100\mbox{ nm}$, and report new theoretical predictions for the equilibrium superfluid phases under strong confinement. The results are based on the strong-coupling formulation of Ginzburg-Landau theory with precise numerical minimization of the free energy functional to identify the equilibrium phases and their regions of stability. We introduce an extension of the standard GL strong-coupling theory that accurately accounts for the phase diagram at high pressures, including the tri-crital point and $T_{AB}(p)$ line defining the region of stability for the bulk A-phase. We also introduce tuneable boundary conditions that allow us to explore boundary scattering ranging from maximal to minimal pairbreaking, and report results for the phase diagram as a function of pressure, temperature, and boundary conditions. Four stable phases are found: a polar phase stable in the vicinity of $T_c$, a strongly anisotropic, cylindrical analog of the bulk B phase stable at sufficiently low temperatures, and two chiral A-like phases with distinctly different orbital symmetry, one of which spontaneously breaks rotation symmetry about the axis of the cylindrical channel. The relative stability of these phases depends sensitively on pressure and the degree of pairbreaking by boundary scattering. The broken symmetries exhibited by these phases give rise to distinct signatures in transverse NMR resonance spectroscopy. We present theoretical results for the transverse NMR frequency shifts as functions of temperature, the r.f. pulse tipping angle and the static NMR field orientation.

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arXiv2015

Anisotropy and Strong-Coupling Effects on the Collective Mode Spectrum of Chiral Superconductors: Application to Sr$_2$RuO$_4$

Recent theories of Sr$_2$RuO$_4$ based on the interplay of strong interactions, spin-orbit coupling and multi-band anisotropy predict chiral or helical ground states with strong anisotropy of the pairing states, with deep minima in the excitation gap, as well as strong phase anisotropy for the chiral ground state. We develop time-dependent mean field theory to calculate the Bosonic spectrum for the class of 2D chiral superconductors spanning $^3$He-A to chiral superconductors with strong anisotropy. Chiral superconductors support a pair of massive Bosonic excitations of the time-reversed pairs labeled by their parity under charge conjugation. These modes are degenerate for 2D $^3$He-A. Crystal field anisotropy lifts the degeneracy. Strong anisotropy also leads to low-lying Fermions, and thus to channels for the decay of the Bosonic modes. Selection rules and phase space considerations lead to large asymmetries in the lifetimes and hybridization of the Bosonic modes with the continuum of un-bound Fermion pairs. We also highlight results for the excitation of the Bosonic modes by microwave radiation that provide clear signatures of the Bosonic modes of an anisotropic chiral ground state.

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arXiv2013

Superfluid phases of $^3$He in a periodic confined geometry

Predictions and discoveries of new phases of superfluid $^3$He in confined geometries, as well as novel topological excitations confined to surfaces and edges of near a bounding surface of $^3$He, are driving the fields of superfluid $^3$He infused into porous media, as well as the fabrication of sub-micron to nano-scale devices for controlled studies of quantum fluids. In this report we consider superfluid $^3$He confined in a periodic geometry, specifically a two-dimensional lattice of square, sub-micron-scale boundaries ("posts") with translational invariance in the third dimension. The equilibrium phase(s) are inhomogeneous and depend on the microscopic boundary conditions imposed by a periodic array of posts. We present results for the order parameter and phase diagram based on strong pair breaking at the boundaries. The ordered phases are obtained by numerically minimizing the Ginzburg-Landau free energy functional. We report results for the weak-coupling limit, appropriate at ambient pressure, as a function of temperature T, lattice spacing L, and post edge dimension, $d$. For all $d$ in which a superfluid transition occurs, we find a transition from the normal state to a periodic, inhomogeneous "polar" phase with $T_{c_1} < T_c$ for bulk superfluid $^3$He. For fixed lattice spacing, L, there is a critical post dimension, d_c, above which only the periodic polar phase is stable. For $d < d_c$ we find a second, low-temperature phase onsetting at $T_{c_2} < T_{c_1}$ from the polar phase to a periodic "B-like" phase. The low temperature phase is inhomogeneous, anisotropic and preserves time-reversal symmetry, but unlike the bulk B-phase has only $D_{4h}$ point symmetry.

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arXiv2013

Majorana Excitations, Spin- and Mass Currents on the Surface of Topological Superfluid $^3$He-B

The B-phase of superfluid $^3$He is a 3D time-reversal invariant (TRI) topological superfluid with an isotropic energy gap separating the ground-state and bulk continuum states. We report calculations of surface spectrum, spin- and mass current densities originating from the Andreev surface states for confined $^3$He-B. The surface states are Majorana Fermions with their spins polarized transverse to their direction of propagation along the surface. The negative energy states give rise to a ground-state helical spin current confined on the surface. The spectral functions reveal the subtle role of the spin-polarized surface states in relation to the ground-state spin current. By contrast, these states do not contribute to the $T=0$ mass current. Superfluid flow through a channel of confined $^3$He-B is characterized by the flow field, $p_s$. The flow field breaks 2D rotational symmetry and time reversal (T). However, the Bogoliubov-Nambu Hamiltonian remains invariant under $T$ combined with a 180 degree rotation about the normal to the film. As a result the B-phase in the presence of a superflow remains a topological phase with a gapless spectrum of Majorana modes on the surface. Thermal excitation of the Doppler shifted Majorana branches leads to a power law suppression of the superfluid mass current, providing a direct signature of the Majorana branches of surface excitations in the fully gapped 3D topological superfluid, $^3$He-B. Results are reported for the superfluid fraction (mass current) and helical spin current for confined $^3$He-B, including the temperature dependences, as well as dependences on confinement, interactions between quasiparticles and pressure.

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arXiv2013

Collective Modes and Nonlinear Acoustics in Superfluid 3He-B

We discuss the relationship of collisionless sound propagation and attenuation to the order parameter collective modes of superfluid 3He-B. These modes, which reflect the symmetries of the normal state as well as the broken gauge and relative rotational symmetries of the condensate, play a central role in the high-frequency acoustic response of the superfluid. We review the theory of acoustic spectroscopy based on the linear coupling of the J=2+/- modes. We develop the theory of the nonlinear acoustic response of superfluid 3He-B for the case of a three-wave resonance between two sound waves of different frequencies and the J=2+/- modes, as well as the generation of higher harmonic sound waves. The J=2+ modes couple nonlinearly to two zero-sound waves. The predictions for the nonlinear absorption and velocity anomalies resulting from the J=2+ modes depend on relatively well understood material properties of 3He.

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arXiv2013

Chiral Phases of Superfluid 3He in an Anisotropic Medium

I report theoretical analysis and predictions for the equilibrium phases of superfluid 3He infused into a low-density, homogeneous uniaxial aerogel. Ginzburg-Landau (GL) theory for a class of equal-spin-pairing (ESP) states in a medium with uniaxial anisotropy is developed and used to analyze recent experiments on uniaxially strained aerogels. For 3He in an axially "stretched" aerogel GL theory predicts a transition from normal liquid into a chiral ABM phase at T_c1 in which the chirality axis, l, is aligned along the strain axis. This orbitally aligned state, is protected from random fluctuations in the anisotropy direction, has a positive NMR frequency shift, a sharp NMR resonance line and is identified with the high-temperature ESP-1 phase of superfluid 3He in axially stretched aerogel. A second transition into a biaxial phase is predicted to onset at a slightly lower temperature, T_c2 < T_c1. This phase is an ESP state, breaks time-reversal symmetry, and is defined by an orbital order parameter that spontaneously breaks axial rotation symmetry. This transition is driven by the coupling of an axially aligned 1D "polar" order parameter to the two time-reversed 2D axial ABM states. The biaxial phase has a continuous degeneracy associated with the projection of its chiral axis in the plane normal to the anisotropy axis. Theoretical predictions for the NMR frequency shifts of the biaxial phase provide an identification of the ESP-2 as the biaxial state, partially disordered by random anisotropy (Larkin-Imry-Ma effect). The "width" of the jump in the NMR frequency shift at T_c2 provides an estimate of the orbital domain size, xi_LIM ~ 5 mu-m at 18 bar.

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