Maxim Dzero

Dynamical Vanishing of the Order Parameter in a Fermionic Condensate

We analyze the dynamics of a condensate of ultracold atomic fermions following an abrupt change of the pairing strength. At long times, the system goes to a nonstationary steady state, which we determine exactly. The superfluid order parameter asymptotes to a constant value. We show that the order parameter vanishes when the pairing strength is decreased below a certain critical value. In this case, the steady state of the system combines properties of normal and superfluid states -- the gap and the condensate fraction vanish, while the superfluid density is nonzero.

Superconductivity in Charge Kondo Systems

We present a theory for superconductivity and charge Kondo fluctuations, i.e., resonant quantum valence fluctuations by two charge units, for Tl-doped PbTe. We show that Tl is very special as it first supplies a certain amount of charge carriers to the PbTe-valence band and then puts itself into a self-tuned resonant state to yield a new, robust pairing mechanism for these carriers.

Electron Cotunneling into a Kondo Lattice

Motivated by recent experimental interest in tunneling into heavy-electron materials, we present a theory for electron tunneling into a Kondo lattice. The passage of an electron into a Kondo lattice is accompanied by a simultaneous spin flip of the localized moments via cotunneling mechanism. We compute the tunneling current with the large-N mean field theory. In the absence of disorder, differential tunneling conductance exhibits two peaks separated by the hybridization gap. Disorder effects lead to the smearing of the gap resulting in a Fano line shape.

Activated events in glasses : The structure of entropic droplets

Using an effective potential approach, we present a replica instanton theory for the dynamics of entropic droplets in glassy systems. Replica symmetry breaking in the droplet interface leads to a length scale dependent reduction of the droplet surface tension and changes the character of the dynamical heterogeneity and activated dynamics in glasses.

Heavy electrons and the symplectic symmetry of spin

The recent discovery of two heavy-fermion materials PuCoGa5 and NpPd5Al2, which transform directly from Curie paramagnets into superconductors, reveals a new class of superconductors where local moments quench directly into the superconducting condensate. Unlike other heavy-electron superconductors, where Cooper pairing is thought to be driven by spin fluctuations, these higher-transition-temperature materials do not seem to be close to a magnetic instability. Large-N expansions have been invaluable in describing heavy-fermion metals, but so far cannot treat superconductivity. Here, we introduce a new class of large-N expansion that uses symplectic symmetry to protect the odd time-reversal parity of spin and sustain Cooper pairs as well-defined singlets. We show that when a lattice of magnetic ions exchange spin with their metallic environment in two distinct symmetry channels, they can simultaneously satisfy both channels by forming a condensate of composite pairs between local moments and electrons. In the tetragonal crystalline environment relevant to PuCoGa5 and NpPd5Al2, the lattice structure selects a natural pair of spin exchange channels, predicting a unique anisotropic paired state with either d- or g-wave symmetry. This pairing mechanism also predicts a large upturn in the NMR relaxation rate above Tc and strong enhancement of Andreev reflection in tunnelling measurements. The Kondo problem—dealing with localized magnetic impurities embedded in a sea of conduction electrons—can be treated on an equal footing with superconductivity for a large system of interacting electrons.

Quantum critical end point for the Kondo volume collapse model.

The Kondo volume collapse describes valence transitions in f-electron metals and is characterized by a line of first order transitions in the pressure-temperature phase plane terminated at critical end points. We analyze the quantum critical end point, when the lower end point is tuned to T=0, and determine the specific heat, thermal expansion, and compressibility. We find that the inclusion of quantum critical fluctuations leads to a novel bifurcation of the first order phase line. Finally, we show that critical strain fluctuations can cause both, superconductivity and non-Fermi liquid behavior near the critical point.

Replica theory for fluctuations of the activation barriers in glassy systems

We consider the problem of slow activation dynamics in glassy systems undergoing a random first order phase transition. Using an effective potential approach to supercooled liquids, we determine the spectrum of activation barriers for entropic droplets. We demonstrate that fluctuations of the configurational entropy and of the liquid glass surface tension are crucial to achieve an understanding of the barrier fluctuations in glassy systems and thus are ultimatively responsible for the broad spectrum of excitations and heterogeneous dynamics in glasses. In particular we derive a relation between the length scale for dynamic heterogeneity and the related barrier fluctuations. Diluted entropic droplets are shown to have a Gaussian distribution of barriers, strongly suggesting that non-Gaussian behavior results from droplet-droplet interactions.

Surface-dominated conduction up to 240[thinsp]K in the Kondo insulator SmB6 under strain

SmB6 is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T* ~ 4 K limiting its scientific investigation and device application. Here, we report the enhancement of T * in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be indued in epitaxial SmB6 films via substrate in potential device applications.

Cooper pair turbulence in atomic Fermi gases

We investigate under what conditions a uniform quench of a superfluid atomic Fermi gas leads to the emergence of spatial inhomogeneities. We demonstrate that, if the system is larger than the coherence length, the superfluid order parameter becomes spatially nonuniform. Spatial modulations develop through a parametric excitations of pairing modes with opposite momenta. Their growth is eventually suppressed by nonlinear effects resulting in a state characterized by a random superposition of wave packets of the superfluid order parameter. This state can be probed by measuring the molecular momentum distribution following a fast sweep to the BEC side of the Feshbach resonance.

Spectroscopic Signatures of Nonequilibrium Pairing in Atomic Fermi Gases

We determine the radio-frequency (rf) spectra for nonstationary states of a fermionic condensate produced by a rapid switch of the scattering length. The rf spectrum of the nonequilibrium state with constant BCS order parameter has two features in contrast with equilibrium where there is a single peak. The additional feature reflects the presence of excited pairs in the steady state. In the state characterized by periodically oscillating order parameter, the rf-absorption spectrum contains two sequences of peaks spaced by the frequency of oscillations. Satellite peaks appear due to a process where a rf photon in addition to breaking a pair emits or absorbs oscillation quanta.