International Journal of Modern Physics B

We examine the effect of core-hole lifetime on the spectral weight of resonant inelastic X-ray scattering (RIXS) in hole-doped cuprates. We calculate the spectral weight by using the exact diagonalization technique for a 4 × 4 doped Hubbard lattice and find that the spin-flip channel detecting single-magnon excitation is less sensitive to the core-hole lifetime while in the non-spin-flip channel the spectral weight is strongly dependent on the lifetime. In the latter, charge and two-magnon excitations predominately contribute to RIXS for short and long core-hole lifetimes, respectively. For a realistic value of the core-hole lifetime in cuprates, both the charge and two-magnon excitations are expected to contribute to the non-spin-flip channel in RIXS when the incident-photon energy is tuned to the main peak of X-ray absorption spectrum.

The interplay between magnetism and superconductivity is tuned as a function of the external pressure in LnFeAsO${}_{1-x}$F_x at $x$ values close to the appearance of superconductivity for lanthanide ion Ln=La${}_{0.7}$Y${}_{0.3}$ and Ce. The results show that in these compounds, both superconductivity and magnetism are reinforced by pressure. Furthermore, muon spin rotation and relaxation measurements reveal qualitative remarkable differences among the samples: Ln=Ce displays nanoscopic coexistence of magnetism and superconductivity in the same volume fraction while Ln=La${}_{0.7}$Y${}_{0.3}$ shows macroscopic phase separation. The Ln-dependent results can be due to a complex combination of the external and chemical pressure, the latter being driven by the different average ionic Ln radius.

We use thermal conductivity to study a unique state in the high field corner of the superconducting phase of CeCoIn₅. The spin-density-wave (SDW) magnetic order in this high field low temperature (HFLT) phase requires superconductivity for its very existence, and both magnetism and superconductivity disappear together at superconducting critical field H${}_{c2}$. We measured thermal conductivity of CeCoIn₅ in a rotating magnetic field to study the nature of the HFLT phase. Our measurements reveal the presence of an additional order inside the HFLT phase that is intimately intertwined with the superconducting $d$-wave and the SDW orders, which we identify as a superconducting $p$-wave pair-density-wave (PDW). CeCoIn₅ displays a hierarchy of interactions within the HFLT phase, where spin–orbit coupling orients the SDW, which then determines orientation of the secondary PDW component.

The ternary fulleride, Rb${}_{0.25}$Cs${}_{2.75}$C${}_{60}$, is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb${}_{0.25}$Cs${}_{2.75}$C${}_{60}$ lies in the Mott–Jahn–Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at $\sim$ 50 K — evident in the evolution of the magnetic susceptibility with temperature — is accompanied by a lattice collapse, $\mathrm{\Delta }$V/V₀ of $-0.5\%$. Bulk superconductivity then emerges on further cooling with a T${}_c$ of 25.9 K. The results permit the extension of the electronic phase diagram of A₃C${}_{60}$ fullerides as close as possible to the metal–insulator (M–I) crossover.

In 1989, Lieb proved two theorems about the Hubbard model. One showed that the ground state of the attractive model was a spin singlet state (S = 0) was unique and was positive definite. The other showed that the ground state of the repulsive model on a bipartite lattice at half-filling has a total spin given by $|$(N${}_A-$N_B)/2$|$, corresponding to the difference of the number of lattice sites on the two sublattices divided by two. In the mid to late 1990s, Shen extended these proofs to show that the pseudospin of the attractive model was minimal until the electron number equaled 2N_A where it became fixed at J = $|$(N${}_A-$N_B)/2$|$ until the filling became 2N_B, where it became minimal again. In addition, Shen showed that a spin tower exists for the spin eigenstates for the half-filled case on a bipartite lattice. The spin tower says the minimal energy state with spin S is higher in energy than the minimal energy state with spin S − 1 until we reach the ground-state spin given above. One long-standing conjecture about this model remains, namely, does the attractive model have such a spin tower for all fillings, which would then imply that the repulsive model has minimal pseudopsin in its ground state. While we do not prove this last conjecture, we provide a quick review of this previous work, provide a constructive proof of the pseudospin of the attractive model ground state, and describe the challenges by proving the remaining open conjecture.

We analyze the theoretical derivation of the beyond-mean-field equation of state for two-dimensional gas of dilute, ultracold alkali-metal atoms in the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensate (BEC) crossover. We show that at zero temperature our theory — considering Gaussian fluctuations on top of the mean-field equation of state — is in very good agreement with experimental data. Subsequently, we investigate the superfluid density at finite temperature and its renormalization due to the proliferation of vortex–antivortex pairs. By doing so, we determine the Berezinskii–Kosterlitz–Thouless (BKT) critical temperature — at which the renormalized superfluid density jumps to zero — as a function of the inter-atomic potential strength. We find that the Nelson–Kosterlitz criterion overestimates the BKT temperature with respect to the renormalization group equations, this effect being particularly relevant in the intermediate regime of the crossover.

We investigate the electronic properties of the ground state of strongly correlated electron systems. We use an optimization variational Monte Carlo method for the two-dimensional Hubbard model and the three-band d-p model. The many-body wavefunction is improved and optimized by introducing variational parameters that control the correlation between electrons. The on-site repulsive Coulomb interaction U induces strong antiferromagnetic (AF) correlation. There is a crossover from weakly to strongly correlated regions as U increases. We show an idea that high-temperature superconductivity occurs as a result of this crossover in the strongly correlated region where U is greater than the bandwidth.

Thermodynamic discussion on the physical properties around the Mott boundary of the dimer-Mott system of $\kappa$-type (BEDT-TTF)₂X system, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes monovalent counter anions is performed by heat capacity measurements of the $\kappa$-(d[n,n] BEDT-TTF)₂Cu[N(CN)₂]Br of which donor molecules are partially deuterated. We compare temperature dependences of heat capacity of d[2,2], d[3,3] and d[4,4] compounds (defined as shown in Fig. 1) obtained in rapidly cooled and slowly cooled conditions. While the heat capacity of d[2,2] compound shows cooling rate dependence with the slight change of the absolute values Cp between 25 K and 50 K, the temperature dependences of the other compounds do not give drastic change by changing the cooling rate. The compounds of d[2,2] and d[3,3] located very close to the Mott boundary show peculiar hump structure in the C${}_p$T${}^{-1}$ that is related to the existence of a kind of lattice instability region around the first-order phase boundary. The temperature and chemical pressure dependences of the anomalous region are discussed in terms of the phase diagram in which the Mott boundary has peculiar rounding at low-temperature region.

The electronic density of states of one- and two-layer FeSe-thin films deposited on SrTiO₃ was calculated using a Real Space Density Functional Theory method. This method has been previously used to calculate X-ray absorption near edge spectroscopy (XANES) and Resonant X-ray Inelastic Scattering (RIXS) in the system FeSe${}_x$Te${}_{1-x}$, finding agreement with experimental spectra. In that system, we also found the same trend in the calculated density of states at the Fermi level and the behavior of T${}_{\mathrm{\text{c}}}$ with varying Te content. In the present case, we find non-negligible contributions of Ti-3d states in the vicinity of the Fermi level, and qualitative differences in the density of states between calculations that assume antiferromagnetic order of the FeSe layers compared with those that assume non-magnetic order. In the magnetic-order calculation, we obtain the highest density of states for the one-layer system, compared with the two-layer or bulk FeSe. Such a trend does not correlate with the experimentally reported increase from T${}_{\mathrm{\text{c}}}$=8 K in bulk FeSe to T${}_{\mathrm{\text{c}}}$=65 K for one-layer FeSe, and absence of superconductivity in two-layer FeSe.

The normal and pairing self-energies are the microscopic quantities which reflect and characterize the underlying interaction in superconductors. The momentum and frequency dependence of the self-energies, therefore, provides the experimental criteria which can single out the long sought-after pairing interaction among many proposed ideas. This line of research to pin down the pairing interaction for the cuprate superconductors has been carried out with some success by analyzing the momentum distribution curves (MDCs) of laser angle-resolved photo-emission spectroscopy (ARPES) data. Some progress and results are presented and compared with theoretical calculations based on leading proposals. Comments are made on the proposed scenarios from the comparisons.

Within the framework of the t-t${}^{\prime }-$J model, the evolution of the charge-order state with the next nearest-neighbor (NN) hopping in the electron-doped cuprate superconductors is studied. It is shown that although the magnitude of the charge-order wave vector increases with the increase of the next NN hopping in the hole-doped case, the charge-order wave vector in the electron-doped side decreases with the increase of the next NN hopping, reflecting an asymmetric next NN hopping dependence of the charge-order state between the hole- and electron-doped cuprate superconductors.

We analyze nonadiabatic non-Abelian holonomic transformations of spin-qubits confined to a linear time-dependent harmonic trap with time-dependent Rashba interaction. For this system, exact results can be derived for spin-rotation angle which also enables exact treatment of white gate-noise effects. We concentrate, in particular, on the reliability of cyclic transformations quantified by fidelity defined by the probability that the qubit after one full cycle remains in the ground-state energy manifold. The formalism allows exact analysis of spin transformations that optimize final fidelity. Various examples of time-dependent fidelity probability distributions are presented and discussed.

We study spin-dependent transport through a quantum nanostructure composed of a quantum dot (QD) surrounded by a quantum nanoring. The nanostructure is side-attached to source and drain electrodes and we assume that the confining potential that forms the nanostructure can be controlled by electrical gating. We demonstrate that depending on the model parameters the system can exhibit positive as well as negative differential conductance (NDC). What is more interesting, these properties can be easily controlled by tuning the applied gate voltage.

In the present work, we study the influence of adsorbed impurities, namely potassium atoms, on the energy spectrum of electrons in graphene. The electron states of the system are described in the frame of the self-consistent multiband strong-coupling model. It is shown that, at the ordered arrangement of potassium atoms corresponding to a minimum of the free energy, the gap arises in the energy spectrum of graphene. It is established that, at the potassium concentration such that the unit cell includes two carbon atoms and one potassium atom, the latter being placed on the graphene surface above a carbon atom at a distance of 0.286 nm, the energy gap is equal to $\sim$0.25 eV. Such situation is realized if graphene is placed on a potassium support.

In order to clarify the high-T${}_c$ mechanism in inhomogeneous cuprate layer superconductors, we deduce and find the correlation strength which has not revealed before, contributing to the formation of the Cooper pair and the two-dimensional density of state, and demonstrate the pairing symmetry in the superconductors which is still controversial. To the open questions, the fitting and analysis of the diverging effective mass with decreasing doping, extracted from the acquired quantum-oscillation data in underdoped YaBa₂Cu₃O${}_{6+x}$ superconductors, using the extended Brinkman–Rice (BR) picture, reveal the nodal constant Fermi energy with the maximum carrier density, a constant Coulomb correlation strength ${\kappa }_{\mathrm{\text{BR}}}$=U/U${}_c>$0.90, and a growing Fermi arc from the nodal Fermi point to the isotropic Fermi surface with an increasing x. The growing of the Fermi arc indicates that a superconducting gap develops with x from the node (underdoped) to the anti-node (optimally or over-doped). The large ${\kappa }_{\mathrm{\text{BR}}}$ results from the $d_{x^2-y^2}$-wave metal–insulator transition for the pseudogap phase in lightly doped superconductors, which can be direct evidence for high-T${}_c$ superconductivity. The quantum critical point is regarded as the nodal Fermi point satisfied with the BR picture. The experimentally measured mass diverging behavior is an average effect and the true effective mass is constant. As an application of the nodal constant carrier density, the superconducting node gap analyzed by an angle-resolved photoemission spectroscopy (ARPES) is a precursor of s-wave symmetry in underdoped cuprates. Furthermore, the half-flux quantum, induced by the circulation of d-wave supercurrent and observed by the phase sensitive Josephson-$\pi$ junction experiments, is not shown due to “anisotropic or asymmetric effect” appearing in superconductors with trapped flux.

The influence of long-range spin and charge fluctuations on spectra of the two-dimensional (2D) fermionic Hubbard model is considered using the strong coupling diagram technique (SCDT). Infinite sequences of diagrams containing ladder inserts, which describe the interaction of electrons with these fluctuations, are summed, and obtained equations are self-consistently solved for the range of Hubbard repulsions 4t $\le$U $\le$ 8t and temperatures 0.3t $\lesssim$ T $\lesssim$ t with t the intersite hopping constant. It was found that a metal–insulator transition curve goes from larger U and T to smaller values of these parameters. The temperature decrease causes the transition to the long-range antiferromagnetic order. It is responsible for the splitting out of a narrow band from a Hubbard subband with doping for U = 8t and low T. This segregated band is located near the Fermi level and forms a pseudogap here.

DNA-nanosensor based on multiwalled carbon nanotubes (MWCNTs) decorated by nanocyclic thiophene-pyrrole complexes of octahedral high-spin Fe has been proposed for high-performance sensing of DNA-hybridization utilizing a surface-enhanced spin-dependent CNT-plasmon resonance. This DNA-nanosensor operates on screening and quenching effects at Raman scattering of light in duplexes DNA-probe/DNA-target at complementary pairing of nucleotides in DNA. Single nucleotide polymorphism (SNP) genotyping has been successfully performed for DNA samples from healthy and cancer-suffering donors.

It is shown that in ferromagnetic films with anisotropy inhomogeneous in thickness, an interphase boundary can be formed for certain values and directions of the magnetic field. This boundary has a stable magnetic configuration. It is oriented parallel to the film plane and separates the regions with different orientations of the magnetization. In an alternating magnetic field, the interphase boundary oscillates, which may be accompanied by a resonance. The field and frequency dependences of the components of the magnetic susceptibility tensor are determined. It is shown that the susceptibility coefficients at a resonance are extremely sensitive to the direction of the external magnetic field which can underlie the development of a highly sensitive sensor.

Construction of a calorimetry system, by which the light irradiation to the surface area of molecular crystals at extremely low temperatures can be possible is reported. The development was performed based on the relaxation calorimetry technique to evaluate electronic structure using small amount of samples of molecular compounds where the electron correlation effects and electron–lattice coupling effects induce drastic change of the ground state. Detection of electronic structure changes occur as the light-induced phenomena becomes possible by this type of calorimetry apparatus. We report on the detailed structure of the small-sized calorimetry cells constructed for this purpose as well as the design of the calorimeter. The experimental results using organic superconductor and magnetic materials are given, which substantiate that the absolute value measurements under light irradiation are promising using the developed systems. Further possibility is also discussed.

We deal with thermodynamic features of organic conductors and superconductors where itinerant $\pi$-electrons/holes released from organic molecules are playing essential roles for electronic properties. Since they are low-dimensional electronic systems with relatively soft lattice framework, they show variety of phenomena related to electron correlations and electron–lattice coupling. The drastic changes of conductive and magnetic properties owing to quantum features of $\pi$-electrons can be induced by external perturbations such as magnetic/electric field, pressure, etc. It is especially emphasized that the possible mechanism and relation with other phenomena of the superconductivity in $\pi$-electrons system remains to be one of the interesting research areas in fundamental condensed matter science. In this review paper, we consider several topics of organic conductors and superconductors from the standpoints of thermodynamic experiments, data analyses and theories performed up to now. Starting from the overall picture of the electronic states in charge transfer complexes, thermodynamic properties of the quasi-one-dimensional systems, quasi-two-dimensional systems and $\pi$–d interacting systems are reviewed. The thermodynamic parameters of the superconductive compounds in them are compared and discussed. The relations with crystal structures, electronic states, phase diagram and other experiments are also discussed in comparison with these thermodynamic properties. The possible pairing symmetries in organic superconductors and some models are mentioned in the last part. This review deals with a wide scope of theoretical and experimental topics in superconductivity in molecule-based conductive systems.