52,523 research outputs found
Stability of ferromagnetism in the Hubbard model on the kagom\'e lattice
The Hubbard model on the kagom\'e lattice has highly degenerate ground states
(the flat lowest band) in the corresponding single-electron problem and
exhibits the so-called flat-band ferromagnetism in the many-electron ground
states as was found by Mielke. Here we study the model obtained by adding extra
hopping terms to the above model. The lowest single-electron band becomes
dispersive, and there is no band gap between the lowest band and the other
band. We prove that, at half-filling of the lowest band, the ground states of
this perturbed model remain saturated ferromagnetic if the lowest band is
nearly flat.Comment: 4 pages, 1 figur
Field-Induced Magnetic Order and Simultaneous Lattice Deformation in TlCuCl3
We report the results of Cu and Cl nuclear magnetic resonance experiments
(NMR) and thermal expansion measurements in magnetic fields in the coupled
dimer spin system TlCuCl3. We found that the field-induced antiferromagnetic
transition as confirmed by the splitting of NMR lines is slightly
discontinuous. The abrupt change of the electric field gradient at the Cl
sites, as well as the sizable change of the lattice constants, across the phase
boundary indicate that the magnetic order is accompanied by simultaneous
lattice deformation.Comment: 4 pages, 5 figure
Single-cycle THz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3
Using the tilted-pulse-intensity-front scheme, we generate single-cycle
terahertz (THz) pulses by optical rectification of femtosecond laser pulses in
LiNbO3. In the THz generation setup, the condition that the image of the
grating coincides with the tilted-optical-pulse front is fulfilled to obtain
optimal THz beam characteristics and pump-to-THz conversion efficiency. The
designed focusing geometry enables tight focus of the collimated THz beam with
a spot size close to the diffraction limit, and the maximum THz electric field
of 1.2 MV/cm is obtained
Effective Temperature in a Colloidal Glass
We study the Brownian motion of particles trapped by optical tweezers inside
a colloidal glass (Laponite) during the sol-gel transition. We use two methods
based on passive rheology to extract the effective temperature from the
fluctuations of the Brownian particles. All of them give a temperature that,
within experimental errors, is equal to the heat bath temperature. Several
interesting features concerning the statistical properties and the long time
correlations of the particles are observed during the transition.Comment: to be published in Philosophical Magazin
Uniaxial pressure dependencies of the phase boundary of TlCuCl_3
We present a thermal expansion and magnetostriction study of TlCuCl_3, which
shows a magnetic-field induced transition from a spin gap phase to a Neel
ordered phase. Using Ehrenfest relations we derive huge and strongly
anisotropic uniaxial pressure dependencies of the respective phase boundary,
e.g. the transition field changes by about GPa depending on the
direction of uniaxial pressure.Comment: 2 pages, e figures; presented at SCES200
A Viscoelastic model of phase separation
We show here a general model of phase separation in isotropic condensed
matter, namely, a viscoelastic model. We propose that the bulk mechanical
relaxation modulus that has so far been ignored in previous theories plays an
important role in viscoelastic phase separation in addition to the shear
relaxation modulus. In polymer solutions, for example, attractive interactions
between polymers under a poor-solvent condition likely cause the transient
gellike behavior, which makes both bulk and shear modes active. Although such
attractive interactions between molecules of the same component exist
universally in the two-phase region of a mixture, the stress arising from
attractive interactions is asymmetrically divided between the components only
in dynamically asymmetric mixtures such as polymer solutions and colloidal
suspensions. Thus, the interaction network between the slower components, which
can store the elastic energy against its deformation through bulk and shear
moduli, is formed. It is the bulk relaxation modulus associated with this
interaction network that is primarily responsible for the appearance of the
sponge structure peculiar to viscoelastic phase separation and the phase
inversion. We demonstrate that a viscoelastic model of phase separation
including this new effect is a general model that can describe all types of
isotropic phase separation including solid and fluid models as its special
cases without any exception, if there is no coupling with additional order
parameter. The physical origin of volume shrinking behavior during viscoelastic
phase separation and the universality of the resulting spongelike structure are
also discussed.Comment: 14 pages, RevTex, To appear in Phys. Rev
Physical Origin of the Boson Peak Deduced from a Two-Order-Parameter Model of Liquid
We propose that the boson peak originates from the (quasi-) localized
vibrational modes associated with long-lived locally favored structures, which
are intrinsic to a liquid state and are randomly distributed in a sea of
normal-liquid structures. This tells us that the number density of locally
favored structures is an important physical factor determining the intensity of
the boson peak. In our two-order-parameter model of the liquid-glass
transition, the locally favored structures act as impurities disturbing
crystallization and thus lead to vitrification. This naturally explains the
dependence of the intensity of the boson peak on temperature, pressure, and
fragility, and also the close correlation between the boson peak and the first
sharp diffraction peak (or prepeak).Comment: 5 pages, 1 figure, An error in the reference (Ref. 7) was correcte
The effect of an imaginary part of the Schwinger-Dyson equation at finite temperature and density
We examined the effect of an imaginary part of the ladder approximation
Schwinger-Dyson equation. We show the imaginary part enhances the effect of the
first order transition, and affects a tricritical point. In particular, a
chemical potential at a tricritical point is moved about 200(MeV). Thus, one
should not ignore the imaginary part. On the other hand, since an imaginary
part is small away from a tricritical point, one should be able to ignore an
imaginary part. In addition, we also examined the contribution of the wave
function renormalization constant.Comment: 12 pages, 14 figure
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