980 research outputs found
Absence of a consistent classical equation of motion for a mass-renormalized point charge
The restrictions of analyticity, relativistic (Born) rigidity, and negligible
O(a) terms involved in the evaluation of the self electromagnetic force on an
extended charged sphere of radius "a" are explicitly revealed and taken into
account in order to obtain a classical equation of motion of the extended
charge that is both causal and conserves momentum-energy. Because the
power-series expansion used in the evaluation of the self force becomes invalid
during transition time intervals immediately following the application and
termination of an otherwise analytic externally applied force, transition
forces must be included during these transition time intervals to remove the
noncausal pre-acceleration and pre-deceleration from the solutions to the
equation of motion without the transition forces. For the extended charged
sphere, the transition forces can be chosen to maintain conservation of
momentum-energy in the causal solutions to the equation of motion within the
restrictions of relativistic rigidity and negligible O(a) terms under which the
equation of motion is derived. However, it is shown that renormalization of the
electrostatic mass to a finite value as the radius of the charge approaches
zero introduces a violation of momentum-energy conservation into the causal
solutions to the equation of motion of the point charge if the magnitude of the
external force becomes too large. That is, the causal classical equation of
motion of a point charge with renormalized mass experiences a high acceleration
catastrophe.Comment: 13 pages, No figure
Rolling friction of a viscous sphere on a hard plane
A first-principle continuum-mechanics expression for the rolling friction
coefficient is obtained for the rolling motion of a viscoelastic sphere on a
hard plane. It relates the friction coefficient to the viscous and elastic
constants of the sphere material. The relation obtained refers to the case when
the deformation of the sphere is small, the velocity of the sphere is
much less than the speed of sound in the material and when the characteristic
time is much larger than the dissipative relaxation times of the
viscoelastic material. To our knowledge this is the first ``first-principle''
expression of the rolling friction coefficient which does not contain empirical
parameters.Comment: 6 pages, 2 figure
Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2
Quantum-mechanical fluctuations in strongly correlated electron systems cause
unconventional phenomena such as non-Fermi liquid behavior, and arguably high
temperature superconductivity. Here we report the discovery of a field-tuned
quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave
ordered heavy fermion metal that exhibits unconventional superconductivity
under ~ 10 GPa of applied pressure. Our finding of the associated quantum
critical spin fluctuations of the antiferromagnetic spin density wave order,
dominating the local fluctuations due to single-site Kondo effect, provide new
information about the underlying mechanism that can be important in
understanding superconductivity in this novel compound.Comment: Heavy Fermion, Quantum Critical Phenomeno
Hole-Doping Effects on a Two-dimensional Kondo Insulator
We study the effects of hole doping on the two-dimensional Heisenberg-Kondo
model around the quantum critical point, where the spin liquid phase (Kondo
insulator) and the magnetically ordered phase are separated via a second-order
phase transition. By means of the self-consistent Born approximation within the
bond operator formalism as well as the standard spin wave theory, we discuss
dynamical properties of a doped hole. It is clarified that a quasi-particle
state stabilized in the spin liquid phase is gradually obscured as the system
approaches the quantum critical point. This is also the case for the
magnetically ordered phase. We argue the similarity and the difference between
these two cases.Comment: 8 pages, 14 figure
Spin diffusion and relaxation in three-dimensional isotropic Heisenberg antiferromagnets
A theory is proposed for kinetic effects in isotropic Heisenberg
antiferromagnets at temperatures above the Neel point. A metod based on the
analysis of a set of Feynman diagrams for the kinetic coefficients is developed
for studying the critical dynamics. The scaling behavior of the generalized
coefficient of spin diffusion and relaxation constant in the paramagnetic phase
is studied in terms of the approximation of coupling modes. It is shown that
the kinetic coefficients in an antiferromagnetic system are singular in the
fluctuation region. The corresponding critical indices for diffusion and
relaxation processes are calculated. The scaling dimensionality of the kinetic
coefficients agrees with the predictions of dynamic scaling theory and a
renormalization group analysis. The proposed theory can be used to study the
momentum and frequency dependence of the kinetic parameters, and to determine
the form of the scaling functions. The role of nonlocal correlations and
spin-liquid effects in magnetic systems is briefly discussed.Comment: 10 pages, RevTeX, 3 EPS figures include
Random Field Models for Relaxor Ferroelectric Behavior
Heat bath Monte Carlo simulations have been used to study a four-state clock
model with a type of random field on simple cubic lattices. The model has the
standard nonrandom two-spin exchange term with coupling energy and a random
field which consists of adding an energy to one of the four spin states,
chosen randomly at each site. This Ashkin-Teller-like model does not separate;
the two random-field Ising model components are coupled. When , the
ground states of the model remain fully aligned. When , a
different type of ground state is found, in which the occupation of two of the
four spin states is close to 50%, and the other two are nearly absent. This
means that one of the Ising components is almost completely ordered, while the
other one has only short-range correlations. A large peak in the structure
factor appears at small for temperatures well above the transition
to long-range order, and the appearance of this peak is associated with slow,
"glassy" dynamics. The phase transition into the state where one Ising
component is long-range ordered appears to be first order, but the latent heat
is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev
Quantum critical effects on transition temperature of magnetically mediated p-wave superconductivity
We determine the behavior of the critical temperature of magnetically
mediated p-wave superconductivity near a ferromagnetic quantum critical point
in three dimensions, distinguishing universal and non-universal aspects of the
result. We find that the transition temperature is non-zero at the critical
point, raising the possibility of superconductivity in the ferromagnetic phase.Comment: 4 pages, 4 figure
NMR study of the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4 : Quantum phase transition and critical dynamics
We present an extensive NMR study of the spin-1/2 antiferromagnetic
Heisenberg ladder Cu2(C5H12N2)2Cl4 in a magnetic field range 4.5 - 16.7 T. By
measuring the proton NMR relaxation rate 1/T_1 and varying the magnetic field
around the critical field H_c1 = Delta / g\mu_B = 7.5 T, we have studied the
transition from a gapped spin liquid ground state to a gapless magnetic regime
which can be described as a Luttinger liquid. We identify an intermediate
regime T > |H-H_c1|, where the spin dynamics is (possibly) only controlled by
the T=0 critical point H_c1.Comment: 4 pages, 3 eps figures, submitted to Phys. Rev. Let
d-wave superconductivity near charge instabilities
We investigate the symmetry of the superconducting order parameter in the
proximity of a phase-separation or of an incommensurate charge-density-wave
instability. The attractive effective interaction at small or intermediate
transferred momenta is singular near the instability. This strongly
-dependent interaction, together with a residual local repulsion between the
quasiparticles and an enhanced density of states for band structures
appropriate for the high temperature superconducting oxides, strongly favors
the formation of -wave superconductivity. The relative stability with
respect to superconductivity in the -wave channel is discussed in detail,
finding this latter hardly realized in the above conditions. The
superconducting temperature is mostly determined by the closeness to the
quantum critical point associated to the charge instability and displays a
stronger dependence on doping with respect to the simple proximity to a Van
Hove singularity. The relevance of this scenario and the generic agreement of
the resulting phase diagram with the properties displayed by high temperature
superconducting oxides is discussed.Comment: 1 revtex file and 12 postscript figure
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