649 research outputs found

### Frequency dependent third cumulant of current in diffusive conductors

We calculate the frequency dispersion of the third cumulant of current in
diffusive-metal contacts. The cumulant exhibits a dispersion at the inverse
time of diffusion across the contact, which is typically much smaller than the
inverse $RC$ time. This dispersion is much more pronounced in the case of
strong electron-electron scattering than in the case of purely elastic
scattering because of a different symmetry of the relevant second-order
correlation functions.Comment: 8 pages, 4 figure

### Current fluctuations near to the 2D superconductor-insulator quantum critical point

Systems near to quantum critical points show universal scaling in their
response functions. We consider whether this scaling is reflected in their
fluctuations; namely in current-noise. Naive scaling predicts low-temperature
Johnson noise crossing over to noise power $\propto E^{z/(z+1)}$ at strong
electric fields. We study this crossover in the metallic state at the 2d z=1
superconductor/insulator quantum critical point. Using a Boltzmann-Langevin
approach within a 1/N-expansion, we show that the current noise obeys a scaling
form $S_j=T \Phi[T/T_{eff}(E)]$ with $T_{eff} \propto \sqrt{E}$. We recover
Johnson noise in thermal equilibrium and $S_j \propto \sqrt{E}$ at strong
electric fields. The suppression from free carrier shot noise is due to strong
correlations at the critical point. We discuss its interpretation in terms of a
diverging carrier charge $\propto 1/\sqrt{E}$ or as out-of-equilibrium Johnson
noise with effective temperature $\propto \sqrt{E}$.Comment: 5 page

### Energy Relaxation of Hot Dirac Fermions in Graphene

We develop a theory for the energy relaxation of hot Dirac fermions in
graphene. We obtain a generic expression for the energy relaxation rate due to
electron-phonon interaction and calculate the power loss due to both optical
and acoustic phonon emission as a function of electron temperature
$T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly
dependent on carrier density and non-vanishing at the Dirac point $n = 0$,
originating from interband electron-optical phonon scattering by the intrinsic
electrons in the graphene valence band. We obtain the total power loss per
carrier $\sim 10^{-12} - 10^{-7} \mathrm{W}$ within the range of electron
temperatures $\sim 20 - 1000 \mathrm{K}$. We find optical (acoustic) phonon
emission to dominate the energy loss for $T_{\mathrm{e}} > (<) 200-300
\mathrm{K}$ in the density range $n = 10^{11}-10^{13} \mathrm{cm}^{-2}$.Comment: 5 page

### Divergence at low bias and down-mixing of the current noise in a diffusive superconductor-normal metal-superconductor junction

We present current noise measurements in a long diffusive
superconductor-normal-metal-superconductor junction in the low voltage regime,
in which transport can be partially described in terms of coherent multiple
Andreev reflections. We show that, when decreasing voltage, the current noise
exhibits a strong divergence together with a broad peak. We ascribe this peak
to the mixing between the ac- Josephson current and the noise of the junction
itself. We show that the junction noise corresponds to the thermal noise of a
nonlinear resistor 4kBT=R with R V = I V and no adjustable parameters

### Statistics of speckle patterns

We develop a general method for calculating statistical properties of the
speckle pattern of coherent waves propagating in disordered media. In some
aspects this method is similar to the Boltzmann-Langevin approach for the
calculation of classical fluctuations. We apply the method to the case where
the incident wave experiences many small angle scattering events during
propagation, but the total angle change remains small. In many aspects our
results for this case are different from results previously known in the
literature. The correlation function of the wave intensity at two points
separated by a distance $r$, has a long range character. It decays as a power
of $r$ and changes sign. We also consider sensitivities of the speckles to
changes of external parameters, such as the wave frequency and the incidence
angle.Comment: 4 pages, 2 figure

### Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory

We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio

### Propagation of coherent waves in elastically scattering media

A general method for calculating statistical properties of speckle patterns
of coherent waves propagating in disordered media is developed. It allows one
to calculate speckle pattern correlations in space, as well as their
sensitivity to external parameters. This method, which is similar to the
Boltzmann-Langevin approach for the calculation of classical fluctuations,
applies for a wide range of systems: From cases where the ray propagation is
diffusive to the regime where the rays experience only small angle scattering.
The latter case comprises the regime of directed waves where rays propagate
ballistically in space while their directions diffuse. We demonstrate the
applicability of the method by calculating the correlation function of the wave
intensity and its sensitivity to the wave frequency and the angle of incidence
of the incoming wave.Comment: 19 pages, 5 figure

### Radiative transfer theory for vacuum fluctuations

A semiclassical kinetic theory is presented for the fluctuating photon flux
emitted by a disordered medium in thermal equilibrium. The kinetic equation is
the optical analog of the Boltzmann-Langevin equation for electrons. Vacuum
fluctuations of the electromagnetic field provide a new source of fluctuations
in the photon flux, over and above the fluctuations due to scattering. The
kinetic theory in the diffusion approximation is applied to the
super-Poissonian noise due to photon bunching and to the excess noise due to
beating of incident radiation with the vacuum fluctuations.Comment: 4 pages, 2 figures, revised version according to referee's comment

### Conductance noise in interacting Anderson insulators driven far from equilibrium

The combination of strong disorder and many-body interactions in Anderson
insulators lead to a variety of intriguing non-equilibrium transport phenomena.
These include slow relaxation and a variety of memory effects characteristic of
glasses. Here we show that when such systems are driven with sufficiently high
current, and in liquid helium bath, a peculiar type of conductance noise can be
observed. This noise appears in the conductance versus time traces as
downward-going spikes. The characteristic features of the spikes (such as
typical width) and the threshold current at which they appear are controlled by
the sample parameters. We show that this phenomenon is peculiar to hopping
transport and does not exist in the diffusive regime. Observation of
conductance spikes hinges also on the sample being in direct contact with the
normal phase of liquid helium; when this is not the case, the noise exhibits
the usual 1/f characteristics independent of the current drive. A model based
on the percolative nature of hopping conductance explains why the onset of the
effect is controlled by current density. It also predicts the dependence on
disorder as confirmed by our experiments. To account for the role of the bath,
the hopping transport model is augmented by a heuristic assumption involving
nucleation of cavities in the liquid helium in which the sample is immersed.
The suggested scenario is analogous to the way high-energy particles are
detected in a Glaser's bubble chamber.Comment: 15 pages 22 figure

### Theory of shot noise in space-charge limited diffusive conduction regime

As is well known, the fluctuations from a stable stationary nonequilibrium
state are described by a linearized nonhomogeneous Boltzmann-Langevin equation.
The stationary state itself may be described by a nonlinear Boltzmann equation.
The ways of its linearization sometimes seem to be not unique. We argue that
there is actually a unique way to obtain a linear equation for the
fluctuations. In the present paper we treat as an example an analytical theory
of nonequilibrium shot noise in a diffusive conductor under the space charge
limited regime. Our approach is compared with that of Schomerus, Mishchenko and
Beenakker [Phys. Rev. B 60, 5839 (1999)]. We find some difference between the
present theory and the approach of their paper and discuss a possible origin of
the difference. We believe that it is related to the fundamentals of the theory
of fluctuation phenomena in a nonequilibrium electron gas.Comment: 17 pages, no figure

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