arXiv subjects

David I. Santiago

Publications and source records attributed to David I. Santiago.

69 records · Page 4Linked to original sources

arXiv2003

Spin Density Wave and D-Wave Superconducting Order Parameter "Coexistence"

We study the properties of a spin-density-wave antiferromagnetic mean-field ground state with d-wave superconducting (DSC) correlations. This ground state always gains energy by Cooper pairing. It would fail to superconduct at half-filling due to the antiferromagnetic gap although its particle-like excitations would be Bogolyubov-BCS quasiparticles consisting of coherent mixtures of electrons and holes. More interesting and relevant to the superconducting cuprates is the case when antiferromagnetic order is turned on weakly on top of the superconductivity. This would correspond to the onset of antiferromagnetism at a critical doping. In such a case a small gap proportional to the weak antiferromagnetic gap opens up for nodal quasiparticles, and the quasiparticle peak would be discernible. We evaluate numerically the absorption by nodal quasiparticles and the local density of states for several ground states with antiferromagnetic and d-wave superconducting correlations.

cond-mat.str-el↗

arXiv2003

Coherent Quantum Tunneling of Bose-Einstein Condensates in Optical Lattices

We study inter-well coherent quantum tunneling of Bose-Einstein condensates (BEC's) in optical lattices. Irrespective of whether the BEC is a superfluid in the whole lattice, it may or may not be a superfluid within each well of the lattice depending on whether the ratio of the well kinetic energy to the interaction energy is small or large. When the bosons in each well are superfluid, we have Josephson tunneling among the wells driven by the phase difference between the superfluids in the different wells. For typical lattice parameters the Josephson tunneling rate will be somewhat enhanced from the naive estimate of (tunneling energy scale)$/\hbar$. Whether it is enhanced or not will depend on lattice parameters, but it can never be zero. When the bosons in each well are not superfluid, there is coherent quantum tunneling of essentially free bosons between the wells. This tunneling is proportional to the boson number difference between the wells.

cond-mat.soft↗

arXiv2003

Quantum States of Matter of Simple Bosonic Systems: BEC's, Superfluids and Quantum Solids

The phase diagram of a single component Bose system in a lattice at zero temperature is obtained. We calculate the variational energies for the Mott insulating and superfluid phases. Below a certain critical density, which depends monotonically on the well depth of the lattice, the Mott insulating phase is stable over the superfluid phase for low enough tunelling amplitude regardless of whether the number of bosons is or is not incommensurate with the lattice. The transition is discontinuous as the superfluid order parameter jumps from a finite value to zero at the Mott transition.

cond-mat.soft↗

arXiv2003

On Gossamer Metals and Insulating Behavior

We extend the Gossamer technique recently proposed to describe superconducting ground states to metallic ground states. The gossamer metal in a single band model will describe a metallic phase that becomes arbitrarily hard to differentiate from an insulator as one turns the Coulomb correlations up. We were motivated by the phase diagram of V$_2$O$_3$ and f-electron systems which have phase diagrams in which a line of first order metal-insulator transition ends at a critical point above which the two phases are indistinguishable. This means that one can go continuously from the metal to the ``insulator'', suggesting that they might be the same phase.

cond-mat.str-el↗

arXiv2003

On Superfluidity and Suppressed Light Scattering in BECs

We show that the suppression of light scattering off a Bose Einstein Condensate is equivalent to the Landau argument for superfluidity and thus is a consequence of the {\it Principle of Superfluidity}. The superfluid ground state of a BEC contains nonseparable, nontrivial correlations between the bosons that make up the system, i. e., it is entangled. The correlations in the ground state entangle the bosons into a coherent state for the lowest energy state. The entanglement is so extreme that the bosons that make up the system cannot be excited at long wavenumbers. Their existence at low energies is impossible. Only quantum sound can be excited, i.e. the excitations are Bogolyubov quasiparticles which do not resemble free bosons whatsoever at low energies. This means that the system is superfluid by the Landau argument and the superfluidity is ultimately the reason for suppressed scattering at low wavelengths.

cond-mat.soft↗

arXiv2003

On Vortices and Phase Coherence in High Tc Superconductors

We show that an array of Josephson coupled Cooper paired planes can never have long range phase coherence at any finite temperature due to an infrared divergence of phase fluctuations. The phase correlations decay in a slow enough manner providing enough local phase coherence as to make possible the nucleation of vortices. The planes then acquire Kosterlitz-Thouless topological order with its intrinsic rigidity and concomitant superfluidity. We thus conclude that the high temperature superconducting cuprates are topologically ordered superconductors rather than phase ordered superconductors. For low enough superfluid densities, as in the underdoped cuprates, the transition temperature, Tc, will be proportional to the superfluid density corresponding to vortex-antivortex unbinding, and not to disappearance of the Cooper pairing amplitude. Above Tc, but below the BCS pairing temperature Tp, we will have a dephased Cooper pair fluid that is a vortex-antivortex liquid. The AC and DC conductivities measured in this region are those corresponding to flux flow. Furthermore there will be vortices above Tc which will lead to Nernst vortexlike response and there will be a measurable depairing field H_c2 above Tc as evidenced by recent experiments.

cond-mat.supr-con↗

arXiv2003

Magnetic Instability in Strongly Correlated Superconductors

Recently a new phenomenological Hamiltonian has been proposed to describe the superconducting cuprates. This so-called Gossamer Hamiltonian is an apt model for a superconductor with strong on-site Coulomb repulsion betweenthe electrons. It is shown that as one approaches half-filling the Gossamer superconductor, and hence the superconducting state, with strong repulsion is unstable toward an antiferromagnetic insulator an can undergo a quantum phase transition to such an insulator if one increases the on-site Coulomb repulsion.

cond-mat.supr-con↗

arXiv2001

The Generalized Uncertainty Principle and Black Hole Remnants

In the current standard viewpoint small black holes are believed to emit radiation as black bodies at the Hawking temperature, at least until they reach Planck size, after which their fate is open to conjecture. A cogent argument against the existence of remnants is that, since no evident quantum number prevents it, black holes should radiate completely away to photons and other ordinary stable particles and vacuum, like any unstable quantum system. Here we argue the contrary, that the generalized uncertainty principle may prevent their total evaporation in exactly the same way that the uncertainty principle prevents the hydrogen atom from total collapse: the collapse is prevented, not by symmetry, but by dynamics, as a minimum size and mass are approached.

gr-qc↗

arXiv2001

Interplay Between Gravity and Quintessence: A Set of New GR Solutions

A set of new exact analytical General Relativity (GR) solutions with time-dependent and spatially inhomogeneous quintessence demonstrate 1) a static non-empty space-time with a horizon-type singular surface; 2) time-dependent spatially homogeneous `spheres' which are completely different in geometry from the Friedmann isotropic models; 3) infinitely strong anti-gravity at a `true' singularity where the density is infinitely large. It is also found that 4) the GR solutions allow for an extreme `density-free' form of energy that can generate regular space-time geometries.

astro-ph↗

arXiv1999

On the detectability of quantum spacetime foam with gravitational-wave interferometers

We discuss a recent provocative suggestion by Amelino-Camelia and others that classical spacetime may break down into ``quantum foam'' on distance scales many orders of magnitude larger than the Planck length, leading to effects which could be detected using large gravitational wave interferometers. This suggestion is based on a quantum uncertainty limit obtained by Wigner using a quantum clock in a gedanken timing experiment. Wigner's limit, however, is based on two unrealistic and unneccessary assumptions: that the clock is free to move, and that it does not interact with the environment. Removing either of these assumptions makes the uncertainty limit invalid, and removes the basis for Amelino-Camelia's suggestion.

gr-qc↗

arXiv1999

On a Generalization in Quantum Theory: Is $\hbar$ Constant?

We here consider a generalization of the Klein-Gordon scalar wave equation which involves a single arbitrary function. The quantization may be viewed as allowing $\hbar$ to be a function of the momentum or wave vector rather than a constant. The generalized theory is most easily viewed in the wave vector space analog of the Lagrangian. We need no reference to spacetime. In the generalized theory the de Broglie relation between wave vector and momentum is generalized, as are the canonical commutation relations and the uncertainty principle. The generalized uncertainty principle obtained is the same as has been derived from string theory, or by a general consideration of gravitational effects during the quantum measurement process. The propagator of the scalar field is also generalized, and an illustrative example is given in which it factors into the usual propagator times a "propagator form factor."

hep-th↗

arXiv1999

On Gravity and the Uncertainty Principle

Heisenberg showed in the early days of quantum theory that the uncertainty principle follows as a direct consequence of the quantization of electromagnetic radiation in the form of photons. As we show here the gravitational interaction of the photon and the particle being observed modifies the uncertainty principle with an additional term. From the modified or gravitational uncertainty principle it follows that there is an absolute minimum uncertainty in the position of any particle, of order of the Planck length. A modified uncertainty relation of this form is a standard result of superstring theory, but the derivation given here is based on simpler and rather general considerations with either Newtonian gravitational theory or general relativity theory.

gr-qc↗

arXiv1999

Observation and Modeling of the Solar Transition Region: II. Solutions of the Quasi-Static Loop Model

In the present work we undertake a study of the quasi-static loop model and the observational consequences of the various solutions found. We obtain the most general solutions consistent with certain initial conditions. Great care is exercised in choosing these conditions to be physically plausible (motivated by observations). We show that the assumptions of previous quasi-static loop models, such as the models of Rosner, Tucker and Vaiana (1978) and Veseckey, Antiochos and Underwood (1979), are not necessarily valid for small loops at transition region temperatures. We find three general classes of solutions for the quasi-static loop model, which we denote, radiation dominated loops, conduction dominated loops and classical loops. These solutions are then compared with observations. Departures from the classical scaling law of RTV are found for the solutions obtained. It is shown that loops of the type that we model here can make a significant contribution to lower transition region emission via thermal conduction from the upper transition region.

astro-ph↗

arXiv1999

On the Energy-Momentum Tensor of the Scalar Field in Scalar--Tensor Theories of Gravity

We study the dynamical description of gravity, the appropriate definition of the scalar field energy-momentum tensor, and the interrelation between them in scalar-tensor theories of gravity. We show that the quantity which one would naively identify as the energy-momentum tensor of the scalar field is not appropriate because it is spoiled by a part of the dynamical description of gravity. A new connection can be defined in terms of which the full dynamical description of gravity is explicit, and the correct scalar field energy-momentum tensor can be immediately identified. Certain inequalities must be imposed on the two free functions (the coupling function and the potential) that define a particular scalar-tensor theory, to ensure that the scalar field energy density never becomes negative. The correct dynamical description leads naturally to the Einstein frame formulation of scalar-tensor gravity which is also studied in detail.

gr-qc↗

arXiv1998

Scalar-Tensor Cosmologies and their Late Time Evolution

We study the asymptotic behavior at late times of Friedmann-Robertson-Walker (uniform density) cosmological models within scalar-tensor theories of gravity. Particularly, we analyze the late time behavior in the present (matter dominated) epoch of the universe. The result of Damour and Nordtvedt that for a massless scalar in a flat cosmology the Universe evolves towards a state indistinguishable from general relativity is generalized. We first study a massless scalar field in an open universe. It is found that, while the universe tends to approach a state with less scalar contribution to gravity, the attractor mechanism is not effective enough to drive the theory towards a final state indistinguishable from general relativity. For the self-interacting case it is found that the scalar field potential dominates the late time behavior. In most cases this makes the attractor mechanism effective, thus resulting in a theory of gravity with vanishingly small scalar contribution even for the open Universe.

gr-qc↗