343 research outputs found
Noise spectra of an interacting quantum dot
We study the noise spectra of a many-level quantum dot coupled to two
electron reservoirs, when interactions are taken into account only on the dot
within the Hartree-Fock approximation. The dependence of the noise spectra on
the interaction strength, the coupling to the leads, and the chemical potential
is derived. For zero bias and zero temperature, we find that as a function of
the (external) frequency, the noise exhibits steps and dips at frequencies
reflecting the internal structure of the energy levels on the dot.
Modifications due to a finite bias and finite temperatures are investigated for
a non-interacting two-level dot. Possible relations to experiments are pointed
out.Comment: Added reference
Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder: a quantum Monte Carlo study
We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and
inequivalent chains. This model is obtained by a twist (\theta) deformation of
the ladder and interpolates between the isotropic ladder (\theta=0) and the
SU(2) ferromagnetic Kondo necklace model (\theta=\pi). We show that the ground
state in the (\theta,J_\perp) plane has a finite string order parameter
characterising the Haldane phase. Twisting the chain introduces a new energy
scale, which we interpret in terms of a Suhl-Nakamura interaction. As a
consequence we observe a crossover in the scaling of the spin gap at weak
coupling from \Delta/J_\| \propto J_\perp/J_\| for \theta < \theta_c \simeq
8\pi/9 to \Delta/J_\| \propto (J_\perp/J_\|)^2 for \theta > \theta_c. Those
results are obtained on the basis of large scale Quantum Monte Carlo
calculations.Comment: 4 page
Multiple Core-Hole Coherence in X-Ray Four-Wave-Mixing Spectroscopies
Correlation-function expressions are derived for the coherent nonlinear
response of molecules to three resonant ultrafast pulses in the x-ray regime.
The ability to create two-core-hole states with controlled attosecond timing in
four-wave-mixing and pump probe techniques should open up new windows into the
response of valence electrons, which are not available from incoherent x-ray
Raman and fluorescence techniques. Closed expressions for the necessary
four-point correlation functions are derived for the electron-boson model by
using the second order cumulant expansion to describe the fluctuating
potentials. The information obtained from multidimensional nonlinear techniques
could be used to test and refine this model, and establish an anharmonic
oscillator picture for electronic excitations
Persistent currents of noninteracting electrons
We thoroughly study the persistent current of noninteracting electrons in
one, two, and three dimensional thin rings. We find that the results for
noninteracting electrons are more relevant for individual mesoscopic rings than
hitherto appreciated. The current is averaged over all configurations of the
disorder, whose amount is varied from zero up to the diffusive limit, keeping
the product of the Fermi wave number and the ring's circumference constant.
Results are given as functions of disorder and aspect ratios of the ring. The
magnitude of the disorder-averaged current may be larger than the
root-mean-square fluctuations of the current from sample to sample even when
the mean free path is smaller, but not too small, than the circumference of the
ring. Then a measurement of the persistent current of a typical sample will be
dominated by the magnitude of the disorder averaged current.Comment: 10 pages, 4 figure
Compressibility of and (M = Rh, Ir and Co) Compounds
The lattice parameters of the tetragonal compounds CeIn and
CeIn(Rh, Ir and Co) have been studied as a function of
pressure up to 15 GPa using a diamond anvil cell under both hydrostatic and
quasihydrostatic conditions at room temperature. The addition of In
layers to the parent CeIn compound is found to stiffen the lattice as the
2-layer systems (average of bulk modulus values is 70.4 GPa) have a
larger than CeIn (67 GPa), while the 1-layer systems with the are
even stiffer (average of is 81.4 GPa). Estimating the hybridization
using parameters from tight binding calculations shows that the dominant
hybridization is in nature between the Ce and In atoms. The values of
at the pressure where the superconducting transition temperature
reaches a maximum is the same for all CeIn compounds. By
plotting the maximum values of the superconducting transition temperature
versus for the studied compounds and Pu-based superconductors, we
find a universal versus behavior when these quantities are
normalized appropriately. These results are consistent with magnetically
mediated superconductivity.Comment: Updated version resubmitted to Phys. Rev.
Coexistence of antiferromagnetic order and unconventional superconductivity in heavy fermion compounds CeRh_{1-x}Ir_xIn_5: nuclear quadrupole resonance studies
We present a systematic ^{115}In NQR study on the heavy fermion compounds
CeRh_{1-x}Ir_xIn_5 (x=0.25, 0.35, 0.45, 0.5, 0.55 and 0.75). The results
provide strong evidence for the microscopic coexistence of antiferromagnetic
(AF) order and superconductivity (SC) in the range of 0.35 \leq x \leq 0.55.
Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of
superconductivity at T_c=0.9 K. T_c reaches a maximum (0.94 K) at x=0.45 where
T_N is found to be the highest (4.0 K). Detailed analysis of the measured
spectra indicate that the same electrons participate in both SC and AF order.
The nuclear spin-lattice relaxation rate 1/T_1 shows a broad peak at T_N and
follows a T^3 variation below T_c, the latter property indicating
unconventional SC as in CeIrIn_5 (T_c=0.4 K). We further find that, in the
coexistence region, the T^3 dependence of 1/T_1 is replaced by a T-linear
variation below T\sim 0.4 K, with the value \frac{(T_1)_{T_c}}{(T_1)_{low-T}}
increasing with decreasing x, likely due to low-lying magnetic excitations
associated with the coexisting magnetism.Comment: 20 pages, 14 figure
The electronic structure of the heavy fermion metal
The electronic structure of the first reported heavy fermion compound without
f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the
result of the trigonal splitting and d-d Coulomb interaction one electron of
the configuration of V ion is localized and the rest partially fills
a relatively broad conduction band. The effective Anderson impurity model was
solved by Non-Crossing-Approximation method, leading to an estimation for the
single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion
phenomenon of Nozi\`eres for the Kondo lattice leads to a remarkable decrease
of the heavy-fermion (or coherence) energy scale (D
is the typical bandwidth), comparable to the experimental result.Comment: 4 pages, RevTeX; 3 figures in format .eps. submitted to PR
Novel approach to description of spin liquid phases in low-dimensional quantum antiferromagnets
We consider quantum spin systems with dimerization, which at strong coupling
have singlet ground states. To account for strong correlations, the excitations
are described as dilute Bose gas of degenerate triplets with infinite on-site
repulsion. This approach is applied to the two-layer Heisenberg model at zero
temperature with general couplings. Our analytic results for the triplet gap,
the excitation spectrum and the location of the quantum critical point are in
excellent agreement with numerical results, obtained by dimer series
expansions.Comment: 4 pages, REVTex, 3 Postscript figure
Quantum Criticality in doped CePd_1-xRh_x Ferromagnet
CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C=
6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a
detailed investigation on the suppression of the ferromagnetic (F) phase in
this alloy using dc-(\chi_dc) and ac-susceptibility (\chi_ac), specific heat
(C_m), resistivity (\rho) and thermal expansion (\beta) techniques. Our results
show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K
at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is
traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration
between x = 0.87 and 0.90. The quantum criticality of this region is recognized
by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one
at x = 0.87. At high temperature, this system shows the onset of valence
instability revealed by a deviation from Vegard's law (at x_V~0.75) and
increasing hybridization effects on high temperature \chi_dc and \rho.
Coincidentally, a Fermi liquid contribution to the specific heat arises from
the MV component, which becomes dominant at the CeRh limit. In contrast to
antiferromagnetic systems, no C_m/T flattening is observed for x > x_cr rather
the mentioned power law divergence, which coincides with a change of sign of
\beta. The coexistence of F and MV components and the sudden changes in the T
dependencies are discussed in the context of randomly distributed magnetic and
Kondo couplings.Comment: 11 pages, 11 figure
Dynamics of disordered heavy Fermion systems
Dynamics of the disordered heavy Fermion model of Dobrosavljevic et al. are
calculated using an expression for the spectral function of the Anderson model
which is consistent with quantum Monte Carlo results. We compute the
self-energy for three distributions of Kondo scales including the distribution
of Bernal et al. for UCu{5-x}Pd{x}. The corresponding low temperature optical
conductivity shows a low-frequency pseudogap, a negative optical mass
enhancement, and a linear in frequency transport scattering rate, consistent
with results in Y{1-x}U{x}Pd{3} and UCu{5-x}Pd{x}.Comment: 5 pages, LaTeX and 4 PS figure
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