James A. Sauls

Louisiana State University | LSU · Department of Physics & Astronomy

The B-phase of superfluid 3He 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 3He-B. The surface states are Majorana Fe...

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 emb...

Nematic superconductivity with spontaneously broken rotation symmetry has recently been reported in doped topological insulators, MxBi2Se3 (M=Cu, Sr, Nb). Here we show that the electromagnetic (EM) response of these compounds provides a spectroscopy for bosonic excitations that reflect the pairing channel and the broken symmetries of the ground sta...

Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk and edge Fermionic excitations, and the structure of the impurity potential. In the case of electronic heat transport, impurities induce an anomalous (zero-field) thermal Hall conductivity...

My contribution to this collection of articles in honor of David Lee and John Reppy on their 90th birthdays is a reflection on the remarkable phenomenology of the excitation spectra of superfluid 3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrs...

Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D) consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum, L z = (N/2) ħ, for N...

The thermodynamic potential for superfluid \(^3\)He-B embedded in a homogeneously distributed random potential is calculated from a quasiclassical reduction of the Luttinger–Ward functional to leading order in \({\mathbf s} =k_{\text {B}}T_c/E_f\). The resulting functional provides an extension of the Ginzburg–Landau free energy functional to all t...

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, the London penetration depth - based quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provi...

We present results for the resonant frequency shift and quality factor of disordered Nb SRF cavities driven out of equilibrium by the resonant microwave field. The theory is based on the nonequilibrium theory of superconductivity for the current response to the electromagnetic field at the vacuum-metal interface. We are able to accurately predict t...

We present a novel transmon qubit fabrication technique that yields systematic improvements in T$_1$ coherence times. We fabricate devices using an encapsulation strategy that involves passivating the surface of niobium and thereby preventing the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying th...

Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. This comprehensive study of the Nb surface oxide decomposition mechanism led by collaborators at Ames National Laboratory may guide future superconducting qubit device optimization through control or minimization of interfacial scatter...

Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, Tc=9.32 K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magnetosensing, radio-frequency caviti...

The thermodynamic potential for superfluid $^3$He-B embedded in a homogeneously distributed random impurity potential is calculated from a quasiclassical reduction of the Luttinger-Ward functional to leading order in $k_{\mbox{$\tiny B$}} T_c/E_f$, i.e. the weak-coupling limit. Theoretical predictions for the heat capacity are shown to be in good a...

Niobium is one of the most researched superconductors, both theoretically and experimentally. It is enormously significant in all branches of superconducting applications, from powerful magnets to quantum computing. It is therefore imperative to understand its fundamental properties in great detail. Here, we use the results of recent microscopic ca...

Recently reported anomalies in frequency shift of order $\delta f\sim 0.1 -10\,\mbox{kHz}$ for Niobium SRF cavities in a narrow temperature region near $T_c\simeq 9\,\mbox{K}$ are sensitive to the surface preparation and Nitrogen doping. We developed methods for calculating the surface current response of Nb SRF cavities, as well as the resonant fr...

NMR experiments on liquid $^3$He infused into uniaxially anisotropic silica aerogels show the stabilisation of two equal-spin-pairing chiral phases on cooling from the normal phase. The alignment of the chiral axis relative to the anisotropy axis for these phases is predicted to depend upon temperature. A chiral A-like phase is also stabilized when...

We report theoretical results for heat transport by quasiparticle excitations in superfluid \(^3\)He infused into silica aerogel engineered with uniaxial anisotropy. For this system, two distinct equal spin pairing (ESP) superfluid phases have been reported based on NMR spectroscopy. Theoretical analysis predicts the first ESP state to be the chira...

Liquid 3He\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^3\mathrm {He}$$\end{document} is a Fermi liquid that undergoes a BCS-type phase transition to a spin-triplet...

Niobium superconducting radio-frequency (SRF) cavities for high energy accelerator applications have been greatly improved in terms of the quality factor $Q$ by techniques such as Nitrogen doping. However, the mechanisms leading improvement in $Q$ are still not fully understood. Quite recently the SRF group at Fermilab measured anomalies in the fre...

Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, $T_{c}=9.33$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magneto-sensing, radio-frequency...

Niobium is one of the most researched superconductors, both theoretically and experimentally. It is enormously significant in all branches of superconducting applications, from powerful magnets to quantum computing. It is, therefore, imperative to understand its fundamental properties in great detail. Here we use the results of recent microscopic c...

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive anal...

Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. The coherence time of the superconducting qubits is mainly limited by microwave dissipation attributed to two-level system defects at interfaces, such as the Nb/Si and Nb/air interface. One way to improve the Nb/air interface quality i...

Radio frequency superconductivity is a cornerstone technology for many future HEP particle accelerators and experiments from colliders to proton drivers for neutrino facilities to searches for dark matter. While the performance of superconducting RF (SRF) cavities has improved significantly over the last decades, and the SRF technology has enabled...

This short article on a thermodynamic signature of Majorana fermions confined on the surface of a 3D topological superfluid in the presence of a moving condensate was circulated to a limited audience in Fall 2013. Since recent experiments are looking at possible signatures of a gas of Majorana excitations confined in two dimensions on the surface o...

My contribution to this collection of articles in honor of David Lee and John Reppy on their 90th birthdays is a reflection on the remarkable phenomenology of the excitation spectra of superfluid $^3$He, in particular the B-phase which was identified by NMR and acoustic spectroscopy as Balian-Werthamer state shown in 1963 to be the ground state of...

We suggest a program to establish theoretical performance limits of srf cavities using modern theories of nonequilibrium superconductivity under a strong electromagnetic field. These theories will be used to calculate the main parameter of merit of srf cavities: the quality factor Q and its dependencies on the field amplitude, temperature and frequ...

I present a review of the theory and basic equations for charge transport in superconducting alloys starting from the Keldysh formulation of the quasiclassical transport equations developed by Eilenberger, Larkin and Ovchinnikov and Eliashberg. This formulation is the natural extension of Landau’s theory of normal Fermi liquids to the superconducti...

The low temperature properties of a wide range of many-fermion systems spanning metals, quantum gases and liquids to nuclear matter are well understood within the framework of Landau’s theory of Fermi liquids. The low-energy physics of these systems is governed by interacting fermionic quasiparticles with momenta and energies near a Fermi surface i...

The low-temperature properties of a wide range of many-fermion systems spanning metals, quantum gases and liquids to nuclear matter are well understood within the framework of Landau’s theory of Fermi liquids. The low-energy physics of these systems is governed by interacting fermionic quasiparticles with momenta and energies near a Fermi surface i...

I present a review of the theory and basic equations for charge transport in superconducting alloys starting from the Keldysh formulation of the quasiclassical transport equations developed by Eilenberger, Larkin and Ovchinnikov and Eliashberg. This formulation is the natural extension of Landau's theory of normal Fermi liquids to the superconducti...

We report results for the superconducting transition temperature and anisotropic energy gap for pure Niobium based on Eliashberg's equations and electron and phonon band structures computed from density functional theory. The electronic band structure is used to construct the Fermi surface and calculate the Fermi velocity at each point on the Fermi...

The discovery of superfluidity in 3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^3$$\end{document}He in 1971, published in 1972, [1, 2] has influenced a wide range o...

We report theoretical results for heat transport by quasiparticle excitations in superfluid $^3$He infused into silica aerogel engineered with uniaxial anisotropy. For this system two distinct equal spin pairing (ESP) superfluid phases have been reported based on NMR spectroscopy. Theoretical analysis predicts the first ESP state to be the chiral A...

The leading corrections to Fermi liquid theory for non-equilibrium quasiparticle transport near a Cooper instability arise from the virtual emission and absorption of incipient Cooper pairs. We formulate the corrections to the Landau-Boltzmann transport equation starting from Keldysh's field theory for non-equilibrium, strongly interacting Fermions...

Liquid $^3\mathrm{He}$ is a Fermi liquid that undergoes a BCS-type phase transition to a spin-triplet superfluid, making it valuable for understanding interacting fermions. When the temperature approaches the transition temperature $T_{\mathrm{c}}$ from above, physical properties can be modified by Cooper pair fluctuations, leading to deviations fr...

The low temperature properties of a wide range of many-fermion systems are well understood within the framework of Landau's theory of Fermi liquids. The low-energy physics of these systems is governed by interacting fermionic quasiparticles with momenta and energies near a Fermi surface in momentum space. Nonequilibrium properties are described by...

The discovery of superfluidity in 3He in 1971, published in 1972, [1, 2] has influenced a wide range of investigations that extend well beyond fermionic superfluids, including electronic quantum ma- terials, ultra-cold gases and degenerate neutron matter. Observation of thermodynamic transitions from the 3He Fermi liquid to two other liquid phases,...

We report theoretical results for the stability of half-quantum vortices (HQVs) in the superfluid phases of $^3$He confined in highly anisotropic Nafen aerogel. Superfluidity of $^3$He confined in Nafen is the realization of a "nematic superfluid" with Cooper pairs condensed into a single p-wave orbital aligned along the anisotropy axis of the Nafe...

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 th...

Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk and edge Fermionic excitations, and the structure of the impurity potential. In the case of electronic heat transport, impurities induce an anomalous (zero-field) thermal Hall conductivity...

Nonequilibrium conditions imposed by neutrino cooling through the liquid-solid transition lead to disorder in the solid crust of neutron stars. Disorder reduces the superfluid fraction, $\rho_s/\rho$, at densities above that of neutron drip, $\rho_d \approx 4 \times 10^{11} \mbox{g/cm}^3$. For an amorphous solid crust the suppression of $\rho_s$ is...

We present a theoretical calculation of the pressure-temperature-field phase diagram for the vortex phases of rotating superfluid He3−B. Based on a strong-coupling Ginzburg-Landau functional that accounts for the relative stability of the bulk A and B phases of He3 at all pressures, we report calculations for the internal structure and free energie...

Nonequilibrium conditions imposed by neutrino cooling through the liquid-solid transition lead to disorder in the solid crust of neutron stars. Disorder reduces the superfluid fraction, $\rho_s/\rho$, at densities above that of neutron drip, $\rho_d \approx 4\times 10^{11}\,g/cm^3$. For an amorphous solid crust the suppression of $\rho_s$ is small,...

We report theoretical results for the electronic contribution to thermal and electrical transport for chiral superconductors belonging to even or odd-parity E$_1$ and E$_2$ representations of the tetragonal and hexagonal point groups. Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topol...

Recent advances in the surface treatments of niobium superconducting radio-frequency (SRF) cavities have led to substantially increased Q factors and a 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 surf...

We present the first theoretical calculation of the pressure-temperature-field phase diagram for the vortex phases of rotating superfluid $^3$He-B. Based on a strong-coupling extension of the Ginzburg-Landau theory that accounts for the relative stability of the bulk A and B phases of $^3$He at all pressures, we report calculations for the internal...

I review some of the ideas that have been proposed for the structure of neutron star interiors, and concentrate on the theoretical arguments for the existence of superfluidity in neutron stars. I also discuss the implications of neutron superfluidity and proton superconductivity for the rotational dynamics of pulsars, and review arguments that have...

Circuit quantization links a physical circuit to its corresponding quantum Hamiltonian. The standard quanti-zation procedure generally assumes any external magnetic flux to be static. Time dependence naturally arises, however, when flux is modulated or when flux noise is considered. In this case, application of the existing quan-tization procedure...

Circuit quantization links a physical circuit to its corresponding quantum Hamiltonian. The standard quantization procedure generally assumes any external magnetic flux to be static. Time dependence naturally arises, however, when flux is modulated or when flux noise is considered. In this case, application of the existing quantization procedure ca...

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...

Liquid helium-3 and helium-4 are remarkable substances. They are quantum liquids, meaning that their behavior is governed by the laws of quantum mechanics. Because of their small atomic mass, each isotope exists in a liquid state down to the temperature of absolute zero. And at sufficiently low temperature, each becomes a superfluid. However, the t...

Nematic superconductivity with spontaneously broken rotation symmetry has recently been reported in doped topological insulators, $M_x$Bi$_2$Se$_3$ ($M$=Cu, Sr, Nb). Here we show that the electromagnetic (EM) response of these compounds provides a spectroscopy for bosonic excitations that reflect the pairing channel and the broken symmetries of the...

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 di...

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 di...

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 extension of the Ginzburg-Landau theory that accurately accounts for the thermodynamics and phase diagram of the bulk phases of sup...

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. Amoung the earliest examples is Andreev reflection of an elect...

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 electr...

Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D) $^3$He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum, $L_z = (N/2...

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...

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. Und...

Superfluid He3 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 subgroup SO(3)S×SO(3)L→SO(3)J. The fermions acquire mass mF≡Δ, where Δ is the BCS gap. There...

Researchers working with superfluid helium in a porous medium have generated vortices with half the quantum unit of fluid flow. Comment: This is a ViewPoint article written to highlight the discovery and publication of the ``Observation of half-quantum vortices in topological superfluid 3He'', by S. Autti et al., appearing in Physical Review Lette...

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,...

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 sup...

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 calculatio...

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 fo...

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 fie...

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...

Predictions and discoveries of new phases of superfluid 3He in confined geometries, as well as novel topological excitations confined to surfaces and edges of near a bounding surface of 3He, are driving the fields of superfluid 3He infused into porous media, as well as the fabrication of sub-micron to nano-scale devices for controlled studies of qu...

The surface excitation spectrum of superfluid ^3He-B is discussed for a translationally invariant interface (specular surface). We report calculations of surface spectral spin-current and mass current densities originating from the Andreev bound state and the continuum response. Two branches of gapless Fermions, bound to the surface, disperse linea...