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At least 19 recordsLinked to original sources

arXiv2010

Non-Fermi-Liquid-Like Behaviors and Superconductivity Driven by Orbital Fluctuations in Iron Pnictides: Analysis by Fluctuation-Exchange Approximation

We study the five-orbital Hubbard-Holstein model for iron pnictides with small electron-phonon interaction due to Fe-ion Einstein oscillators. Using the fluctuation-exchange (FLEX) approximation, orbital fluctuations evolve inversely proportional to the temperature, and therefore the resistivity shows linear or convex T-dependence for wide range of temperatures. We also analyze the Eliashberg gap equation, and show that s-wave superconducting state without sign reversal (s_{++}-wave state) emerges when the orbital fluctuations dominate the spin fluctuations. When both fluctuations are comparable, their competition gives rise to a nodal s-wave state. The present study offers us a unified explanation for both the normal and superconducting states.

cond-mat.supr-con↗

arXiv2012

Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES

Despite many ARPES investigations of iron pnictides, the structure of the electron pockets is still poorly understood. By combining ARPES measurements in different experimental configurations, we clearly resolve their elliptic shape. Comparison with band calculation identify a deep electron band with the dxy orbital and a shallow electron band along the perpendicular ellipse axis with the dxz/dyz orbitals. We find that, for both electron and hole bands, the lifetimes associated with dxy are longer than for dxz/dyz. This suggests that the two types of orbitals play different roles in the electronic properties and that their relative weight is a key parameter to determine the ground state.

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arXiv2007

Multi-Higgs U(1) Lattice Gauge Theory in Three Dimensions

We study the three-dimensional compact U(1) lattice gauge theory with $N$ Higgs fields numerically. This model is relevant to multi-component superconductors, antiferromagnetic spin systems in easy plane, inflational cosmology, etc. For N=2, the system has a second-order phase transition line $\tilde{c}_1(c_2)$ in the $c_2$(gauge coupling)$-c_1$(Higgs coupling) plane, which separates the confinement phase and the Higgs phase. For N=3, the critical line is separated into two parts; one for $c_2 \alt 2.25$ with first-order transitions, and the other for $c_2 \agt 2.25$ with second-order transitions.

cond-mat.supr-con↗

arXiv2007

New fluctuation-driven phase transitions and critical phenomena in unconventional superconductors

Using the renormalization group method, new type of fluctuation-driven first order phase transitions and critical phenomena are predicted for certain classes of ferromagnetic superconductors and superfluids with unconventional (spin-triplet) Cooper pairing. The problem for the quantum phase transitions at extremely low and zero temperatures is also discussed. The results can be applied to a wide class of ferromagnetic superconductive and superfluid systems, in particular, to itinerant ferromagnets as UGe2 and URhGe.

cond-mat.supr-con↗

arXiv2008

Corbino experimental set-up for Cooper pair mass spectroscopy and determination of mobility in normal phase

We are suggesting an electronic method for Cooper pair mass spectroscopy of thin superconducting films. The method can be applied, for example, for 100 nm thin high-T_c films grown on insulator substrate 10 mm X 10 mm. In Corbino geometry two Ohmic contacts have to be made on the film evaporating Ag or using silver paste: one circle with radius r_a (1) and a ring electrode with internal radius r_b (2). In the in-between space (r_a,r_b) a ring electrode from mylar assures a capacitive connection between the superconducting layer and the metallized surface (3) of the mylar. In such a way we have an field effect transistor (FET) type structure with a circular gate. When at low enough temperatures T << T_c an AC current with frequency omega is applied between the circle source (1) and the ring-shaped drain (2) an AC Bernoulli voltage with double frequency 2f appears between the gate (3) and the source (1). The 2f signal depends on Cooper pair effective mass and its systematic investigation gives a Cooper pair mass spectroscopy. In the normal phase 2f gives logarithmic derivative of the density of states (DOS) with respect of Fermi energy. Applying a gate voltage in the same structure gives the mobility of the normal phase.

cond-mat.supr-con↗

arXiv2007

On the formation of gyration-like excitations in solid solutions

Measurements of thermal excitations of V-O and Y-Ba-Cu-O solid solutions have been performed by a method of inelastic neutron scattering for low-energy transfers range. Features at energy transfer about 3meV and about 5meV for V-O and Y-Ba-Cu-O accordingly were observed in neutron scattering spectra. The reason of appearance of the observed low-energy excitations are the formation of an effective potential of oxygen with broad weakly upwards bottom at the xy plane of tetragonal symmetry octahedron, which the interstitial atom occupies. The experimental observed features can be understood in the representation of hindered quantum gyration of the interstitial atom. It was assumed that O1 and O4 atoms in Y-Ba-Cu-O, as well as in V-O system, are in two-dimensional quantum-well, which has almost flat bottom formed by the surrounding atoms Cu and Ba. The possibility of formation of collective excitations of gyration-type as a result of exchange interaction of light atoms in matrix lattice is considered. The assumption is made that the existence of gyration-like excitations can play an important role for explanation of some phenomena in solid state physics, in particular in realization of high values of temperatures of superconducting transitions in high temperature superconductors.

cond-mat.supr-con↗

arXiv2007

The critical temperature for the BCS equation at weak coupling

For the BCS equation with local two-body interaction $λV(x)$, we give a rigorous analysis of the asymptotic behavior of the critical temperature as $λ\to 0$. We derive necessary and sufficient conditions on $V(x)$ for the existence of a non-trivial solution for all values of $λ>0$.

cond-mat.supr-con↗

arXiv2011

Superconducting antenna for detection of gravitational waves

Combining the principle of magnetic flux quantization inside a superconducting loop and existence of rigid platforms (i.e., solids, non-deformable under the action of gravitational waves) a design for gravitational wave antenna is suggested. This design could yield a non-resonant detector, with modest sizes and capability to generate detectable signals for gravitational waves from astrophysical sources.

cond-mat.supr-con↗

arXiv2007

s-wave symmetry along the c-axis and s+d in-plane superconductivity in bulk YBa_2Cu_4O_8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth $λ$ was measured along the crystallographic directions $a$, $b$, and $c$ in single crystals of YBa$_2$Cu$_4$O$_8$ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of $λ_a^{-2}$ and $λ_b^{-2}$ exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with $s+d-$wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.

cond-mat.supr-con↗

arXiv2007

The Origin of Fermi Arcs in Cuprate Pseudogap States and Strong Constraints on Viable Theories of High-Temperature Superconductivity

A full Fermi surface exists in underdoped high-temperature superconductors if the temperature T lies above the pseudogap temperature T*. Below T* only arcs of Fermi surface survive, scaling with T/T* as T -> 0, with T* displaying strong doping dependence. There is no accepted explanation for this behavior. We show that generalizing the BCS theory of normal superconductivity to include d-wave pairs and antiferromagnetism leads to the origin and doping dependence of the T* scale, and a quantitative description of Fermi arcs. These results place strong constraints on viable theories of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2007

Is relaxation correlated in superconducting qubits?

We consider coupled quantum two-state systems (qubits) exposed to a global relaxation process. The global relaxation refers to the assumption that qubits are coupled to the same quantum bath with approximately equal strengths, appropriate for long-wavelength environmental fluctuations. We show that interactions do not spoil the picture of Dicke's subradiant and superradiant states where quantum interference effects lead to striking deviations from the independent relaxation picture. Remarkably, the system possess a stable entangled state and a state decaying faster than single qubit excitations. We propose a scheme how these effects can be experimentally accessed in superconducting flux qubits and, possibly, used in constructing long-lived entangled states.

cond-mat.mes-hall↗

arXiv2007

Effect of superlattice modulation of electronic parameters on superconducting density of states in cuprate superconductors

Recent scanning tunneling microscopy on BSCCO 2212 has revealed a substantial spatial supermodulation of the energy gap in the superconducting state. We propose that this gap modulation is due to the superlattice modulations of the atoms in the structure, and hence the parameters in a microscopic model of the CuO2 plane. The gap modulation is estimated using renormalized mean field theory for a t-t'-J model on a superlattice. The results compare well with experiment.

cond-mat.supr-con↗

arXiv2007

Collective Modes in Two-band Superconductors

We analyze collective modes in two-band superconductors in the dirty limit. It is shown that these modes exist at all temperatures $T$ below $T_{c}$ provided the frequency of the modes is higher than the inelastic scattering rate and lower than the energy gaps $Δ_{a,b}$. At low temperatures these modes are related to counterphase oscillations of the condensate currents in each band. The spectrum of the collective oscillations is similar to the spectrum of the Josephson ''plasma'' modes in a tunnel Josephson junction but the velocity of the mode propagation in the case under consideration is much lower. At higher temperatures ($Δ_{b}<T<T_{c}$) the spectrum consists of two branches. One of them is gapless (sound-like) and the second one has a threshhold that depends on coupling between the bands. We formulate the conditions under which both types of collective modes can exist. The spectrum of the collective modes can be determined by measuring the I-V characteristics of a Josephson junction in a way as it was done by Carlson and Goldman.

cond-mat.supr-con↗

arXiv2008

Evidences for pairing of nearly-free quasiparticles from paraconductivity in layered superconducting cuprates

We revisit the Aslamazov-Larkin theory of paraconductivity in two dimensions, to distinguish its universal features from the specific features of nearly-free paired fermions. We show that both the numerical prefactor and the temperature dependence of the experimental paraconductivity in underdoped La_{2-x}Sr_xCuO_4 are only compatible with pairing of nearly-free fermionic quasiparticles. This conclusion is strengthened by the analysis of paraconductivity data in the presence of a finite magnetic field, from which we extract a rather low value of the critical field H_{c2}(T=0).

cond-mat.supr-con↗

arXiv2007

Theory of Neutron Scattering in High-T$_c$ Cuprates: Two Component Spin-Fermion Model

Recent neutron scattering experiments have revealed that the generic form of the magnetic excitations in the high-Tc cuprates of wide range of doping has the so-called "hourglass" shape; it features both upward and downward excitations at the incommensurate (IC) momenta spanning from the resonance peak at the commensurate momentum $(π,π)$. We propose the two-component spin-fermion model as a minimal phenomenological model which has both local spins and itinerant fermions as independent degrees of freedom. Our calculations of the dynamic spin correlation function provide good agreement with experiments and show: (1) the upward dispersion branch of magnetic excitations is mostly due to the local spin excitations; (2) the downward dispersion branch is from collective spin excitations of fermions; (3) the resonance mode is a mixture of both degrees of freedom.

cond-mat.str-el↗

arXiv2007

Nonlocal effect on the magnetic penetration depth in multigapped superconductors

A brief discussion is given on the nonlocal effect in multigapped superconductivity. It is pointed out that the effective magnetic penetration depth at lower external fields may be reduced by the nonlocal effect associated with the presence of small energy gap. A crude estimation of the effect in double gap system is provided and compared with the data obtained by muSR in MgB2.

cond-mat.supr-con↗

arXiv2007

Josephson-Like Behaviour of Granular Carbon Films

This work presents the study of some new anomalous electromagnetic effects in graphite-like thin carbon films. These are: The fast switching (1nanosecond) of electrical conductivity The detection of microwave radiation and its temperature dependence The oscillations of film stack magnetization in the magnetic field of 1-5 T. The optical radiation under process of spasmodic switching of conductivity Results of magnetic force microscopy (MFM), DC SQUID magnetization, reversed Josephson effect (RJE), and resistance measurements in thin carbon arc (CA) films are presented. The observation of a RJE induced voltage as well as its rf frequency, input amplitude, and temperature dependence reveals the existence of Josephson-like Junction arrays. Oscillating behavior of the DC SQUID magnetization reminiscent of the Fraunhofer-like behavior of superconducting (SC) critical current in the range of 10000-50000 Oe has been observed. The DC SQUID magnetization measurement indicates a possible elementary 102 nm SC loop; this is compared to MFM direct observations of magnetic clusters with a median size of 165 nm. The results obtained provides a basis for non-cryogenic elecrtonic devices utilizing the Josephson effect.

cond-mat.supr-con↗

arXiv2008

Quantum Oscillations in Magnetic Field Induced Antiferromagnetic Phase of Underdoped Cuprates : Application to Ortho-II YBa2Cu3O6.5

Magnetic field induced antiferromagnetic phase of the underdoped cuprates is studied within the t-t'-J model. A magnetic field suppresses the pairing amplitude, which in turn may induce antiferromagnetism. We apply our theory to interpret the recently reported quantum oscillations in high magnetic field in ortho-II YBa2Cu3O6.5 and propose that the total hole density abstracted from the oscillation period is reduced by 50% due to the antiferromagnetism.

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