8,243 research outputs found
Some Experimental Signatures to look for Time-reversal Violating superconductors
We discuss some experimental signatures associated with the topological
structures of unconventional superconductor order parameters of form
, where , or . Specifically, we study
the topological surface states on the and equivalent surfaces of such
superconductors which are observable in Andreev tunneling experiments, as well
as evaluate the magnetic flux trapped in superconducting rings of such
superconductors with multiple grain-boundary Josephson junctions. Previous
experiments are examined and several new experiments suggested.Comment: 11 pages, 3 figure
Detection and Implications of a Time-reversal breaking state in underdoped Cuprates
We present general symmetry considerations on how a Time-reversal breaking
state may be detected by angle-resolved photoemission using circularly
polarized photons as has been proposed earlier. Results of recent experiments
utilizing the proposal in underdoped cuprates are analysed and found to be
consistent in their symmetry and magnitude with a theory of the Copper-Oxides.
These togather with evidence for a quantum critical point and marginal
Fermi-liquid properties near optimum doping suggest that a valid microscopic
theory of the phenomena in the cuprates has been found.Comment: A statement on detecting the Anyon state is added and some typos are
subtracte
Collective Modes in the Loop Current Ordered Phase of Cuprates
Recently two branches of weakly dispersive collective modes have been
discovered in under-doped cuprates by inelastic neutron scattering.
Polarization analysis reveals that the modes are magnetic excitations. They are
only visible for temperatures below the transition temperature to a broken
symmetry phase which was discovered earlier and their intensity increases as
temperature is further decreased. The broken symmetry phase itself has
symmetries consistent with ordering of orbital current loops within a unit-cell
without breaking translational symmetry. In order to calculate the collective
modes of such a state we add quantum terms to the Ashkin-Teller (AT) model with
which the classical loop current order has been described. We derive that the
mean field ground state of the quantum model is a product over all unit-cells
of linear combination of the four possible classical configurations of the loop
current order in each unit-cell. The collective modes are calculated by using a
generalized Holstein-Primakoff boson representation of orbital moment operators
and lead to three branches of gapped weakly dispersive collective modes. The
experimental results are consistent with the two lower energy branches; the
third mode is at a higher energy than looked for by present neutron scattering
experiments and might also be over-damped. Implications of the discovery of the
collective modes are discussed.Comment: 16 pages, 6 figure
A Theory of the Pseudogap State of the Cuprates
The phase diagram for a general model for Cuprates is derived in a mean-field
approximation. A phase violating time-reversal without breaking translational
symmetry is possible when both the ionic interactions and the local repulsions
are large compared to the energy difference between the Cu and O
single-particle levels. It ends at a quantum critical point as the hole or
electron doping is increased. Such a phase is necessarily accompanied by
singular forward scattering such that, in the stable phase, the density of
states at the chemical potential, projected to a particular point group
symmetry of the lattice is zero producing thereby an anisotropic gap in the
single-particle spectrum. It is suggested that this phase occupies the
"pseudogap" region of the phase diagram of the cuprates. The temperature
dependence of the single-particle spectra, the density of states, the specific
heat and the magnetic susceptibility are calculated with rather remarkable
correspondence with the experimental results. The importance of further direct
experimental verification of such a phase in resolving the principal issues in
the theory of the Cuprate phenomena is pointed out. To this end, some
predictions are provided.Comment: 41 pages, 8 figure
Theory of the Quantum Critical Fluctuations in Cuprates
The statistical mechanics of the time-reversal and inversion symmetry
breaking order parameter, possibly observed in the pseudogap region of the
phase diagram of the Cuprates, can be represented by the Ashkin-Teller model.
We add kinetic energy and dissipation to the model for a quantum generalization
and show that the correlations are determined by two sets of charges, one
interacting locally in time and logarithmically in space and the other locally
in space and logarithmically in time. The quantum critical fluctuations are
derived and shown to be of the form postulated in 1989 to give the marginal
fermi-liquid properties. The model solved and the methods devised are likely to
be of interest also to other quantum phase transitions
Heavy-Fermions in a Transition-Metal Compound:
The recent discovery of heavy-Fermion properties in Lithium Vanadate and the
enormous difference in its properties from the properties of Lithium Titanate
as well as of the manganite compounds raise some puzzling questions about
strongly correlated Fermions. These are disscussed as well as a solution to the
puzzles provided.Comment: late
Collective Modes in the Loop Ordered Phase of Cuprates
We show that the two branches of collective modes discovered recently in
under-doped Cuprates with huge spectral weight are a necessary consequence of
the loop-current state. Such a state has been shown in earlier experiments to
be consistent with the symmetry of the order parameter competing with
superconductivity in four families of Cuprates. We also predict a third branch
of excitations and suggest techniques to discover it. Using parameters to fit
the observed modes, we show that the direction of the effective moments in the
ground state lies in a cone at an angle to the c-axis as observed in
experiments
Narasimham Committee Report - Some Further Ramifications and Suggestions
This paper while agreeing with the general thrust of the Narasimham Committee Report. Calls attention to some logical corollaries of the Report and analyses some possible fallout from implementing the Report. We agree with the view that control of banking system should be under an autonomous body supervised by the RBI. However at the level of individual banks, closer scrutiny of lending procedures may be called for than is envisaged in the Report. In a freely functioning capital market the potential of government bonds is enormous, but this necessitates restructuring of the government bond market. The government bonds may then also be used as suitable hedging mechanisms by introducing options and futures trading. We recommend freeing up the operation of pension and provident fund to enable at least partial investment of such funds in risky securities. In the corporate sector, we believe that the current 2:1 debt equity norm is too high and not sustainable in the long term. We envisage that high debt levels and higher interest rates, combined with higher business risk may result in greater incidence of corporate sickness. This may call for various schemes for retrenched workers and amendment to land laws for easy exit of companies. On account of interdependencies across different policies, any sequencing of their implementation may be highly problematic. We therefore suggest a near simultaneity in the implementation of various reforms in order to build up a momentum which would be irreversible if people are to have confidence that the reforms will endure, and if we are to retain our credibility with international financial institutions.
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