40,764 research outputs found

### Theory of Raman scattering from Leggett's collective mode in a multiband superconductor: Application to MgB$_2$

In 1966 Leggett used a two-band superconductor to show that a new collective
mode could exist at low temperatures, corresponding to a counter-flow of the
superconducting condensates in each band. Here, the theory of electronic Raman
scattering in a superconductor by Klein and Dierker (1984) is extended to a
multiband superconductor. Raman scattering creates particle/hole pairs. In the
relevant $A_{1g}$\ symmetry, the attraction that produces pairing necessarily
couples excitations of superconducting pairs to these p/h excitations. In the
Appendix it is shown that for zero wave vector transfer $% q$ this coupling
modifies the Raman response and makes the long-range Coulomb correction null.
The 2-band result is applied to MgB$_{2}$ where this coupling activates
Leggett's collective mode. His simple limiting case is obtained when the
interband attractive potential is decreased to a value well below that given by
LDA theory. The peak from Leggett's mode is studied as the potential is
increased through the theoretical value: With realistic MgB$_{2}$\ parameters,
the peak broadens through decay into the continuum above the smaller ($\pi$
band) superconducting gap. Finite $q$ effects are also taken into account,
yielding a Raman peak that agrees well in energy with the experimental result
by Blumberg \textit{et el.} (2007). This approach is also applied to the $q=0$,
2-band model of the Fe-pnictides considered by Chubukov \textit{et al.}(2009).Comment: 10 pages, 3 figures. To appear in Physical Review

### Metastability in stochastic dynamics of disordered mean-field models

We study a class of Markov chains that describe reversible stochastic
dynamics of a large class of disordered mean field models at low temperatures.
Our main purpose is to give a precise relation between the metastable time
scales in the problem to the properties of the rate functions of the
corresponding Gibbs measures. We derive the analog of the Wentzell-Freidlin
theory in this case, showing that any transition can be decomposed, with
probability exponentially close to one, into a deterministic sequence of
``admissible transitions''. For these admissible transitions we give upper and
lower bounds on the expected transition times that differ only by a constant.
The distribution rescaled transition times are shown to converge to the
exponential distribution. We exemplify our results in the context of the random
field Curie-Weiss model.Comment: 73pp, AMSTE

### Metastability and low lying spectra in reversible Markov chains

We study a large class of reversible Markov chains with discrete state space
and transition matrix $P_N$. We define the notion of a set of {\it metastable
points} as a subset of the state space \G_N such that (i) this set is reached
from any point x\in \G_N without return to x with probability at least $b_N$,
while (ii) for any two point x,y in the metastable set, the probability
$T^{-1}_{x,y}$ to reach y from x without return to x is smaller than
$a_N^{-1}\ll b_N$. Under some additional non-degeneracy assumption, we show
that in such a situation: \item{(i)} To each metastable point corresponds a
metastable state, whose mean exit time can be computed precisely. \item{(ii)}
To each metastable point corresponds one simple eigenvalue of $1-P_N$ which is
essentially equal to the inverse mean exit time from this state. The
corresponding eigenfunctions are close to the indicator function of the support
of the metastable state. Moreover, these results imply very sharp uniform
control of the deviation of the probability distribution of metastable exit
times from the exponential distribution.Comment: 44pp, AMSTe

### Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors

Electronic Raman scattering from high- and low-energy excitations was studied
as a function of temperature, extent of hole doping, and energy of the incident
photons in Bi_2Sr_2CaCu_2O_{8 \pm \delta} superconductors. For underdoped
superconductors, short range antiferromagnetic (AF) correlations were found to
persist with hole doping, and doped single holes were found to be incoherent in
the AF environment. Above the superconducting (SC) transition temperature T_c,
the system exhibits a sharp Raman resonance of B_{1g} symmetry and energy of 75
meV and a pseudogap for electron-hole excitations below 75 meV, a manifestation
of a partially coherent state forming from doped incoherent quasi particles.
The occupancy of the coherent state increases with cooling until phase ordering
at T_c produces a global SC state.Comment: 6 pages, 4 color figures, PDF forma

### Non-resonant Raman response of inhomogeneous structures in the electron doped $t-t'$ Hubbard model

We calculate the non-resonant Raman response, the single particle spectra and
the charge-spin configuration for the electron doped $t-t'$ Hubbard model using
unrestricted Hartree-Fock calculations. We discuss the similarities and
differences in the response of homogeneous versus inhomogeneous structures.
Metallic antiferromagnetism dominates in a large region of the $U-n$ phase
diagram but at high values of the on-site interaction and for intermediate
doping values, inhomogeneous configurations are found with lower energy. This
result is in contrast with the case of hole doped cuprates where
inhomogeneities are found already at very low doping. The inhomogeneities found
are in-phase stripes compatible with inelastic neutron scattering experiments.
They give an incoherent background in the Raman response. The $B_{2g}$ signal
can show a quasiparticle-like component even when no Fermi surface is found in
the nodal direction.Comment: 8 pages, 10 figures, accepted for publication in Phys. Rev.

### Evidence for electron-phonon interaction in Fe$_{1-x}$M$_{x}$Sb$_{2}$ (M=Co, Cr) single crystals

We have measured polarized Raman scattering spectra of the
Fe$_{1-x}$Co$_{x}$Sb$_{2}$ and Fe$_{1-x}$Cr$_{x}$Sb$_{2}$ (0$\leq x\leq$0.5)
single crystals in the temperature range between 15 K and 300 K. The highest
energy $B_{1g}$ symmetry mode shows significant line asymmetry due to phonon
mode coupling width electronic background. The coupling constant achieves the
highest value at about 40 K and after that it remains temperature independent.
Origin of additional mode broadening is pure anharmonic. Below 40 K the
coupling is drastically reduced, in agreement with transport properties
measurements. Alloying of FeSb$_2$ with Co and Cr produces the B$_{1g}$ mode
narrowing, i.e. weakening of the electron-phonon interaction. In the case of
A$_{g}$ symmetry modes we have found a significant mode mixing

### Identification of Bare-Airframe Dynamics from Closed-Loop Data Using Multisine Inputs and Frequency Responses

Amethod is presented for computing multiple-input multiple-output frequency responses of bare-airframe dynamics for systems excited using orthogonal phase-optimized multisines and including correlated data arising from control mixing or feedback control. The estimation was posed as the solution to an underdetermined system of linear equations, for which additional information was supplied using interpolation of the frequency responses. A simulation model of the NASA T-2 aircraft having two inputs and two outputs was used to investigate the method in the open-loop configuration and under closed-loop control. The method was also applied to flight test data from the X-56A aeroelastic demonstrator having five inputs and ten outputs and flying under closed-loop control with additional control allocation mixing. Results demonstrated that the proposed method accurately estimates the bare airframe frequency responses in the presence of correlated data from control mixing and feedback control. Results also agreed with estimates obtained using different methods that are less sensitive to correlated inputs

### Photoinduced Fano-resonance of coherent phonons in zinc

Utilizing femtosecond optical pump-probe technique, we have studied transient
Fano-resonance in zinc. At high excitation levels the Fourier spectrum of the
coherent E$_{2g}$ phonon exhibits strongly asymmetric line shape, which is well
modeled by the Fano function. The Fano parameter (1/Q) was found to be strongly
excitation fluence dependent while depending weakly on the initial lattice
temperature. We attribute the origin of the Fano-resonance to the coupling of
coherent phonon to the electronic continuum, with their transition
probabilities strongly renormalized in the vicinity of the photoinduced
structural transition.Comment: 5 pages, 3 figures, to be published in Physical Review

### Evolution of the Kondo resonance feature and its relationship to spin-orbit coupling across the quantum critical point in Ce2Rh{1-x}CoxSi3

We investigate the evolution of the electronic structure of Ce2Rh{1-x}CoxSi3
as a function of x employing high resolution photoemission spectroscopy. Co
substitution at the Rh sites in antiferromagnetic Ce2RhSi3 leads to a
transition from an antiferromagnetic system to a Kondo system, Ce2CoSi3 via the
Quantum Critical Point (QCP). High resolution photoemission spectra reveal
distinct signature of the Kondo resonance feature (KRF) and its spin orbit
split component (SOC) in the whole composition range indicating finite Kondo
temperature scale at the quantum critical point. We observe that the intensity
ratio of the Kondo resonance feature and its spin orbit split component,
KRF/SOC gradually increases with the decrease in temperature in the strong
hybridization limit. The scenario gets reversed if the Kondo temperature
becomes lower than the magnetic ordering temperature. While finite Kondo
temperature within the magnetically ordered phase indicates applicability of
the spin density wave picture at the approach to QCP, the dominant temperature
dependence of the spin-orbit coupled feature suggests importance of spin-orbit
interactions in this regime.Comment: 6 figure

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