302 research outputs found
An analytical treatment of in-plane magnetotransport in the Falicov-Sievert model
We derive an analytical expression which allows efficient computation of the
effect of all the Fermi surface trajectories induced by a combination of Bragg
scattering and magnetic breakdown on the in-plane components of the resistivity
tensor. The particular network of coupled orbits which we consider was first
formulated by Falicov and Sievert, who studied the problem numerically. Our
approach, based upon a method used previously to derive an analytical solution
for interlayer transport, allows us to show that the conductivity tensor can be
written as a sum of a matrix representing the effect of total magnetic
breakdown and one representing a combination of complex electronic
trajectories, and we find a compact expression for the in-plane components of
the resistivity tensor that can be evaluated straightforwardly.Comment: 4 pages, 4 figure
Hypothalamic control of food intake in cats and monkeys
The role of the central nervous system in regulating food intake was probably suggested first by the discovery that either obesity or emaciation may occur in patients with nervous diseases. For a while these observations were not properly evaluated, because emphasis was laid upon the obesity as such, or the leanness, rather than upon the changed eating habits responsible for the clinical picture. Interest was focused on the hypothalamic region by the experimental studies of many workers (Hetherington, 1941; Hetherington & Ranson, 1940, 1942 a, b; Brobeck, Tepperman & Long, 1943; Kennedy, 1950; Ranson, Fisher & Ingram, 1938) who showed that bilateral lesions in the medial hypothalamus, especially lesions in or ventro-lateral to the ventromedial nucleus, resulted in obesity. The confusion introduced by the notion that pituitary disturbances caused obesity was also clarified by Hetherington (1943), who showed that the hypophysis is in no way directly concerned with the pathogenesis of obesity following injury to the base of brain. Brobeck et al. (1943) demonstrated that this hypothalamic obesity was due to increased food intake (hypothalamic hyperphagia) rather than to disturbances in the fat, carbohydrate or intermediary metabolism. From the time of its discovery this hyperphagia was assumed to be a release phenomenon brought about through the destruction of an inhibitory mechanism.The existence of another mechanism in the lateral hypothalamus of the rat, which controls the 'instinct' or the 'urge' to eat, was demonstrated by Anand & Brobeck (1951 a, b). They showed that bilateral destruction of a well localized area in the lateral hypothalamus, at the same rostro-caudal level as the ventro-medial nucleus, produces complete aphagia and death due to starvation, in spite of the availability of food. It was also observed that of the two mechanisms the lateral one exerts the more basic type of control over food intake and the medial one (inhibitory) produces its effects only when the lateral is intact. The lateral mechanism is designated a 'feeding centre', or even an 'appetite centre', while the medial one is called a 'satiety centre'. Joliffe named the two, together, the 'appestat'. The present study was undertaken to determine, whether similar mechanisms exist in the hypothalamic regions of higher mammals, cats and monkeys, and also whether they are modified by the more highly evolved higher nervous centres
Large bulk resistivity and surface quantum oscillations in the topological insulator Bi2Te2Se
Topological insulators are predicted to present novel surface transport
phenomena, but their experimental studies have been hindered by a metallic bulk
conduction that overwhelms the surface transport. We show that a new
topological insulator, Bi2Te2Se, presents a high resistivity exceeding 1 Ohm-cm
and a variable-range hopping behavior, and yet presents Shubnikov-de Haas
oscillations coming from the surface Dirac fermions. Furthermore, we have been
able to clarify both the bulk and surface transport channels, establishing a
comprehensive understanding of the transport in this material. Our results
demonstrate that Bi2Te2Se is the best material to date for studying the surface
quantum transport in a topological insulator.Comment: 4 pages, 3 figure
Magnetic oscillations in a two-dimensional network of compensated electron and hole orbits
The FS of (ET)8Hg4Cl12(C6H5Br)2 can be regarded as a 2D network of
compensated electron and hole orbits coupled by magnetic breakthrough.
Simultaneous measurements of the interlayer magnetoresistance and magnetic
torque have been performed up to 28 T. Magnetoresistance and de dHvA
oscillations spectra exhibit frequency combinations typical of such a network.
Even though some of the observed magnetoresistance oscillations cannot be
interpreted on the basis of neither conventional SdH oscillations nor quantum
interference, the temperature and magnetic field (both orientation and
magnitude) dependence of all the Fourier components of the dHvA spectra can be
consistently accounted for by the LK formula. This behaviour is at variance
with that currently reported for compounds illustrating the linear chain of
coupled orbits model.Comment: accepted for publication in europhysics Letter
Fermi Surface Reconstruction in CeRhCoIn
The evolution of the Fermi surface of CeRhCoIn was studied as
a function of Co concentration via measurements of the de Haas-van Alphen
effect. By measuring the angular dependence of quantum oscillation frequencies,
we identify a Fermi surface sheet with -electron character which undergoes
an abrupt change in topology as is varied. Surprisingly, this
reconstruction does not occur at the quantum critical concentration ,
where antiferromagnetism is suppressed to T=0. Instead we establish that this
sudden change occurs well below , at the concentration x ~ 0.4 where long
range magnetic order alters its character and superconductivity appears. Across
all concentrations, the cyclotron effective mass of this sheet does not
diverge, suggesting that critical behavior is not exhibited equally on all
parts of the Fermi surface.Comment: 4 pages, 4 figure
Magnetization of small lead particles
The magnetization of an ensemble of isolated lead grains of sizes ranging
from below 6 nm to 1000 nm is measured. A sharp disappearance of Meissner
effect with lowering of the grain size is observed for the smaller grains. This
is a direct observation by magnetization measurement of the occurrence of a
critical particle size for superconductivity, which is consistent with
Anderson's criterion.Comment: 7 pages, 5 figures, Submitted to PR
Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite
Electric field effect devices based on mesoscopic graphite are fabricated for
galvanomagnetic measurements. Strong modulation of magneto-resistance and Hall
resistance as a function of gate voltage is observed as sample thickness
approaches the screening length. Electric field dependent Landau level
formation is detected from Shubnikov de Haas oscillations in
magneto-resistance. The effective mass of electron and hole carriers has been
measured from the temperature dependant behavior of these oscillations.Comment: 4 pages, 4 figures included, submitted to Phys. Rev. Let
Pressure induced softening of YB_6: pressure effect on the Ginzburg-Landau parameter \kappa=\lambda/\xi
Measurements of the transition temperature T_c, the second critical filed
H_{c2} and the magnetic penetration depth \lambda under hydrostatic pressure
(up to 9.2 kbar) in the YB_6 superconductor were carried out. A pronounced and
{\it negative} pressure effects (PE) on T_c and H_{c2} with dT_c/dp=-0.0547(4)
K/kbar and \mu_0dH_{c2}(0)/dp =-4.84(20) mT/kbar, and zero PE on \lambda(0)
were observed. The PE on the coherence length d\xi(0)/dp=0.28(2) nm/kbar was
calculated from the measured pressure dependence of H_{c2}(0). Together with
the zero PE on the magnetic penetration depth \lambda(0), our results imply
that the Ginzburg-Landau parameter \kappa(0)=\xi(0)/\lambda(0) depends on
pressure and that pressure "softens" YB_6, e.g. moves it to the type-I
direction.Comment: 4 pages, 2 figure
Quantum oscillations in graphene in the presence of disorder and interactions
Quantum oscillations in graphene is discussed. The effect of interactions are
addressed by Kohn's theorem regarding de Haas-van Alphen oscillations, which
states that electron-electron interactions cannot affect the oscillation
frequencies as long as disorder is neglected and the system is sufficiently
screened, which should be valid for chemical potentials not very close to the
Dirac point. We determine the positions of Landau levels in the presence of
potential disorder from exact transfer matrix and finite size diagonalization
calculations. The positions are shown to be unshifted even for moderate
disorder; stronger disorder, can, however, lead to shifts, but this also
appears minimal even for disorder width as large as one-half of the bare
hopping matrix element on the graphene lattice. Shubnikov-de Haas oscillations
of the conductivity are calculated analytically within a self-consistent Born
approximation of impurity scattering. The oscillatory part of the conductivity
follows the widely invoked Lifshitz-Kosevich form when certain mass and
frequency parameters are properly interpreted.Comment: Appendix A was removed, as the content of it is already contained in
Ref. 17. Thanks to M. A. H. Vozmedian
Condon Domain Phase Diagram for Silver
We present the Condon domain phase diagram for a silver single crystal
measured in magnetic fields up to 28 T and temperatures down to 1.3 K. A
standard ac method with a pickup coil system is used at low frequency for the
measurements of the de Haas-van Alphen effect (dHvA). The transition point from
the state of homogeneous magnetization to the inhomogeneous Condon domain state
(CDS) is found as the point where a small irreversibility in the dHvA
magnetization arises, as manifested by an extremely nonlinear response in the
pickup voltage showing threshold character. The third harmonic content in the
ac response is used to determine with high precision the CDS phase boundary.
The experimentally determined Condon domain phase diagram is in good agreement
with the theoretical prediction calculated by the standard Lifshitz-Kosevich
(LK) formula
- …