37 research outputs found
Quantum Critical Point in the Spin Glass-Kondo Transition in Heavy Fermion Systems
The Kondo-Spin Glass competition is studied in a theoretical model of a Kondo
lattice with an intra-site Kondo type exchange interaction treated within the
mean field approximation, an inter-site quantum Ising exchange interaction with
random couplings among localized spins and an additional transverse field in
the x direction, which represents a simple quantum mechanism of spin flipping.
We obtain two second order transition lines from the spin-glass state to the
paramagnetic one and then to the Kondo state. For a reasonable set of the
different parameters, the two second order transition lines do not intersect
and end in two distinct QCP.Comment: 20 pages; 1 figure; to appear in Physical Review
Quantum and classical criticality in a dimerized quantum antiferromagnet
A quantum critical point (QCP) is a singularity in the phase diagram arising
due to quantum mechanical fluctuations. The exotic properties of some of the
most enigmatic physical systems, including unconventional metals and
superconductors, quantum magnets, and ultracold atomic condensates, have been
related to the importance of the critical quantum and thermal fluctuations near
such a point. However, direct and continuous control of these fluctuations has
been difficult to realize, and complete thermodynamic and spectroscopic
information is required to disentangle the effects of quantum and classical
physics around a QCP. Here we achieve this control in a high-pressure,
high-resolution neutron scattering experiment on the quantum dimer material
TlCuCl3. By measuring the magnetic excitation spectrum across the entire
quantum critical phase diagram, we illustrate the similarities between quantum
and thermal melting of magnetic order. We prove the critical nature of the
unconventional longitudinal ("Higgs") mode of the ordered phase by damping it
thermally. We demonstrate the development of two types of criticality, quantum
and classical, and use their static and dynamic scaling properties to conclude
that quantum and thermal fluctuations can behave largely independently near a
QCP.Comment: 6 pages, 4 figures. Original version, published version available
from Nature Physics websit
Kondo effect in a quantum critical ferromagnet
We study the Heisenberg ferromagnetic spin chain coupled with a boundary
impurity. Via Bethe ansatz solution, it is found that (i) for J>0, the impurity
spin behaves as a diamagnetic center and is completely screened by 2S bulk
spins in the ground state, no matter how large the impurity spin is; (ii) The
specific heat of the local composite (impurity plus 2S bulk spins which form
bound state with it) shows a simple power law . (iii)For
J<0, the impurity is locked into the critical behavior of the bulk. Possible
phenomena in higher dimensions are discussed.Comment: 6page Revtex, no figure, final version in PRB, Jun 1 issue, 199
Symmetries of Pairing Correlations in Superconductor-Ferromagnet Nanostructures
Using selection rules imposed by the Pauli principle, we classify pairing
correlations according to their symmetry properties with respect to spin,
momentum, and energy. We observe that inhomogeneity always leads to mixing of
even- and odd-energy pairing components. We investigate the superconducting
pairing correlations present near interfaces between superconductors and
ferromagnets, with focus on clean systems consisting of singlet superconductors
and either weak or half-metallic ferromagnets. Spin-active scattering in the
interface region induces all of the possible symmetry components. In
particular, the long-range equal-spin pairing correlations have odd-frequency
s-wave and even-frequency p-wave components of comparable magnitudes. We also
analyze the Josephson current through a half-metal. We find analytic
expressions and an interesting universality in the temperature dependence of
the critical current in the tunneling limit.Comment: 20 pages, 5 figures, added citations, corrected typo
Coexistence of antiferromagnetism and superconductivity in the Anderson lattice
We study the interplay between antiferromagnetism and superconductivity in a
generalized infinite- Anderson lattice, where both superconductivity and
antiferromagnetic order are introduced phenomenologically in mean field theory.
In a certain regime, a quantum phase transition is found which is characterized
by an abrupt expulsion of magnetic order by d-wave superconductivity, as
externally applied pressure increases. This transition takes place when the
d-wave superconducting critical temperature, , intercepts the magnetic
critical temperature, , under increasing pressure. Calculations of the
quasiparticle bands and density of states in the ordered phases are presented.
We calculate the optical conductivity in the clean limit. It
is shown that when the temperature drops below a double peak structure
develops in .Comment: 18 pages, 13 figure
Correlated electrons in the presence of disorder
Several new aspects of the subtle interplay between electronic correlations
and disorder are reviewed. First, the dynamical mean-field theory
(DMFT)together with the geometrically averaged ("typical") local density of
states is employed to compute the ground state phase diagram of the
Anderson-Hubbard model at half-filling. This non-perturbative approach is
sensitive to Anderson localization on the one-particle level and hence can
detect correlated metallic, Mott insulating and Anderson insulating phases and
can also describe the competition between Anderson localization and
antiferromagnetism. Second, we investigate the effect of binary alloy disorder
on ferromagnetism in materials with -electrons described by the periodic
Anderson model. A drastic enhancement of the Curie temperature caused by
an increase of the local -moments in the presence of disordered conduction
electrons is discovered and explained.Comment: 17 pages, 7 figures, final version, typos corrected, references
updated, submitted to Eur. Phys. J. for publication in the Special Topics
volume "Cooperative Phenomena in Solids: Metal-Insulator Transitions and
Ordering of Microscopic Degrees of Freedom
Hall effect in the vicinity of quantum critical point in Tm1-xYbxB12
The angular, temperature and magnetic field dependences of Hall resistance
roH for the rare-earth dodecaboride solid solutions Tm1-xYbxB12 have been
studied in a wide vicinity of the quantum critical point (QCP) xC~0.3. The
measurements performed in the temperature range 1.9-300 K on high quality
single crystals allowed to find out for the first time in these fcc compounds
both an appearance of the second harmonic contribution in ro2H at QCP and its
enhancement under the Tm to ytterbium substitution and/or with increase of
external magnetic field. When the Yb concentration x increases a negative
maximum of a significant amplitude was shown to appear on the temperature
dependences of Hall coefficient RH(T) for the Tm1-xYbxB12 compounds. Moreover,
a complicated activation type behavior of the Hall coefficient is observed at
intermediate temperatures for x>0.5 with activation energies Eg~200K and
Ea~55-75K in combination with the sign inversion of RH(T) at low temperatures
in the coherent regime. The density of states renormalization effects are
analyzed within the variation of Yb concentration and the features of the
charge transport in various regimes (charge gap formation, intra-gap manybody
resonance and coherent regime) are discussed in detail in Tm1-xYbxB12 solid
solutions.Comment: 38 pages including 10 figures, 70 reference
Quantum phase transitions
In recent years, quantum phase transitions have attracted the interest of
both theorists and experimentalists in condensed matter physics. These
transitions, which are accessed at zero temperature by variation of a
non-thermal control parameter, can influence the behavior of electronic systems
over a wide range of the phase diagram. Quantum phase transitions occur as a
result of competing ground state phases. The cuprate superconductors which can
be tuned from a Mott insulating to a d-wave superconducting phase by carrier
doping are a paradigmatic example. This review introduces important concepts of
phase transitions and discusses the interplay of quantum and classical
fluctuations near criticality. The main part of the article is devoted to bulk
quantum phase transitions in condensed matter systems. Several classes of
transitions will be briefly reviewed, pointing out, e.g., conceptual
differences between ordering transitions in metallic and insulating systems. An
interesting separate class of transitions are boundary phase transitions where
only degrees of freedom of a subsystem become critical; this will be
illustrated in a few examples. The article is aimed on bridging the gap between
high-level theoretical presentations and research papers specialized in certain
classes of materials. It will give an overview over a variety of different
quantum transitions, critically discuss open theoretical questions, and
frequently make contact with recent experiments in condensed matter physics.Comment: 50 pages, 7 figs; (v2) final version as publishe