Bose Condensation Temperature Research Articles

On the Bipolaronic Mechanism of High-Temperature Superconductivity in "Ginzburg Sandwiches" FeSe-SrTiO<sub>3</sub>; SrTiO<sub>3</sub>-FeSe-SrTiO<sub>3</sub>

The mechanism of experimentally observed high-temperature superconductivity (HTSC) in thin FeSe films on SrTiO₃-type substrates has been theoretically investigated. Applying the theory of large-radius bipolaronic states developed based on the exact Hamiltonian of electron-phonon interaction for arbitrary multilayer structures, the bipolaronic mechanism of Cooper pairing of polarons in FeSe monolayers on SrTiO₃ substrates is investigated and in three-layer structures SrTiO₃–FeSe–SrTiO₃, which are typical "Ginzburg sandwiches". Approach proposed by Ginzburg to enhance the electron-phonon interaction and achieve HTSС by separating the regions where electrons are located (forming Cooper pairs or bipolarons) with the regions in which excitons are excited (or inertial polarization is induced), made it possible to implement the criteria for the formation of bipolaronic states in multilayer structures with high binding energy, due to the possibility of selecting optimal geometric and material parameters (layer thicknesses, dielectric permittivity, optical frequencies, effective masses). It is shown that the binding energy of bipolarons (<i>E_bp</i>) in these structures is in the range of values for which bipolarons remain stable quasiparticles and can exist at temperatures significantly higher than their Bose condensation temperature. The formation of bipolarons with high binding energy in the FeSe monolayer on the SrTiO₃ substrate provides the emergence of a bipolaronic HTS with a critical temperature (<i>T_c</i>) more than an order of magnitude higher than <i>T_c</i> for massive FeSe crystals. At the same time, the binding energy of the bipolaron in the FeSe layer with thickness <i>d</i> on the SrTiO₃ substrate increases exponentially with decreasing <i>d</i> (<i>E_bp</i>~<i>exp</i>(-<i>d </i>/ <I>R<SUB>S</SUB></I>) <I>R<SUB>S</SUB></I> is the radius of the polaron) and reaches its maximum value in the limit of the multilayer film FeSe (d→0). The presented theory allows modeling a multilayer system and determining the range of values of the material and geometric parameters of layers forming a multilayer structure with a large number of FeSe layers in which <i>T_c</i> values in the room temperature range can be achieved.

Modified Padé–Borel Summation

We revisit the problem of calculating amplitude at infinity for the class of functions with power-law behavior at infinity by means of a resummation procedure based on the truncated series for small variables. Iterative Borel summation is applied by employing Padé approximants of the “odd” and “even” types modified to satisfy the power-law. The odd approximations are conventional and are asymptotically equivalent with an odd number of terms in the truncated series. Even approximants are new, and they are constructed based on the idea of corrected approximants. They are asymptotically equivalent to the even number of terms in truncated series. Odd- and even-modified Padé approximants could be applied with and without a Borel transformation. The four methods are applied to some basic examples from condensed matter physics. We found that modified Padé–Borel summation works well in the case of zero-dimensional field theory with fast-growing coefficients and for similar examples. Remarkably, the methodology of modified Padé–Borel summation appears to be extendible to the instances with slow decay or non-monotonous behavior. In such situations, exemplified by the problem of Bose condensation temperature shift, the results are still very good.

The effects of next-to-nearest-neighbour hopping on Bose-Einstein condensation in cubic lattices

In this paper, we present results of our calculations on the effects of next to nearest neighbor boson hopping ($t^{\prime}$) energy on Bose-Einstein condensation in cubic lattices. We consider both non-interacting and repulsively interacting bosons moving in the lowest Bloch band. The interacting bosons are studied making use of the Bogoliubov method. We find that the bose condensation temperature is enhanced with increasing $t^{\prime}$ for bosons in a simple cubic (sc) lattice and decreases for bosons in body-centered cubic (bcc) and face-centered cubic (fcc) lattices. We also find that interaction induced depletion of the condensate is reduced for bosons in a sc lattice while is enhanced for bosons in bcc and fcc lattices.

DOES HIGH TEMPERATURE IONIC SUPERCONDUCTIVITY EXISTS?

The basic idea is that we try to find some materials in which bosonic ions with sufficiently small effective mass are used as charge carriers instead of Cooper's pairs in order to provide high temperature ionic superconductivity. Ionic crystals LiCl , LiF , LiBr and LiI were considered with lithium isotope Li 6. Calculations show that Bose condensation temperature for lithium ions in these crystals is of the order of 10-34–10-43 K. If, however, the crystal is compressed so that the wave functions of neighboring lithium ions are sufficiently overlapped, then Bose-condensation temperature of Li 6-ions can be increased significantly. Our estimates show that by compressing the crystals by 20–22% in all three directions, one can raise the Bose-condensation temperature in all crystals considered to above room temperature. To realize materials with room temperature superconductivity in practice, the use of molecular beam epitaxy is proposed for the formation of heterostructures from thin and thick layers of thoughtfully chosen composition.

Joule Expansion of a Pure Many-Body State

We derive the Joule expansion of an isolated perfect gas from the principles of quantum mechanics. Contrary to most studies of irreversible processes which consider composite systems, the interesting and ignored degrees of freedom are here described by operators acting in the same many-body Hilbert space. Moreover, the expansion of the gas into the entire accessible volume is obtained for pure states. Still, the number particle density is characterized by a chemical potential and a temperature. We discuss the special case of a boson gas below the Bose condensation temperature.

Bose-Einstein condensation in tight-binding bands

We present a theoretical study of condensation of bosons in tight binding bands corresponding to simple cubic, body centered cubic, and face centered cubic lattices. We have analyzed non-interacting bosons, weakly interacting bosons using Bogoliubov method, and strongly interacting bosons through a renormalized Hamiltonian approach valid for number of bosons per site less than or equal to unity. In all the cases studied, we find that bosons in a body centered cubic lattice has the highest Bose condensation temperature. The growth of condensate fraction of non-interacting bosons is found to be very close to that of free bosons. The interaction partially depletes the condensate at zero temperature and close to it, while enhancing it beyond this range below the Bose-Einstein condensation temperature. Strong interaction enhances the boson effective mass as the band-filling is increased and eventually localizes them to form a Bose-Mott-Hubbard insulator for integer filling.

Specific heat of bosons in a lattice

We present a theoretical study of specific heat of bosons ( C v ) in a simple cubic lattice. We have studied the non-interacting bosons and the Tonks gas. For both cases, the C v above the Bose condensation temperature shows considerable temperature dependence compared to that of free bosons. For Tonks gas, we find that the low-temperature specific heat increases as the system gets closer to the Mott transition.

Temperature variation of the fluctuation field in Co∕Pt

Measurements of the fluctuation field of a perpendicular Co∕Pt multilayer medium was carried out from 4K to room temperature, using a procedure that for each holding field matches, in log-time, the aftereffect curve with the major loop. This is done by translation and scaling using the entire curve. The fluctuation field at each temperature is obtained from the slope of the required translation as a function of the holding field. The fluctuation field is directly proportional to the absolute temperature below the Bose condensation temperature, TBE, has a sharp peak at the TBE, then decreases until it approaches a constant value asymptotically from below for higher temperatures. This paper presents both the technique used to obtain this data and a theory of this magnetic aftereffect.

Specific heat of an ideal Bose gas above the Bose condensation temperature

An approximation for the specific heat of an ideal Bose as an explicit function of temperature above the condensation temperature Tc is derived. The formula yields the exact values of the specific heat and its first derivative at Tc and has an accuracy of at least 0.3% over the full range of temperatures.

Effective mass of atom and the excitation spectrum in liquid helium-4 at T=0 K

A self-consistent approach is applied for calculations within the two-time temperature Green function formalism in the random phase approximation for superfluid He4. The effective mass of the He4 atom is computed as m*=1.58m. The excitation spectrum is found to be in satisfactory agreement with experiment. The sound velocity is calculated as 230 m/s. The Bose-condensation temperature with the effective mass taken into consideration is estimated as 1.99 K.

Atomic Bose condensation and the lattice

I show how interaction corrections to the Bose condensation temperature of an atomic gas can be computed using a combination of perturbative effective field theory and lattice techniques.

Bose–Einstein condensation in atomic trap: gravity shift of critical temperature

The 2D Bose condensation of trapped atoms in a gravitational field is considered. The deformation of the finite parabolic potential in this field leads to the cut-off of a trap spectrum and the redefinition of its states. A shift of Bose condensation temperature T c via these factors is calculated in the simplified model. The tunneling of atoms is taken into account, and its contribution to the T c shift by the noncondensate atoms may be found important. The condensate atoms do not contribute to tunneling because they are almost motionless.

Holon pair bose condensation based on the SU(2) slave-boson approach to the t– J Hamiltonian; instability of the normal state against holon pairing

Based on the application of the SU(2) slave-boson theory to the Bethe–Salpeter equation for the boson (holon) sector, we derive the bose condensation temperature T c as a function of hole doping rate for high T c cuprates. It is shown that the computed results of the t-matrix yielded good agreement with the functional integral approach of the SU(2) slave-boson theory in reproducing the bose condensation temperature T c of an arch shape as a function of doping rate.

Isobaric heat capacity of an ideal Bose gas

It is demonstrated that the heat capacity of an ideal Bose gas at constant pressure increases infinitely when its temperature approaches the Bose condensation temperature from above and is infinite for the phase with a Bose condensate.

The Bose–Einstein condensation in random box

The Bose–Einstein condensation in the presence of quenched disorder due to random boundary conditions is considered. We have shown that for bosons confined in a box with volume L d and for random L the Bose condensation temperature is lowered by the presence of disorder and the onset of condensation may even be completely prohibited. Similar results hold for bosons confined in a harmonic oscillator potential well, the geometry used in recent experiments on Bose–Einstein condensation in atomic traps. Comments on possible consequences of quenched disorder in boundary conditions for many fermion systems are also given.

Structural ordering of supercooled hydrogen in porous Vycor glass

Raman scattering measurements were performed on porous Vycor filled with H 2 and cooled using two different cooling rates. For the slowest cooled sample a transition from the liquid phase to a solid phase which is neither hcp, nor fcc was observed. For the faster cooled sample, the H 2 molecules probably freeze into a structurally disordered phase. Our measurements show that no substantial liquid fraction is present below the predicted Bose condensation temperature.

Bose condensation and relaxation explosion in magnetically trapped atomic hydrogen.

We predict and analyze nontrivial relaxational behavior of magnetically trapped gases near the Bose-condensation temperature ${\mathit{T}}_{\mathit{c}}$. Due to strong compression of the condensate by the inhomogeneous trapping field, particularly at low densities, the relaxation rate shows a strong, almost jumplike, increase below ${\mathit{T}}_{\mathit{c}}$. As a consequence the maximum fraction of condensate particles is limited to a few percent. This phenomenon can be called a ``relaxation explosion.'' We discuss its implication for the detectability of Bose-Einstein condensation in atomic hydrogen.

A real space pair model for the NMR behavior in high-T c cuprates

A model for real space singlet pairs (RSP), hopping on the quasi 2D antiferromagnetic background of localized spins, is presented. It is found that the “gas parameter” of the RSP's reaches unity at low enough temperatures, however small the concentration of RSP's. This happens due to the local coupling of the RSP to the growing antiferromagnetic fluctuations in the 2D magnetic layers. The low temperature increase of the magnetic correlation length ξ is suppressed significantly by the low energy density fluctuations in the quasi 2D RSP system, which develop already far above the Bose condensation temperature T c. The effective “magnetic mass” of the RSP is found to be proportional to (In ξ/ a) 2, where a is the magnetic lattice spacing. Both the magnetic energy shift and the size of the “magnetic cloud” of the RSP are calculated. The results are applied to the explanation of the NMR behavior observed in the high- T c cuprates.

Electron-phonon interaction in a strongly correlated Hubbard system

The electron-phonon (local) interactions have been considered in a single-band Hubbard model with strong on-site correlation. It has been shown that when no holes are present (i.e., one electron per site) the ground state of the system corresponds to the conventional coherent state of the phonon subsystem and the polaron has high effective mass, wheras for non-zero hole concentration the two-phonon coherent state of the phonon subsystem corresponds to the ground state of the system and the effective mass of the resulting squeezed polaron is reduced. If the superconductivity is due to Bose condensation of bipolarons the effective mass of the bisqueeps (squeezed bipolarons) at appropriate hole concentration may be reduced by 100 times or more in comparison to the conventional bipolarons and the corresponding Bose condensation temperature would be high.

Reexamination of magnetic effects in the Bose gas

The breakdown of the Meissner-Ochsenfeld effect in the three-dimensional Bose gas as the applied field passes through its critical value is an entropy-driven weakly first-order transition, rather than the second-order transition usually ascribed to the system. The transition is second order at the usual Bose condensation temperature ${T}_{c}$ as well as at T=0, with a line of first-order transitions connecting these critical points. The first-order transitions make the Bose gas resemble familiar superconductors, and a Landau-Ginzburg analysis indicates that the Bose gas is always a type-I superconductor.