TRANSPORT AND MAGNETORESISTANCE
IN LOW CARRIER DENSITY
FERROMAGNETS
P.B. LittlewoodCavendish Laboratory, Madingley Road, Cambridge
CB3 0HE, England
and Bell Laboratories, Lucent Technologies, Murray Hill,
NJ 07974, USA
In metallic magnets, scattering from magnetic fluctuations above
and
near TC may provide a substantial contribution to the electrical
resistance rho(T). However, this effect is usually small because the
dominant fluctuations are near q approx 0, which does not produce
substantial backscattering across the Fermi surface unless 2kF is
itself small; such a situation can be realised in metallically-doped
ferromagnetic semiconductors. A simple adaptation of the theory of
deGennes
and Friedel shows the low field magnetoresistance scales with the ratio
of field induced magnetisation m(H) to the saturation magnetisation
msat: Delta rho / rho approx
C(m/msat)2, where C approx
x-2/3, with x the number of charge carriers per magnetic unit cell.
Comparison to data on very different ferromagnetic metals and doped
semiconductors is in broad quantitative agreement with this trend, with
the prime exception of the perovskite manganese oxides, already understood
to involve the extra physics of dynamic lattice distortions. At very low
doping,
the physics should involve ferromagnetic polarons, and polaron formation
and transport are discussed.
PACS numbers: 75.70.Pa
|
CONDUCTANCE OF MESOSCOPIC MAGNETIC SYSTEMS
S. KrompiewskiInstitute of Molecular Physics, Polish Academy of
Sciences
Smoluchowskiego 17, 60-179 Poznań, Poland
Theoretical studies of electrical conductance of various nanowires
are performed within the framework of a semi-realistic s-d tight-binding
model. The presented results concern both homogeneous paramagnetic and
ferromagnetic systems as well as trilayers composed of 2 magnetic slabs
separated by a nonmagnetic spacer. On the one hand, in the case of the
homogeneous systems the attention is focused on conductance
quantization,
which manifests itself when a contact gets open and conduction decays in
a spectacular stepwise way. A new approach is developed by assuming that
in the
last stage of the breaking of the contact between wires there are fewer
and fewer, distributed at random, conduction paths passing through the
nanowire
cross-section. The corresponding conductances are calculated within the
quasiballistic regime, using the Kubo formula and a recursion Green
function
technique. The results for weak ferromagnets (when both majority and
minority
bands intersect the Fermi surface) are qualitatively different from
those for
strong ferromagnets (only the minority bands do), which may explain
experimental cumulative conductance histograms of Fe and Ni. On the
other
hand, giant magnetoresistances of magnetic trilayers are studied for
both
current-perpendicular-to-plane and current-in-plane geometries. The
corresponding magnetoresistances are compared with each other and with
the
interlayer exchange coupling.
PACS numbers: 73.40.Jn, 75.70.-i, 75.70.Pa, 73.23.Ad
|
ANOMALOUS TRANSPORT AND THERMAL PROPERTIES OF THE SOLID SOLUTIONS
U1-xThxCu5M (M =3D Al, Ga, In AND Sn)
R. Troc, V.H. Tran, D. Kaczorowski and A. CzopnikW.
Trzebiatowski
Institute of Low Temperature and Structure Research
Polish Academy of
Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland
UCu5M-type compounds are moderate heavy fermion systems with an
antiferromagnetic order below TN=3D18, 16-25, and 25 K for M =3D Al,
(Ga, In), and In, respectively, and a ferrimagnetic order with TC=3D
54 K
for M =3D Sn. We present the results of magnetoresistance measurements
as a function of temperature and applied magnetic field for the pure
compounds and
for their solid solutions containing Th, i.e. U1-xThxCu5M.
Only in the case of UCu5In a substantial positive contribution to the
magnetoresistance was observed at low temperatures, as for normal
antiferromagnets. For the Al- and Sn-based alloys the magnetoresistance
at low temperatures is negative, and its field variation proves the
existence of
Kondo-type interactions. On Th-substitution the magnetic phase
transition is
shifted to lower temperature and simultaneously the magnetoresistance
becomes
less negative. The field dependence of the magnetoresistance of these
alloys
can be well described by the Coqblin-Schrieffer model. Interestingly,
the
heat capacity measurements have revealed a general tendency to increase
the linear coefficient gamma with magnetic dilution of a given UCu5M
compound by Th-substitution. This feature reflects an enlargement of the
effective mass of conduction electrons with the increase in both the
unit
cell volume and magnetic dilution. Such a tendency is also observed for
the system UCu5(In1-xGax), though the unit cell is here suppressed
with increasing x. Thus, a similar physical picture is reached in all
these
systems but involving probably different mechanisms.
PACS numbers: 75.30.Mb,
72.15.Qm, 75.20.Hr
|
SEEBECK COEFFICIENT OF CePd3
G.D. Mahana and M. Bartkowiaka,ba Department of Physics and
Astronomy, University of Tennessee
Knoxville, TN 37996, USA, and
Solid
State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6032, USA
b Institute of Physics, A. Mickiewicz University
Umultowska 85,
61-614 Poznań, Poland
Rare earth compounds have potential applications in thermoelectric
devices due to their large value of conductivity and Seebeck
coefficient. CePd3 has the highest reported Seebeck coefficient
(S sim 125 µV/K),
when doped appropriately, among all rare-earth compounds. This high
value is
explained as a result of the placement of the cerium f1 level.
PACS numbers: 72.15.Jf,
72.15.Qm, 75.20.Hr
|
Ce3Cu3Sb4: A CANTED ANTIFERROMAGNETIC SEMIMETAL
P. Wachter, L. Degiorgi, G. Wetzel, H. Schwer, K.
MattenbergerLaboratorium für Festkörperphysik, Eidgenössische
Technische Hochschule Zürich
8093 Zürich, Switzerland
T. Herrmannsdörfer and P. FischerLaboratory for Neutron
Scattering,
Paul Scherrer Institute PSI
5232 Villigen, Switzerland
It has been claimed by Patil et al., that Ce3Cu3Sb4 is the first Ce-based
semiconducting ferromagnet. In this paper it is shown, mainly with Hall
effect and far infrared spectroscopy that no gap in the excitation
spectrum
exists, as well above as below the magnetic ordering temperature. A
maximum
in the resistivity near TC is due to trapped magnetic polarons. The
resistivity is an effect of the mobility of the charge carriers. The
structure of the magnetic unit cell has been determined with elastic
neutron
scattering.
PACS numbers: 75.30.-m, 78.20-e, 72.15.Eb, 72.15.Gd
|
BAND GAP STABILITY IN KONDO INSULATORS
A. SlebarskiInstitute of Physics, University of Silesia,
Uniwersytecka 4, 40-007 Katowice, Poland
A. JezierskiInstitute of Molecular Physics, Polish Academy of
Sciences
Smoluchowskiego 17, 60-179 Poznań, Poland
and A. ZygmuntInstitute for Low Temperature and Structure
Research,
Polish Academy of Sciences
Okólna 2, 50-950 Wroclaw, Poland
We report on magnetic measurements and electronic structure
investigations of CeNiSn and CeRhSb. Both belong to the group of Kondo
insulators. The magnetic susceptibility shows the nonmagnetic ground
state
for these compounds and their alloys. The 3d X-ray photoemission
spectroscopy spectra show evidence for the mixed valence state of Ce in
CeRhSb alloys, as also seen for CeNiSn, whereas the spectra for the La
substituted (Ce,La)NiSn compounds show only evidence for a pure Ce3+
ground state. We suggest the presence of Kondo-hole states in
(Ce,La)RhSb.
The location of the pseudogap in CeRhSb varies with the number of free
electron, the valence of Ce, and the f-d hybridization. We discuss the
similar crystallographic properties and the closed electronic structures
of ZrNiSn-type semi-Heusler alloys and CeNiSn-type Kondo
insulators.
PACS numbers: 71.20.-b, 71.20.Eh
|
KONDO-LATTICE METALLIC
AND SEMICONDUCTING STATES
AND THEIR
INSTABILITIES
J. SpalekMarian
Smoluchowski Institute of Physics, Jagiellonian University
Reymonta 4,
30-059 Kraków, Poland
We review briefly recent results concerning the stability of
para-,
ferro-, and antiferromagnetic states in the Kondo-lattice limit. Both
the
macroscopic quantities such as the specific heat and the electrical
resistivity, as well as the magnitude of the magnetic moment and the
mass enhancement (together with its spin dependence) are
discussed.
PACS numbers: 71.27.+a, 71.10.-w, 71.10.Fd
|
CRITICAL AND MASSIVE PHASES
IN SPIN CHAINS AND SPIN LADDERS
J. SólyomResearch Institute for Solid State Physics, Hungarian
Academy of Sciences
P.O. Box 49, 1525 Budapest, Hungary
Spin chains and spin ladders can have a variety of gapless
(critical)
or gapped (massive) phases depending on the length of the spin and the
type
of coupling. A brief review of the results on some simple models is
given
with emphasis on the generation of the Haldane gap in anisotropic spin
ladders.
PACS numbers: 75.10.Jm
|
MAGNON AND SOLITON EXCITATIONS
IN THE CARRIER-POOR,
ONE-DIMENSIONAL
S=3D1/2 ANTIFERROMAGNET Yb4As3
F. Steglich, M. Köppen, P. Gegenwart, T. Cichorek, B. Wand,
M. Lang,
P. Thalmeier, B. SchmidtMax-Planck Institute for Chemical Physics of
Solids, 01187 Dresden, Germany
H. Aoki and A. OchiaiDepartment of Material Science and
Technology, Niigata 950-21, Japan
The semimetallic quasi-one-dimensional S=3D1/2 Heisenberg
antiferromagnet Yb4As3 was studied by low-temperature measurements of
the specific heat C(T,B), thermal expansion alpha(T,B), and thermal
conductivity kappa(T,B). At finite magnetic fields (B<=12 T) we
observed the following distinct anomalies: (1) the magnon contribution
to C(T,0), gamma T, with large coefficient
gamma approx 200 mJ/(K2mol), becomes strongly reduced with field,
and (2) a broad hump in C(T,B=3Dconst) is induced at slightly higher
temperatures. (3) The latter corresponds to a pronounced peak in
alpha(T,B=3Dconst) as well as (4) to a broad minimum in kappa(T,B=3D
const)/kappa(T,0). These anomalies are well described by the classical
sine-Gordon solution of a one-dimensional Heisenberg antiferromagnet
with a weak easy-plane anisotropy. However, the soliton-rest energy deduced
from the experimental results depends on the magnetic field like ES ~ Bnu,
with an exponent \nu\approx 0.66, while the classical sine-Gordon model
requires nu=3D1. Thus, our results suggest an alternative description of soliton
excitations in an antiferromagnetic S=3D1/2 Heisenberg chain in terms
of the quantum sine-Gordon model, for which an exponent nu=3D2/3 is
appropriate.
PACS numbers: 75.30.Mb, 65.50.+m, 65.70.+y
|
SPIN POLARONS IN TWO-DIMENSIONAL SYSTEMS
J.A. MorkowskiInstitute of Molecular Physics, Polish Academy of
Sciences
Smoluchowskiego 17, 60-179 Poznań, Poland
The problem of motion of holes on a square lattice of
antiferromagnetically ordered spins is considered. An overview of the
theory
of hole dynamics in the antiferromagnetic background is presented.
Motion of
a single hole is rather well understood both by numerical and analytical
methods. For small but finite concentration of holes a treatment
analogous to
the standard polaron theory is discussed starting with an exact mapping
of the t-J Hamiltonian into holon and pseudospin variables.
PACS numbers: 71.27.+a,
72.10.Di, 74.20.Mn
|
THE SPIN-VALVE TRANSISTOR -
A NEW MAGNETOELECTRONIC DEVICE
P.S. Anil Kumar and J.C. LodderInformation Storage Technology
Group,
MESA+ Research Institute
University of Twente, 7500 AE Enschede, The
Netherlands
A spin-valve transistor showing high sensitivity at low fields was
developed. A large magnetocurrent, above 500% is realized by a magnetic
field change of 2 to 4 Oe at 80 K. Hot electrons are injected into the
spin-valve layer through a Si-Pt Schottky diode. These hot electrons,
while
traversing through the spin-valve, are spin-dependently scattered. Those
electrons with right energy and momentum are collected by a collector
(an Au-Si Schottky diode) constituting a collector current. The relative
orientation of the magnetic layer in the spin-valve is changed by the
application of a magnetic field and causes a change in collector current
giving a large magnetocurrent.
PACS numbers: 75.70.-i, 75.70.Pa
|
SCATTERING THEORY OF THE JOHNSON
SPIN TRANSISTOR
L.S. Geux, A. Brataas and G.E.W. BauerDelft University of
Technology,
Laboratory of Applied Physics and DIMES
2628 CJ Delft, The Netherlands
We discuss a simple, semiclassical scattering theory for
spin-dependent
transport in a many-terminal formulation, with special attention to the
four terminal device of Johnson referred to as spin transistor.
PACS numbers: 73.40.Gk,
73.23.Hk, 75.70.Pa
|
FROM MONOATOMIC MULTILAYERS
TO ORDERED ALLOYS
J. Koreckia,b, M. Kubika, N. Spiridisb and T.
Slezakaa Department of Solid State Physics, Faculty of Physics and
Nuclear Techniques
University of Mining and Metallurgy, Al. Mickiewicza
30, 30-059 Kraków, Poland
b Institute of Catalysis and Surface Chemistry,
Polish Academy of Sciences
Niezapominajek, 30-239 Kraków, Poland
Recent progress in UHV preparation and characterization methods
resulted in a large variety of novel materials. Among them, magnetic
multilayers have become one of the mostly investigated system due to
interesting phenomena like oscillating indirect exchange coupling, spin
dependent electron transport, or large perpendicular anisotropies. An
attractive possibility given by the molecular beam epitaxy is to grow
the multilayer structures on atomic scale by the so-called atomic layer
deposition. At the low thickness limit, a multilayer structure, in which
few atomic layers of different metals are stacked alternately, is expected
to be an artificial ordered alloy. Such artificial material, which does not
exist in the equilibrium bulk phase, was constructed for the first time as the
AuFe ordered alloy of the L10 structure. Our conversion electron Mössbauer
spectroscopy studies of this system verified the existence of the
tetragonal phase, which is responsible for the perpendicular anisotropy. The
ordering process is influenced by the complicated growth of Fe on Au, as shown by
the atomic scale scanning tunneling microscopy investigations. Other systems
to be presented are FeAl (strong ordering mechanism in the bulk) and FeCr
(miscible in the wide concentration range) monoatomic
PACS numbers: 75.70.Cn, 75.50.Bb, 76.80.+y
|
CORRELATION-DRIVEN METAL-INSULATOR TRANSITIONS
J.M. HonigDepartment of Chemistry, Purdue University, West
Lafayette, 47907-1393, USA
The effect of correlation-driven electronic transitions are
described
for the V2O3, NiS2-xSex,
and Fe3O4 systems. The various
tranformations can all be rationalized in terms of elementary concepts
pertaining to the Mott-Hubbard intraatomic electronic
interactions or in terms of an order-disorder formalism
involving Coulomb interactions among electrons on adjacent sites.
Attention
is directed to some outstanding issues that require further
resolution.
PACS numbers: 71.30.+h, 71.45.Gm
|
THE PSEUDOGAP PHENOMENON
IN HIGH-Tc SUPERCONDUCTORS
J. Ranningera and A. Romanoba Centre
de Recherches sur les
Trčs Basses Températures
Laboratoire Associéą l'Université
Joseph Fourier
Centre National de la Recherche Scientifique, B.P. 166, 38042,
Grenoble Cédex 9, France
b Dipartimento di Scienze Fisiche ``E.R.
Caianiello'', Universitą di Salerno
Unitą I.N.F.M. di Salerno,
84081 Baronissi (Salerno), Italy
We review the possible mechanisms leading up to the phenomenon of
a pseudogap which characterizes the normal-state properties of high-Tc
superconductors. We suggest that this pseudogap is not due to superconducting
phase fluctuations and hence is not related to a crossover between a BCS
state of Cooper pairs and a Bose-Einstein condensation of local pairs.
We rather argue that it is due to uncorrelated pairing which is already
manifest
in the local electronic structure and accessible by photoemission and
tunneling experiments.
PACS numbers: 74.20.-z, 74.25.-q, 74.25.Jb
|
NON-COLLINEAR MAGNETISM
IN ITINERANT-ELECTRON SYSTEMS
J. KüblerDarmstadt University of Technology, Institute of Solid
State Physics
64289 Darmstadt, Germany
The Heisenberg model cannot uncritically be applied to
itinerant-electron magnets, including those that show non-collinear
order.
Density functional theory is therefore generalized to apply to
non-collinear
itinerant-electron magnets. The appropriate Kohn-Sham equations are
discussed and the total energy of spiral magnetic order is used to
determine
the magnon spectrum in the adiabatic approximation. The energy spectrum
of transverse spin fluctuations can also be estimated with the total energy
of spiral magnetic order and allows a determination of thermal properties
of itinerant-electron magnets. Calculated results for Fe, Co, Ni, and FeCo
are discussed and compared with experimental results.
PACS numbers: 71.15.Mb, 75.10.Lp,
75.30.Ds, 75.30.Kz
|
CONDUCTIVITY AND MAGNETISM
OF MAGNETIC OXIDES
G.A. GehringDepartment of Physics and Astronomy, University of
Sheffield
Sheffield S3 7RH, U.K.
In a stoichiometric oxide the energy for the magnetic ordering is
due to superexchange. This depends on the virtual transfer of a d electron
from the transition ion to the neighbouring oxygen. When the oxide is p-doped
there are compensating holes on the oxygen or the transition ion becomes
mixed valent. The oxide may then conduct. The same transfer integral
enters both the expression for the antiferromagnetic superexchange and the band
width of the mobile carriers. Thus materials with a large
antiferromagnetic
exchange energy will be expected to have a relatively wide conduction
band in
the doped state and hence to have a high conductivity. In this paper the
difference is explored between the materials in which there is true
antiferromagnetism and those which are ferrimagnetic. In the
antiferromagnets
the carriers must destroy the magnetic order as they move. This
behaviour is
well known from the cuprates. In ferrimagnets the carriers may be able
to move entirely on one sublattice. This occurs in Fe3O4
and probably in
the doped garnets. In the case where motion is on one sublattice then
doping
does not destroy the magnetism and there is a relatively small
magnetoresistance. An interesting feature is that unlike the cuprates
the ferrimagnets do not become good metals at any doping but exhibit hopping
conductivity. We explain the apparent paradox that the best conductivity
is actually observed in materials where the conduction is only allowed when
the antiferromagnetism has been quenched and that the conductivity in
ferrimagnets is low.
PACS numbers: 75.50.Pp, 75.50.Gg
|
``COMB-LIKE'' POLARONS AND BIPOLARONS
IN HIGH-Tc MATERIALS
E.M. Hankiewicz, R. Buczko and Z. WilamowskiInstitute of Physics,
Polish Academy of Sciences
Al. Lotników 32/46, 02-668 Warszawa, Poland
Diagonalization of Hamiltonian composed from d-d exchange
interactions between localized d spins of antiferromagnetic cluster and
p-d interaction with the spin of carriers indicates a possibility of
formation of pure magnetic polarons. The most energetically favorable
solution occurs when the carrier density in CuO2 planes is distributed
on every second spin. These ``comb-like'' polarons have a tendency to bind
into
pairs (bipolarons) ``glued'' by the antiferromagnetic
medium.
PACS numbers: 74.20.-z, 75.10.-b, 74.20.Mn
|
SUBSTITUTION STUDY ON THE COHERENT KONDO STATE IN CeCu5 In
D. Kaczorowskia, Ya.M. Kalychakb and V.I.
Zarembaba W. Trzebiatowski Institute of Low Temperature
and Structure Research
Polish
Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland
b Department
of Inorganic Chemistry, Lviv State University, 290 005 Lviv, Ukraine
Ternary alloys CeCu5-xIn1+x were studied by means of
magnetic and electrical resistivity measurements. In CeCu5In a coherent
Kondo state without long-range magnetic order develops at low
temperatures.
It transforms into an incoherent Kondo state upon small substitution of
In for Cu.
PACS numbers: 75.30.Mb, 72.15.Qm, 75.20.Hr
|
SPIN EXCITATIONS IN DOPED MANGANITES
A.M. Olesa and L.F. Feinerba Institute of Physics,
Jagiellonian University, Reymonta 4, 30-059 Kraków,
Poland
and
Max-Planck-Institut FKF, Heisenbergstrasse 1, 70569
Stuttgart, Germany
b Institute for Theoretical Physics, Utrecht University
3584
CC Utrecht, The Netherlands
and
Philips Research Labs, Prof. Holstlaan 4,
5656 AA Eindhoven, The Netherlands
We generalize the double exchange to describe holes doped in
degenerate
eg orbitals in La1-xAxMnO3 manganites,
where A are divalent alkaline earth ions. Assuming an orbital liquid of disordered eg
orbitals we find isotropic ferromagnetic exchange interactions which
increase with hole doping. The magnon dispersion agrees very well with
the experimental data in ferromagnetic metallic manganites.
PACS numbers: 75.30.Ds,
75.30.Et, 71.27.+a
|
SPECTRAL FUNCTIONS OF THE QUANTUM DOT COUPLED TO NORMAL
AND/OR
SUPERCONDUCTING LEADS
M. Krawiec and K.I. WysokińskiInstitute of Physics, M.
Curie-Sklodowska University
Radziszewskiego 10, 20-031 Lublin, Poland
We study the spectrum of the quantum dot coupled to two external
leads, which may be normal and/or superconducting. The dot is described by the
Anderson-Hubbard impurity model in the infinity U limit. The impurity
spectral function shows complex behavior depending on the temperature
and the
state of the leads. For normal leads and at low temperature we observe
the appearance of the Kondo resonance, while for BCS-like superconducting
leads the bound states emerge due to absence of low energy excitations in the
leads.
PACS numbers: 73.40.Gk, 72.15.Qm, 71.27.+a
|
DETERMINING THE FERMI SURFACE
FROM ARPES INTEGRALS
K. MathoCentre de
Recherches
sur les Trčs Basses Températures
Laboratoire associéą
l'Université Joseph Fourier, CNRS
B.P. 166, 38042 Grenoble cedex 9,
France
ARPES integrals are related to the momentum distribution nk. In
case
of a metal, points kF on the Fermi surface were identified by: (i) a
change of sign in the temperature variation of the ARPES integrals or
(ii)
maximum slope in its angular variation. These criteria are based on the
assumption of particle-hole symmetry in the vicinity of the Fermi edge.
Here, we check (i) and (ii) on the level of the momentum distribution,
for an electronic structure with most of its incoherent weight below the Fermi
edge. Evaluating nk(T) up to T approx Deltap*,
a Fermi liquid coherence energy, we find: criterion (i) remains stable, while (ii) deviates from
kF propto m*T/kF. Published data on the hole doped t-J
model are examined in this light.
PACS numbers: 71.10.Ay, 71.18.+y, 79.60.-i
|
NEW S=3D1/2 ALTERNATING CHAIN COMPOUND
- HIGH PRESSURE FORM OF
(VO)2P2O7-
M. Azumaa, T. Saitoa, Z. Hiroib,
M. Takanoa, Y. Narumic
and K. Kindoca Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan
b Institute for Solid State
Physics, University of Tokyo
Roppongi 7-22-1, Minato-ku, Tokyo
106-8666,
Japan
c KYOKUGEN, Osaka University, Toyonaka, Osaka 560-8531, Japan
The high-pressure phase of (VO)2P2O7 comprises
a unique kind of spin-1/2 Heisenberg alternating antiferromagnetic chain, in contrast
with
the ambient pressure phase containing two crystallographically different
chains. Magnetic susceptibility and high-field magnetization data showed
the presence of a single spin gap of 23 (magnetization) ~27
(susceptibility) K, not double as observed for the ambient pressure phase. This result
shows that the two kinds of chains of the ambient pressure phase have single,
different spin gaps.
PACS numbers: 75.10.Jm, 75.40.Cx, 75.50.Ee
|
BOUND STATES OF FERMIONS ON 2D LATTICE IN A DILUTE LIMIT
M. Bak and R. MicnasInstitute of Physics A. Mickiewicz
University
Umultowska 85, 61-614 Poznań, Poland
We examine extended bound states in a dilute limit of the extended
Hubbard model on the two-dimensional square lattice. By solving exactly
the two-body problem we have determined the binding energies, mobilities,
and dispersion curves across the Brillouin zone for bound states of various
symmetries. It turns out that the d-wave pairing is strongly favored by
the nnn hopping and the intersite local pairs can have small effective
masses, even in the case of strong binding.We have also found a possibility of
extended s-dx2-y2 mixing of the
bound states.
PACS numbers: 74.20.-z, 03.65.Ge, 71.10.Fd
|
CORRELATION FUNCTIONS
IN THE SPIN-FERMION MODEL
J. Bala and A.M. OlesInstitute of Physics, Jagellonian
University, Reymonta 4, 30-059 Kraków, Poland
We show that the quasiparticle states found in a self-consistent
Born approximation for the spin-fermion model which describes CuO2
planes agree
well with experimental data in insulating Sr2CuO2Cl2.
It is found
that the antiferromagnetic correlations are reduced in the neighbourhood
of a moving oxygen hole and the correlation functions can exhibit similar
dipolar
distortions as reported in the context of the t-J model.
PACS numbers: 71.27.+a,
75.10.-b, 74.72.-h, 79.60.-i
|
SUPERCONDUCTING CHARACTERISTICS
OF THE PENSON-KOLB MODEL
W.R. Czart and S. RobaszkiewiczInstitute of Physics, Adam
Mickiewicz University, Umultowska 85, 61-614 Poznań
We study superconducting properties of the Penson-Kolb model,
i.e. the
tight-binding model with the pair-hopping (intersite charge exchange)
interaction J. The evolution of the critical fields, the coherence
length, the Ginzburg ratio, and the London penetration depth with particle
concentration n and pairing strength are determined. The results are
compared with those found earlier for the attractive Hubbard
model.
PACS numbers: 74.20.-z, 71.28.+d, 74.25.Ha
|
BREAKDOWN OF THE DE GENNES SCALING
IN Y1-xDyxNi2B2C
Z. Drzazga, A. Winiarska, A. WiniarskiInstitute of Physics,
University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
and D. KaczorowskiInstitute of Low Temperature and Structure
Research,
Polish Academy of Sciences
P.O. Box 937, Wroclaw 2, Poland
Single crystals of Y1-xDyxNi2B2C
(1<=x\lt;=0.7) were
examined by X-ray diffraction and stoichiometry was attested by XPS
measurements. The superconducting transition temperature Tc and Neel
temperature TN were determined by means of magnetisation measurements.
A dramatic breakdown of the de Gennes scaling in the Dy-rich compounds
was found when Tc<TN.
PACS numbers: 74.25.Ha, 74.72.-h, 82.80.Pv
|
STRIPE PHASES IN THE HUBBARD MODEL
D. Góra, K. Rosciszewski and A.M. OlesInstitute of Physics,
Jagellonian University, Reymonta 4, 30-059 Kraków, Poland
We investigate the magnetic and charge ordering in the stripe
phases obtained while using correlated wave functions for finite 8×8
clusters
described by the Hubbard model with extended hopping. Nonmagnetic
vertical
(01) site-centered domain walls and (11) bond-centered stripes are found
for the parameters of La2-xSrxCuO4 and
YBa2Cu3O6+x,
respectively, at doping delta=3D1/8. The obtained half-filled domain
nonmagnetic walls reproduce the maxima observed in neutron scattering
for La2-xSrxCuO4.
PACS numbers: 71.27.+a, 74.72.-h, 75.10.-b, 64.60.-i
|
ON ELECTRONS IN QUANTUM CHAOS STATE
IN DOPED FULLERENE CRYSTAL
A. Koper and M. MuchaInstitute of Physics, Adam Mickiewicz
University
Umultowska 85, 61-614 Poznań, Poland
We show that band electrons in AnC60
crystal (C60 fullerene doped with alkali ions A) are in highly
chaotic quantum state. We
describe intensity of the chaos by means of the Shannon information
entropy,
which we calculate using single particle Bloch functions. The entropy
provides a quantitative measure of scars as well as degree of electrons
delocalization in gaps between C60 molecules. Implications of our
results for conductivity in A3C60 are discussed.
PACS numbers: 05.45.+b,
61.48.+c
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SIMPLE BAND MODEL OF DOPED
FULLERENE CRYSTAL
A. Koper and M. MuchaInstitute of Physics, Adam Mickiewicz
University
Umultowska 85, 61-614 Poznań, Poland}
We use quantum
billiard with many scattering centers to describe conducting electrons
properties in AnC60 crystals, where A denotes alkali metal.
We focus our attention on the A3C60 crystal, for which we
calculate the band
structure, density of states, and conductivity for normal electrons.
Conductivity shows linear dependence on temperature in this model, which
agrees well with experimental data. We also discuss consequences of our
results for superconductivity mechanism in A3C60 and possibilities
of analogous approach to describe electron properties in fused fullerenes
and multiply connected carbon clusters.
PACS numbers: 74.20.Mn, 74.25.Kc, 74.25.Jb
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TUNNELING CURRENT
INTO THE VORTEX LATTICE STATES
OF s- AND
p-WAVE SUPERCONDUCTORS
L. Kowalewski, M.M. Nogala, M. Thomas and R.J.
WojciechowskiInstitute
of Physics, A. Mickiewicz University
Umultowska 85, 61-614 Poznań, Poland
The tunneling current between the metallic tip of a scanning
microscope
and s-wave and p-wave superconductors in a quantizing magnetic field is
investigated. The differential conductance is calculated both as a
function
of bias voltage at the centre of the vortex line and for varying
position of
the scanning tunneling microscope tip at a stable voltage.
PACS numbers: 74.50.+r,
74.60.-w, 71.70.Di
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SUPERCONDUCTING CRITICAL TEMPERATURE OF LAYERED MERCUROCUPRATE FAMILY
A.L. Kuzemskya, I.G. Kuzemskayab and A.A.
Cheglokovaa Bogolyubov Laboratory of Theoretical Physics, Joint
Institute for Nuclear Research
141980 Dubna, Moscow Region, Russia
b High Pressure Physics Institute, Russian Academy of
Sciences
142092 Troitsk, Moscow Region, Russia
We study the physics of the mercurocuprate family from the
structural
point of view. We present a phenomenological approach determining the
critical temperatures for homologous series
HgBa2Can-1CunO2n+2+delta
depending on the number of layers n. The model
is based on the Lawrence- Doniach theory of layered superconductors
modified for the case of inequivalent layers. The redistribution of
charge was taken into account. This leads to observable nonmonotonic
``bell''-shaped
dependence of Tc (n) and provides a quantitative explanation of the
experiments.
PACS numbers: 74.25.Nf, 74.72.Bk, 74.72.Gr
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DYNAMIC CRYSTAL FIELD IN CePb3
S. Lipiński and A. SzajekInstitute of Molecular Physics, Polish
Academy of Sciences
Smoluchowskiego 17, 60-179 Poznań, Poland
The crystal field of valence fluctuating systems is time dependent
due to f-electron transfers to the conduction band. We examine the effects
of crystal field fluctuations on the neutron spectra, the specific heat and
susceptibility of CePb3.
PACS numbers: 75.10.Dg, 75.20.Hr, 75.30.Mb
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VAN HOVE SINGULARITY AND SUPERCONDUCTIVITY IN DISORDERED Sr2RuO4
G. Litaka, J.F. Annettb and B.L.
Györffyba Department of
Mechanics, Technical University of Lublin
Nadbystrzycka 36, 20-618
Lublin, Poland
b H.H. Wills Physics Laboratory, University of Bristol
Tyndall
Ave, Bristol BS8 1TL, United Kingdom
On the basis of a simple model we analyse the influence of
disorder on
critical temperature Tc in p-wave superconductors. The disorder is
treated by means of the coherent potential approximation and we focus
our attention on the effect of a van Hove singularity near Fermi energy EF.
For the appropriate values of its parameters our model reproduces the
experimentally found behaviour of Sr2 RuO4.
PACS numbers: 74.62.Dh, 74.25.Dw,
74.25.Fy
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CHEMICAL POTENTIAL EVIDENCE FOR PHASE TRANSITIONS IN MAGNETIC AND
SUPERCONDUCTING COMPOUNDS AND ALLOYS
M. Matlaka and M.
Pietruszkaba Institute of Physics, University of Silesia,
Uniwersytecka 4, 40-007 Katowice, Poland
b Faculty of Biology and
Environmental Protection, University of Silesia
Jagiellońska 28,
40-032 Katowice, Poland
We announce that all phase transitions (induced by temperature or
concentration) including structural ones and transitions between
metastable
or ``exotic'' states can be detected by the chemical potential
critical behaviour, as well as, from the average occupation numbers of the
electronic
system (critical electron redistribution).
PACS numbers: 74.70.-b, 75.30.Kz, 75.40.Cx
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