2,182 research outputs found
Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective
This work provides a ground for a quantitative interpretation of experiments
on step bunching during sublimation of crystals with a pronounced
Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step
bunching instability takes place when the kinetic length is larger than the
average distance between the steps on the vicinal surface. In the opposite
limit the instability is weak and step bunching can occur only when the
magnitude of step-step repulsion is small. The central result are power law
relations of the between the width, the height, and the minimum interstep
distance of a bunch. These relations are obtained from a continuum evolution
equation for the surface profile, which is derived from the discrete step
dynamical equations for. The analysis of the continuum equation reveals the
existence of two types of stationary bunch profiles with different scaling
properties. Through a mathematical equivalence on the level of the discrete
step equations as well as on the continuum level, our results carry over to the
problems of step bunching induced by growth with a strong inverse ES effect,
and by electromigration in the attachment/detachment limited regime. Thus our
work provides support for the existence of universality classes of step
bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103
(2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure
Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order.Comment: 15 pages, 9 figure
On Nonlinear Diffusion with Multiplicative Noise
Nonlinear diffusion is studied in the presence of multiplicative noise. The
nonlinearity can be viewed as a ``wall'' limiting the motion of the diffusing
field. A dynamic phase transition occurs when the system ``unbinds'' from the
wall. Two different universality classes, corresponding to the cases of an
``upper'' and a ``lower'' wall, are identified and their critical properties
are characterized. While the lower wall problem can be understood by applying
the knowledge of linear diffusion with multiplicative noise, the upper wall
problem exhibits an anomaly due to nontrivial dynamics in the vicinity of the
wall. Broad power-law distribution is obtained throughout the bound phase.Comment: 4 pages, LaTeX, text and figures also available at
http://matisse.ucsd.edu/~hw
Anisotropy of the paramagnetic susceptibility in LaTiO: The electron-distribution picture in the ground state
The energy-level scheme and wave functions of the titanium ions in
LaTiO are calculated using crystal-field theory and spin-orbit coupling.
The theoretically derived temperature dependence and anisotropy of the magnetic
susceptibility agree well with experimental data obtained in an untwinned
single crystal. The refined fitting procedure reveals an almost isotropic
molecular field and a temperature dependence of the van Vleck susceptibility.
The charge distribution of the 3d--electron on the Ti positions and the
principle values of the quadrupole moments are derived and agree with NMR data
and recent measurements of orbital momentum and crystal-field splitting.
The low value of the ordered moment in the antiferromagnetic phase is
discussed.Comment: 6 pages, 2 figures, 3 table
THz spectroscopy in the pseudo-Kagome system Cu3Bi(SeO3)2O2Br
Terahertz (THz) transmission spectra have been measured as function of
temperature and magnetic field on single crystals of Cu3Bi(SeO3)2O2Br. In the
time-domain THz spectra without magnetic field, two resonance absorptions are
observed below the magnetic ordering temperature T_N~27.4 K. The corresponding
resonance frequencies increase with decreasing temperature and reach energies
of 1.28 and 1.23 meV at 3.5 K. Multi-frequency electron spin resonance
transmission spectra as a function of applied magnetic field show the field
dependence of four magnetic resonance modes, which can be modeled as a
ferromagnetic resonance including demagnetization and anisotropy effects.Comment: 5 pages, 3 figures. All comments are welcome and appreciate
Anisotropic Exchange in LiCuVO probed by ESR
We investigated the paramagnetic resonance in single crystals of LiCuVO
with special attention to the angular variation of the absorption spectrum. To
explain the large resonance linewidth of the order of 1 kOe, we analyzed the
anisotropic exchange interaction in the chains of edge-sharing CuO
octahedra, taking into account the ring-exchange geometry of the
nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The
exchange parameters, which can be estimated from theoretical considerations,
nicely agree with the parameters obtained from the angular dependence of the
linewidth. The anisotropy of this magnetic ring exchange is found to be much
larger than it is usually expected from conventional estimations which neglect
the bonding geometry. Hence, the data yield the evidence that in copper oxides
with edge-sharing structures the role of the orbital degrees of freedom is
strongly enhanced. These findings establish LiCuVO as one-dimensional
compound at high temperatures.
PACS: 76.30.-v, 76.30.Fc, 75.30.EtComment: 18 pages, 6 figure
Unconventional magnetostructural transition in CoCr2O4 at high magnetic fields
The magnetic-field and temperature dependencies of ultrasound propagation and
magnetization of single-crystalline CoCr2O4 have been studied in static and
pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies
with significant changes in the sound velocity and attenuation are found in
this spinel compound at the onset of long-range incommensurate spiral-spin
order at T_s = 27 K and at the transition from the incommensurate to the
commensurate state at T_l = 14 K, evidencing strong spin-lattice coupling.
While the magnetization evolves gradually with field, steplike increments in
the ultrasound clearly signal a transition into a new magneto-structural state
between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a
high-symmetry phase with only the longitudinal component of the magnetization
being ordered, while the transverse helical component remains disordered. This
phase is metastable in an extended H-T phase space.Comment: 5 pages, 4 figure
Transition from KPZ to Tilted Interface Critical Behavior in a Solvable Asymmetric Avalanche Model
We use a discrete-time formulation to study the asymmetric avalanche process
[Phys. Rev. Lett. vol. 87, 084301 (2001)] on a finite ring and obtain an exact
expression for the average avalanche size of particles as a function of
toppling probabilities depending on parameters and . By mapping
the model below and above the critical line onto driven interface problems, we
show how different regimes of avalanches may lead to different types of
critical interface behavior characterized by either annealed or quenched
disorders and obtain exactly the related critical exponents which violate a
well-known scaling relation when .Comment: 10 page
Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls magnet CuGeO3
The fraction of the paramagnetic phase surviving at the impurity-induced
antiferromagnetic order transition of the doped spin-Peierls magnet
Cu(1-x)Mg(x)GeO3 (x < 5%) is found to increase with an external magnetic field.
This effect is qualitatively explained by the competition of Zeeman energy and
exchange interaction between local antiferromagnetic clustersComment: 4 pages 4 figure
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