Pseudogap Ground State and Its Doping Evolution in Bi2Sr2?x La x CuO6+δ : Removing the Superconducting State by the Application of Very High Magnetic Fields

We present results for the pseudogap ground state and its doping evolution in single-layered copper-oxide Bi₂Sr_2?x La _x CuO_6+?? . We apply very high magnetic fields up to 44 T to remove the superconducting state and reveal the hidden low temperature (T) normal state. Through ⁶³Cu-NMR Knight shift and spin-lattice relaxation rate measurements, we find that there remains a finite density of states (DOS) at the Fermi level in the zero-T limit when the superconductivity is removed, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x), but remains quite large even at the vicinity of the magnetically ordered phase of x?? 0.8. The result indicates that the superconductivity emerges from the remaining Fermi surface and coexists with the pseudogap.     

Interfacial Superconductivity on the Topological Semimetal Tungsten Carbide Induced by Metal Deposition

Interfaces between materials with different electronic ground states have become powerful platforms for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the interface may introduce additional electronic states. Among the emergent phenomena, superconductivity is of particular interest. Here, by depositing metal films on a newly identified topological semimetal tungsten carbide (WC) single crystal, interfacial superconductivity is obtained, evidenced from soft point-contact spectroscopy. This very robust phenomenon is demonstrated for a wide range of metal/WC interfaces, involving both nonmagnetic and ferromagnetic films, and the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin films. This method offers an opportunity to explore the long-sought topological superconductivity and has potential applications in topological-state-based spin devices.     

Shape Resonances in Superconducting Gaps and Complexity in Superstripes

The emerging scenario of superstripes for high temperature superconductors is presented. The complexity of the electronic structure of doped cuprates results from two electronic components and nanoscale phase separation. In this lattice, charge and magnetic complexity the unconventional high temperature superconductivity emerges in a broken symmetry. Shape resonances in superconducting gaps are discussed.     

Effects of Spin Fluctuations and Superconductivity in Quasi-One-Dimensional Organic Conductors

We study the electronic states using the single band Hubbard model at half-filling. We treat the effects of the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and calculate the phase diagram and physical properties. The calculated pressure dependence of the magnetic transition temperature coincides well with the experimental one. We also show that a pseudogap is formed in the density of states near the chemical potential and that the NMR relaxation rate increases on cooling in the low-temperature region, and that d-wave superconductivity appears next to the antiferromagnetic state.     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

Superfluid, Superconducting, and Magnetic Ordering in Mesoscopic Quantum Dots

The nature of superfluid, superconducting, and magnetic ordering is elucidated for mesoscopic systems in which the single-particle level spacing is much larger than both the temperature and the critical temperature of ordering. Ordering is defined as a spontaneous breaking of symmetry, the gauge invariance and time reversal being by definition symmetries broken in superfluidity (superconductivity) and magnetism contexts, respectively. Superfluidity and superconductivity are realized in thermodynamic equilibrium states with a nonintegral average number of particles and are accompanied by the spontaneous breaking of time homogeneity. In Fermi systems, two types of superfluidity and superconductivity are possible which are characterized by the presence of pair or single-particle condensates. The latter is remarkable in that spontaneous breaking of fundamental symmetries such as spatial 2 rotation and double time reversal takes place. Possible experiments on metallic nanoparticles and ultracold atomic gases in magnetic traps are discussed.     

From Mott state to superconductivity in 1T-TaS2

The search for the coexistence between superconductivity and other collective electronic states in many instances promoted the discovery of novel states of matter. The manner in which the different types of electronic order combine remains an ongoing puzzle. 1T-TaS(2) is a layered material, and the only transition-metal dichalcogenide (TMD) known to develop the Mott phase. Here, we show the appearance of a series of low-temperature electronic states in 1T-TaS(2) with pressure: the Mott phase melts into a textured charge-density wave (CDW); superconductivity develops within the CDW state, and survives to very high pressures, insensitive to subsequent disappearance of the CDW state and, surprisingly, also the strong changes in the normal state. This is also the first reported case of superconductivity in a pristine 1T-TMD compound. We demonstrate that superconductivity first develops within the state marked by a commensurability-driven, Coulombically frustrated, electronic phase separation.     

Evidence for Skyrmions in the High-Temperature Superconductors

We claim that the charge density wave recently found by resonant soft X-ray scattering in layered copper oxides is the tetragonal symmetry defined by the distance between neighboring Cu³⁺ ions in the CuO₂ layer that determines the critical temperature. We find evidence that this tetragonal symmetry is a skyrmionic state, which is responsible for an unusual magnetic order and charge flow in the layers that leads to the breaking of the time reversal symmetry below the pseudogap line. The core of the skyrmions form pockets of local magnetic field piercing the superconducting layers in opposite direction to the rest of the unit cell.     

Testing Polaron Coherence and the Pairing Symmetry in Cuprate Superconductors by Local Probe Methods

A variety of local structural probes have demonstrated that local lattice distortions take place in cuprates, which correlate with the onset of the pseudogap phase (PG) and superconductivity (SC). We show here that these lattice responses can be a consequence of polaron formation, local coherence in the pseudogap phase, and global coherence in the superconducting phase. In addition, we demonstrate that the results are consistent with a complex s+d wave order parameter in the SC phase.     

Correlation between electronic structure, mechanical properties and phase stability in intermetallic compounds

From the detailed electronic structure studies on intermetallic compounds, it has been found that these materials have low heat of formation and large glass-forming ability, if the Fermi level falls on the peak in the density of states (DOS) curve. On the other hand, if theE _F falls on the pseudogap in the DOS curve, the ordering energy will be larger and the system prefers to form an ordered alloy. The first principles electronic structure calculations performed on Ni₃Al show that it is possible to vary the filling of bonding, nonbonding and antibonding states in the DOS curve and this in turn shows gradual structural transformation as well as formation of multiphases by ternary alloying. Possibilities of tailoring the properties of materials by tuning the Fermi level is discussed in this paper.     

Fermi-surface and charge-spin excitations in strongly correlated electron systems

Fermi-surface and low-energy excitations in a variety of strongly correlated electron systems are studied by using the numerically exact diagonalization method in two-dimensional square lattice. In the one-band Hubbard and extended Hubbard models, the electronic structure in both insulating and metallic states are examined. The structure in the periodic Anderson model is also studied and compared with that in the above models. In the tJ model as well as the negative U Hubbard model, the excitation spectra in the superconducting states are calculated. In particular, in the tJ model the superconductivity with d(x² – y²)-wave pairing is found to be mapped onto the BCS state. It is shown that the systematic study in a variety of models provides an opportunity to construct a unified picture of strongly correlated systems.     

Particle–Hole Asymmetry and the Pseudogap Phase of the High-<Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> Superconductors

In the pseudogap phase of the copper oxide superconductors, a significant portion of the Fermi surface is still gapped at temperatures above the superconducting transition temperature T _C. Instead of a closed Fermi surface, the low-energy electronic excitations appear to form unconnected Fermi arcs separated by gapped regions. It is generally believed that the spectral function is particle–hole symmetric (at low energies) in both regions—with a peak at the Fermi level on the Fermi arcs and a local minimum at the Fermi level in the gapped regions. Here, using high resolution angle-resolved photoemission and new techniques of analysis, we show that on a sizable portion of the Fermi surface, the electronic structure in the immediate vicinity of the Fermi level is not particle–hole symmetric in the pseudogap phase. This is clear evidence that an alternative ground state competes with the superconductivity. The observations are also consistent with the possibility that the Fermi arcs are, in fact, the inner surface of the predicted Fermi pockets.     

Optical Evidence for the Existence of a Stripe Structure in the Normal and Superconducting Phases of YBa2cu3O6+x

We study optical interband absorption spectra of insulator and metallic YBa₂Cu₃O_{6+ x} films in a wide temperature region which includes the pseudogap and superconducting states. We show that in the pseudogap state a stripe structure appears with polaron-like charge carriers having an antiferromagnetic core.     

Superfluid Density and Angular Dependence of the Energy Gap in Optimally Doped (BiPb)2(SrLa)2CuO6+δ

We present a muon-spin rotation study of the optimally doped cuprate superconductor (BiPb)₂(SrLa)₂ CuO_6+δ . The measured magnetic field dependence of the in-plane magnetic penetration λ _ab suggests superconductivity with a dominant d-wave order parameter. The comparison of the temperature dependence of λ _ab with calculations, assuming the angular gap symmetry as obtained from photoemission measurements, is consistent with a partial suppression of the quasi-particle weight towards the anti-nodal region of the Fermi surface. This suggests that the superconducting and the pseudogap state are dominated by different parts of the Fermi surface.     

The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.     

Proximity Effect of Magnesium Diboride on Single-Walled Carbon Nanotube: an Ab Initio Study

Magnesium diboride (MgB₂) superconductor with excellent physical properties continues to attract the attention of researchers since its discovery. It derives its versatility from the absence of weak links, large coherence length, and small anisotropy. On the other hand, reports of superconductivity in small-diameter single-walled carbon nanotubes (SWCNTs) suspended between superconducting contacts and proximity induced supercurrents in Ta/SWNTs/Au junctions have also aroused great interest in the scientific community. Proximity induced superconductivity in SWCNTs has opened up new frontiers of research which will lead to many novel discoveries. This paper reports ab initio investigations on the proximity effect of MgB₂ on the electronic structure of a SWCNT. Condensation of electronic states is observed in the electronic band structure of the pristine SWCNT when MgB₂ is held in proximity. An additional band gap is generated below the lowest energy state of the valence band of the pristine CNT which we suggest, is due to Cooper pair formation. This leads to the prediction that SWCNTs will show superconducting properties in proximity of MgB₂. We envision MgB₂-coated SWCNTs as a novel nanomaterial that has a combination of proximity induced superconductivity and inherently unique mechanical and optical properties of SWCNTs.     

The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.     

Density functional theory calculations for [C(2)H(4)N(2)O(6)]((n)) (n=0, +1, -1)

The structural and electronic properties of neutral and mono ionic structures of isolated ethylene glycol dinitrate (EGDN) [C(2)H(4)N(2)O(6)]((n)) (n=0, +1, -1) have been investigated by performing density functional theory calculations at B3LYP level. The optimum geometry, vibrational frequencies, electronic structure and some thermo dynamical values of the structures considered have been obtained in their ground states. The calculations reveal that as the charge develops the bond lengths and angles change. In the anionic case charge accumulation causes NO(2) elimination as a result of esteric O-N bond cleavage.     

Electronic and structural trends in small GaAs clusters

Using the Hartree-Fock method followed by second-order perturbation theory, the structural and electronic properties of the Ga_nAs_m (n + m ⪯ 8) clusters and its positive and negative ions are examined. The ground state structures are obtained through geometry optimization calculations without spatial symmetry constraints. Electronic and structural properties are then calculated. In particular it is verified an alternating behavior in the ionization potential beyond three-atom clusters. A structural pattern is noticed where the embryonic forms of these clusters are observed to present highly symmetrical configurations formed by the central As atoms. The Ga atoms appear at positions that enhance a hybridization and chemical ordering which tends to that presented by the bulk. The change for a layer-type structure is observed to occur already in the eight-atom stoichiometric cluster. A theoretical explanation of previously obtained mass spectrometry results is suggested.     

Anomalous raman scattering in the normal state of uederdoped crystals of YBA2Cu3O7-x

The integrated intensity of the electronic background between 200 and 700 cm^-1 observed in the Raman spectra of the YBa₂Cu₃O_7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO₂ nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity occur at temperatures greater thanT _c . The one at lower temperature is proportional toT _c and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements in other HT_c material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at T_D>- T_c (where T_D decreases as T_c increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity in cuprates.