Temperature Dependence Of Gap Research Articles

Temperature Dependence of Energy Gaps in Spin-1/2 Dimerized Heisenberg Chain

We present a method which combines a thermal coherent state approach with aself-consistent quantum theory to investigate the spin-1/2 dimerizedantiferromagnetic Heisenberg chain. It is found that both excitation gapsbetween the ground state and two lowest excited modes, the tripletone-magnon excitation and the singlet two-magnon bound state decreasemonotonically with increasing temperature. Our results are consistent withthose obtained from the other approximations.

Long-range isotropic and dipolar spin-spin interactions in the square planar rotator model

The simultaneous presence of a long-range isotropic antiferromagnetic interaction decaying as ${1/r}^{3}$ (where r is the spin-spin distance) and of a dipolar interaction in the square planar rotator model is studied. The pure isotropic antiferromagnetic interaction does not support long-range order, at variance with the pure dipole interaction. The model is investigated analytically at low temperature and by Monte Carlo (MC) simulations at higher temperature. The Luttinger-Tisza method provides a ground-state configuration affected by continuous degeneracy. Thermal fluctuations accounted for by the linear spin-wave approximation lift the degeneracy and lead to order by thermal disorder. Different kinds of order are found going from a pure antiferromagnetic to a pure dipole interaction. The mechanism that restores long-range order is understood by using the renormalized spin-wave theory. The accidental soft mode related to the continuous degeneracy of the ground state is replaced by a temperature-dependent gap. Temperature-driven first-order phase transitions between different ordered phases are investigated via a Landau functional. The phase diagram of the model is obtained by study of the order parameter, specific heat, and staggered magnetization evaluated by MC simulations.

Temperature-dependent gaps in the half-filled Hubbard model on a triangular lattice

The evolution of gaps in the one-electron density of states for the half-filled Hubbard model on a triangular lattice is studied as a function of both the temperature and the coupling constant (Hubbard U) using quantum Monte Carlo. The formation of gaps (or pseudogaps) at finite temperature allows us to distinguish between three regimes: (1) A strong-coupling Mott-Hubbard regime, characterized by a gap, which persists even at high temperatures; (2) a weak-coupling paramagnetic regime, characterized by the absence of a pseudogap at any finite temperature; and (31 an intermediate-coupling spin-density-wave regime, characterized by a pseudogap, which appears when U is increased beyond a critical (temperature-dependent) value. The behavior of the √3 X √3 adlayer structures on fourth-group semiconductor surfaces is briefly commented upon in the light of the above discussion.

Dynamics of the quantum one-dimensional Heisenberg antiferromagnet with spin S=1 at low temperature

We report low-temperature dynamic properties of the quantum one-dimensional antiferromagnetic Heisenberg model with spin S=1. The dynamic correlation function is calculated using a procedure that combines a projection operator technique proposed by Reiter and a modified spin wave theory proposed by Takahashi. This modified spin wave theory predicts a temperature-dependent gap whose behavior is compared to the behavior obtained experimentally and also from different approaches. Our calculation suggests that the gap has strong effects on the shape of the dynamical correlation function.

NaV2O5: AN EXOTIC EXCITON SYSTEM

We show that the phase transition which sodium vanadate undergoes at Tc=34 K is driven by a charge ordering. The relevant effective Hamiltonian is of the Frenkel exciton type, with a very large bandwidth to molecular energy ratio. This causes stron non-Heitler-London effects and a temperature dependent gap that vanishes at Tc. In addition to the phase transition, the model qualitatively explains the observed absorption spectrum and the anomaly in the static dielectric constant. Within our model, the observed spin-gap opening at Tc results from exciton-spinon coupling.

Temperature dependence of the YBa2Cu3O7 energy gap in differently oriented tunnel junctions

We have applied the break-junction technique to highly biepitaxial c-axis oriented YBa2Cu3O7 thin films with T C (ρ=0) = 91 K. Mechanically adjustable junctions with a good stability and tunneling current favored along the ab-planes have been realized. The conductance characteristics of these junctions show the presence of gap related maxima that move towards zero bias for increasing temperatures. Considering the misorientation angle α≈ 45 ° ± 5 ° of the junction, a maximum gap value at the Fermi level Δ ≈ 22 meV is inferred at T = 13 K. The temperature dependence of the gap related structures, shows a quasilinear behavior for T > 0.4 T C similar to that observed in c-axis oriented, S-I-N type YBa2Cu3O7 planar junctions.

Interaction Strength between the Highly Localised Nitrogen States and the Extended Semiconductor Matrix States in GaInNAs

We have investigated the temperature dependence of photoluminescence (PL) emission from sequentially grown Ga 0.8 In 0.2 As and Ga 0.8 In 0.2 N 0.015 As 0.985 quantum wells between 2 K and room temperature. A significant reduction in the temperature dependence of the GaInNAs bandgap compared to nitrogen-free GaInAs is observed. The results are analysed using the band-anticrossing model, which accurately predicts the temperature dependence of the GaInNAs energy gap from the behaviour of the GaInAs energy gap. We also compare the band-anticrossing interaction parameter C NM used to fit our data with other published values for GaNAs and GaInNAs. The results suggest that C NM may not be independent of indium fraction.

Temperature-dependent fermi gap opening in the c(6x4)-C60/Ag(100) two-dimensional superstructure.

High-resolution angle-integrated photoemission of one monolayer of C (60) chemisorbed on Ag(100) shows the reversible opening of a gap at the Fermi level at temperatures 25< or =T< or =300 K. The gap reaches a maximum value of approximately 10 meV at T< or =70 K. This finding is the first evidence of an electronic phase transition in C60 monolayers and has implications on the ongoing debate about surface superconductivity in C60-based bulk materials.

Peierls transition in pyrene radical cation salts

PY 12 (SbF 6 ) 7 and PY 7 PY 4 (AsF 6 ) 4 (CH 2 Cl 2 ) 4 belong to the group of quasi-one dimensional organic conductors. Measurements of the microwave electrical conductivity show that both substances undergo a Peierls transition (metal-insulator transition) at about T P =116 K and T P =74 K, respectively. The 3D-locking of the Peierls transition was investigated by following the temperature dependence of the conduction band energy gap, which was calculated from measurements of the magnetic susceptibility.

Ultrahigh-resolution photoemission study of CePd3: absence of Kondo insulator gap

Abstract We have performed ultrahigh-resolution (Δ E =8 meV) photoemission spectroscopy to examine the possibility of CePd 3 being a ‘Kondo insulator’ as suggested by a recent optical measurement [Phys. Rev. B 53 (1996) R2948]. We found that the photoemission spectrum of CePd 3 does not show any indication of (pseudo) gap at E F , differing from the well-established temperature-dependent (pseudo) gap in Kondo insulators. The Ce 4 f spectrum clearly shows the ‘Kondo resonance peak’ at E F , indicating the intrinsic metallic nature of CePd 3 . These results strongly suggest that CePd 3 is not a Kondo insulator but a conventional Kondo metal as suggested by thermodynamic measurements.

Superconducting properties of LuNi 2B 2C films and junctions

LuNi 2B 2C films have been prepared by a Pulsed Laser Deposition technique. The deposition parameters have been optimized to reach good superconducting, structural and morphological properties with high reliability. In order to deposit different patterned layers for in-situ junctions production, a special set-up for the in situ interchanging of shadow masks has been developed. We studied the growth of different materials to be used as barrier layers. New results on the temperature dependence of the LuNi 2B 2C gap using S/N contact junctions are presented and discussed.

Nuclear shape transition at finite temperature in a relativistic mean field approach

The relativistic Hartree-BCS theory is applied to study the temperature dependence of nuclear shape and pairing gap for $^{166}Er$ and $^{170}Er$. For both the nuclei, we find that as temperature increases the pairing gap vanishes leading to phase transition from superfluid to normal phase as is observed in nonrelativistic calculation. The deformation evolves from prolate shapes to spherical shapes at $T\sim 2.7$ MeV. Comparison of our results for heat capacity with the ones obtained in the non-relativistic mean field framework indicates that in the relativistic mean field theory the shape transition occurs at a temperature about 0.9 MeV higher and is relatively weaker. The effect of thermal shape fluctuations on the temperature dependence of deformation is also studied. Relevant results for the level density parameter are further presented. PACS numbers: 21.10.Ma, 21.60.-n, 27.70.+q

Dipolar interaction and long-range order in the square planar rotator model

A continuous manifold of inequivalent spin configuration minimize the energy of the square planar rotator model when the spins are coupled by dipolar interactions. This continuous degeneracy is accidental in nature and causes a soft mode at $\mathbf{q}=(2\ensuremath{\pi}/a)(1/2,1/2)$ in the simple spin-wave spectrum. Consequently no long-range order is obtained in simple spin-wave approximation in contrast with the expectation based on Monte Carlo simulation. The solution of the puzzle is found going beyond the harmonic approximation. Indeed a careful treatment of the magnon-magnon interaction replaces the accidental soft mode by a temperature-dependent gap so that long-range order is restored. Satisfactory agreement between renormalized spin-wave theory and Monte Carlo simulation is obtained in the low-temperature regime. Critical properties investigated by finite size scaling of Monte Carlo data differ from both mean-field and two-dimensional Ising model critical behavior.

Temperature dependence of the band gap in InAsyP1−y

The temperature dependence of the band gap in InAsyP1−y (y=0–0.67) has been determined by photoluminescence, x-ray diffraction, and absorption spectra measurements. We found that the measured data within the temperature range of 77–300 K can be expressed by the equation proposed by O’Donnell and Chen. The band gap at 77 K is given by Eg=1.407−1.073y+0.089y2, while the compositional dependence of the band gap observed at 300 K, agrees with the values previously reported. We confirmed that changes in temperature caused a slight change in the bowing parameters, and hence found that the band gap temperature dependence of InAsyP1−y (y=0–1) varies very little with changes in composition (2.5–3.5×10−4 eV/K).

Mutual influence of superconductivity and magnetism in strongly correlated cuprates

The interplay between superconductivity (SC) and anti-ferromagnetism (AFM) is studied in strongly correlated systems: R 2− x M x CuO 4 (R=Nd, La, Pr, Gd; M=Sr, Ce). It is assumed that SC arises due to BCS pairing mechanism in the presence of AFM in Cu lattices of Cu–O planes. Temperature dependence of SC gap as well as staggered magnetic field are calculated analytically and solved self-consistently with respect to half-filled band situation for different model parameters ( λ 1 and λ 2 being the SC and AFM coupling parameters, respectively). The SC gap is studied in the co-existent phase of SC and AFM.

Temperature-dependent energy gap in the spin-1/2 alternating Heisenberg antiferromagnet in one dimension

We calculate the temperature-dependent excitation gap between the ground state and the first excited state of the one-dimensional spin-1/2 alternating antiferromagnetic Heisenberg model, using a self-consistent harmonic approximation.

Spin gap state in α'-NaV2O5 studied by far infrared spectroscopy

We present far infrared transmission measurements of α'-NaV 2 O 5 single crystals. This material is known to undergo a spin-Peierls like transition at Tsp=35K. Below the transition temperature we observed together with expected new sharp phonon lines a strong gap-like depression of absorption occurring in the low energy part of a continuum likely connected with two-magnon excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taking the optical gap as a fingerprint of the spin gap, we found that the relationship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems.

Point-contact spectroscopy of the charge-density-wave gap along the chains inNbSe3

We have measured the differential current-voltage $(\mathrm{IV})$ characteristics of Au-${\mathrm{NbSe}}_{3}$ point contacts formed in the direction along the conducting chains. We clearly observed charge density wave (CDW) gap features below the upper Peierls transition temperature in the variation of the differential resistance as a function of bias voltage for a number of contacts with very different zero-bias contact resistances. The temperature dependence of the upper CDW gap, ${\ensuremath{\Delta}}_{p1},$ extracted from the data follows the BCS dependence with ${\ensuremath{\Delta}}_{p1}(0)=100$ meV, similarly to previous measurements with the current applied perpendicularly to the chains. For contacts with direct conductivity, we found an excess resistance when the applied voltage is less than $\ifmmode\pm\else\textpm\fi{}\ensuremath{\Delta}/e.$ This is attributed to an analog of Andreev reflection, in which the incident electron returns along its original path with its charge unchanged.

Singularity of two-electron density of states and anomalous t-dependence of sc gap of high-tc superconducting crystals

It is shown theoretically that complicated structure of electron bands crossing the Fermi level and presence of features (singularities) in two-electron density of states can give rise to arising deeply inside of band far from ef new coupled states which may also be characterized by SC gap and which are responsible for SC of crystals too. The temperature dependence of both SC gaps as traditional (located near ef) and “new” ones are strongly changed when wave vector of electron pairs increases

Temperature and field dependence of the spin wave gap in NdCu2

NdCu 2 shows a complex magnetic phase diagram below its Néel temperature of TN=6.5 K and for magnetic fields applied in the easy b direction. This complicated behavior is expected to be due to Ruderman–Kittel–Kasuya–Yoshida interaction of the 4f shells of the Nd ions in the presence of crystal field splitting. In order to obtain deeper insight into the mechanism leading to such different magnetic phases we started to study the spin wave spectra in a NdCu2 single crystal. In zero field as well as in fields applied in the easy b direction of magnetization, a pronounced minimum in the excitation spectrum has been observed and it forms an energy gap. The position of this spin wave gap does not coincide with any magnetic ordering wave vector in NdCu2, indicating anisotropic magnetic coupling. The temperature and field dependence of the spin wave gap have been studied. The position of the gap remains constant at qgap=(0.35, 0, 0), whereas the character of the excitations changes with temperature and the value of the energy gap changes with the magnetic field. Applying a field of 10 T in the hard c direction of magnetization changes the position of the minimum to qgap=(0.6, 0, 0) and gives proof of magnetic anisotropy.