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We investigate an optimal regime of negative-valued conductance, emerging in a resistively and capacitively shunted Josephson junction, which is driven simultaneously by both, a time-periodic (ac) and a constant (dc) current. We analyze the current-voltage characteristics in the regime of absolute negative conductance. We additionally explore the stability of the negative response with respect to the ac-current frequency.
We study a trapped system of fermions with an attractive zero-range two-body interaction using the Shell-Model Monte Carlo method. The method provides {\em ab initio} results in the low $N$ limit where mean-field theory is not applicable. The energy and pairing properties are presented as functions of interaction strength, particle number, and temperature. In the interesting region where typical matrix elements of the two-body interaction are comparable to the level spacing of the trap we find large odd-even effects and signatures of shell structure. As a function of temperature, we observe the disappearance of these effects as in a phase transition.
Like orbitals in atoms and molecules, persistent currents may appear in larger aggregates. Though classically, equilibrium magnetic moment of non-rotating objects is forbidden, it is known that equilibrium quantum circulating currents are allowed. Such currents were predicted to be periodic with the external magnetic field. It is shown that in general, whenever a periodic component is found, there must be a large aperiodic component forcing aperiodic contribution to the equilibrium magnetic moment as a function of the external field.
Inelastic neutron scattering for Nd$_{2-x}$Ce$_x$CuO$_{4+δ}$ near optimal doping ($x \approx 0.155$, $T_{c} = 25 \mathrm{K}$) reveals that the dynamic magnetic susceptibility at the antiferromagnetic zone center exhibits two characteristic energies in the superconducting state: $ω_1 \approx 6.4 \mathrm{meV}$ and $ω_2 \approx 4.5 \mathrm{meV}$. These two magnetic energies agree $quantitatively$ with the $B_{1g}$ / $B_{2g}$ and $A_{1g}$ features previously observed in electronic Raman scattering, where the former is believed to indicate the maximum electronic gap and the origin of the smaller $A_{1g}$ feature has remained unexplained. The susceptibility change upon cooling into the superconducting state is inconsistent with previous claims of the existence of a magnetic resonance mode near 10 meV, but consistent with a resonance at $ω_2$.
The effect of the 16O-->18O substitution on the superconducting transition temperatures Tc of NaxCoO2.yH2O has been studied for several sets of samples. Each set has two kinds of samples, one with the oxygen sites in the CoO2 layers filled with 16O atoms and another with these sites partially substituted with 18O isotope. These two kinds of samples were prepared in parallel in whole preparation processes. For all the sample sets, we have not found any significant difference of the Tc values between the non-substituted and substituted samples, which indicates that the oxygen isotope effect is not appreciable in NaxCoO2.yH2O. The result, however, does not necessarily exclude the possibility of the phonon mechanism of the superconductivity, because the strong correlation effect suppresses the isotope effect as pointed out previously.
The BCS theory has been extended by us to the strong-coupling regime where carriers are small lattice polarons and bipolarons. Here I review the multi-polaron strong-coupling theory of superconductivity. Attractive electron correlations, prerequisite to any superconductivity, are caused by an almost unretarded electron-phonon interaction sufficient to overcome the direct Coulomb repulsion in this regime. Low energy physics is that of small polarons and bipolarons, which are itinerant quasiparticles existing in the Bloch states at temperatures below the characteristic phonon frequency. I identify the long-range Froehlich electron-phonon interaction with c-axis polarised optical phonons as the most essential for pairing in superconducting cuprates. A number of key observations have been predicted or explained with polarons and bipolarons including unusual isotope effects and upper critical fields, normal state (pseudo)gaps, kinetic properties and diamagnetism, and giant proximity effects. These and some other observations provide strong evidence for a novel state of electronic matter in layered cuprates, which is a charged Bose-liquid of small mobile bipolarons.
We determine the critical Zeeman energy splitting for Fermi superfluidity at infinite s-wave scattering length according to the Monte Carlo and experimental results of the equations of state. Based on the universality hypothesis, we show that there exist two critical fields $H_{c1}$ and $H_{c2}$, between which a superfluid-normal mixed phase is energetically favored, and model-independent formulae for $H_{c1}$, $H_{c2}$ and the critical population imbalance $P_c$ are derived. Using recent Monte Carlo and experimental results of $P_c$, $H_{c1}$ and $H_{c2}$ are determined. It is found $H_{c1}=0.41ε_{\text F}$ and $H_{c2}=0.50ε_{\text F}$, with $ε_{\text F}$ being the Fermi energy of non-interacting gas.
We have performed angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles electronic structure calculations on the electron-doped high-Tc superconductors Ln1.85Ce0.15CuO4 (Ln = Nd, Sm, and Eu). The observed Fermi surface and band dispersion show such changes that with decreasing ionic size of Ln3+, the curvature of the Fermi surface or -t'/t decreases, where t and t' are transfer integrals between the nearest-neighbor and next-nearestneighbor Cu sites, respectively. The increase of t with chemical pressure is found to be significant, which may explain the apparently inconsistent behavior seen in the hole-doped La2-xSrxCuO4 under epitaxial strain [M. Abrecht et al., Phys. Rev. Lett. 91, 057002 (2003)]. A gap due to the antiferromagnetism opens even in the nodal region for the Sm and Eu compounds, and the gap size increases in going from Ln = Sm to Eu.
The electronic structure of the new superconductor, SmO$_{1-x}$F$_x$FeAs ($x=0.15$), has been studied by angle-integrated photoemission spectroscopy. Our data show a sharp feature very close to the Fermi energy, and a relative flat distribution of the density of states between 0.5 eV and 3 eV binding energy, which agrees best with band structure calculations considering an antiferromagnetic ground state. No noticeable gap opening was observed at 12 Kelvin below the superconducting transition temperature, indicating the existence of large ungapped regions in the Brillouin zone.
Different element substitution effects in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, were studied. Similar to the La- or Ce-based systems, we found that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. Electron doping by F increases T$_c$ to about 50 K. While in the case of Gd, the T$_c$ is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity was found when La was replaced by Ce in both cases, instead a ferromagnetic ordering transition was likely to form at low temperature in F-doped sample. We also synthesized LaO$_{1-x}$F$_x$RuAs and CeO$_{1-x}$F$_x$RuAs compounds. Metallic behavior was observed down to 4 K.
We report temperature dependent Andreev reflection measurements of Co/ Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) heterostructure samples with junction areas of 1 $μ$m diameter. Modelling of the 5-70 K conductivity data according to a modified Blonder-Tinkham-Klapwijk theory yields a spin polarization in Co film amounting to 34% which is almost constant up to 70 K. The YBCO films have been grown by pulsed laser deposition on sapphire substrates. The Co films are deposited by thermal evaporation on YBCO. The film is characterized by powder X-ray diffraction measurements which shows YBCO is grown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$, in YBCO remains nearly constant after deposition of Co at zero field and 77 K.
The electron-phonon coupling of LaO$_{1-x}$F$_x$FeAs is re-investigated on the basis of density functional theory in local density approximation. The implications of the $(π,π)$ nesting of the Fermi surfaces are carefully studied and found to lead to a non-standard electron-phonon coupling of the corresponding Fe in-plane breathing mode, which might make a strong contribution to the high superconducting transition temperature. The semi-metallic behavior of the undoped material is also further illuminated.
Non-compact three-dimensional QED is studied by computer simulations to understand its chiral symmetry breaking features for different values of the number of fermion flavors N_f. We consider the four-component formulation for the fermion fields, which arises naturally as the continuum limit of the staggered fermion construction in (2+1) dimensions. We present preliminary results for the equation of state of the theory in an effort to understand the properties of the chiral phase transition of the theory at a critical number of fermion flavors N_{fc}. Our preliminary results indicate that N_{fc}~1.5.
On the basis of the Bogoliubov--de Gennes theory we study the transformation of the quasiparticle spectrum in the mixed state of a mesoscopic superconductor, governed by an external magnetic field. We analyze the low energy part of the excitation spectrum and investigate the field--dependent behavior of anomalous spectral branches crossing the Fermi level. Generalizing the Caroli--de Gennes--Matricon approach we present an analytical solution describing the anomalous branches in a vortex with an arbitrary winding number. We also study the spectrum transformation caused by the splitting of a multiquantum vortex into a set of well separated vortices focusing mainly on a generic example of a two--vortex system. For vortices positioned rather close to the sample surface we investigate the effect of the quasiparticle reflection at the boundary on the spectrum and the density of states at the Fermi level. Considering an arbitrary surface curvature we study the disappearance of an anomalous spectral branch for a vortex leaving the sample. The changes in the vortex configuration and resulting transformation of the anomalous branches are shown to affect strongly the density of states and the heat conductance along the magnetic field direction.
This paper has been withdrawn by the authors due to errors in the X-ray diffraction data. Other measured data are not affected; however, the errors significantly change the interpretation and conclusions, and thus warrant withdrawal and later resubmission.
A brief review of microscopic studies of high-Tc superconductors HgBa2CuO4+d and HgBa2CaCu2O6+d is presented. The topics concerned are the pseudo spin-gap, the antiferromagnetic spin fluctuations near the quantum critical point, and the mechanism of the high Tc via NMR. The crystal structure with the flat CuO2 plane, two-dimensional electrical resistivity, Cu NQR, NMR experimental results are discussed.
There continues to be enormous interest in the BCS to BEC transition as there still is no exact theory. We recently reported a revealing reinterpretation of the condensed phase Boson-Fermion Model (BFM) by comparing it to a cold atom formulation [1]. While the ground and singly excited states appear to remain continuous in all models we have examined, the collective modes contain a singularity due to a Feshbach resonance (tuned by doping) causing a breakdown of the Migdal theorem. As a result of vertex corrections, there is a fundamental change in the nature of the superconductivity due to the formation of preformed pairs as the previously suggested location [1] of a quantum critical point in the fulleride phase diagram is passed. The result is a quantum phase transition (QPT) between BCS and BEC-like (or Feshbach resonance) superconductivity (SC). We discuss features of the resonance and the role of the experimentally observed preformed pair formation in fullerides, essential to the Boson-Fermion Model (BFM), and often speculated since the work of Nozieres and Schmitt-Rink [17]. Here, we present arguments to establish a model of the preformed pair which can be favorably compared to a circular charge density wave (CDW) isolated on a fulleride molecule. The binding is much larger than a Cooper pair. The CDW seems to be stabalized by splitting of the Jahn-Teller active vibrational modes to reduce Coulomb repulsions. Our conclusions are: 1) the doping of two electrons into triply degenerate orbitals results in the experimentally observed singlet state (CDW); and 2) this CDW (preformed pair) has a dual role as doping is varied: suppression of BCS SC and enabling a Feshbach resonance form of SC.
High temperature superconductors with a Tc above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara et al. discovered a Tc up to 26 K in the layered La(O1-xFx)FeAs. By replacing La with tri-valence rare-earth elements RE of smaller ionic radii, Tc has subsequently been raised to 41-52 K. Many theoretical models have been proposed emphasizing the important magnetic origin of superconductivity in this compound system and a possible further Tc-enhancement in RE(O1-xFx)FeAs by compression. This later prediction appears to be supported by the pressure-induced Tc-increase in La(O0.89F0.11)FeAs observed. Here we show that, in contrast to previous expectations, pressure can either suppress or enhance Tc, depending on the doping level, suggesting that a Tc exceeding 50's K may be found only in the yet-to-be discovered compound systems related to but different from R(O1-xFx)FeAs and that the Tc of La(O1-xFx)FeAs and Sm(O1-xFx)FeAs may be further raised to 50's K.