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arXiv2009

New Theoretical Approach to Quantum Size Effects of Interactive Electron-hole in Spherical Semiconductor Quantum Dots

The issue of quantum size effects of interactive electron-hole systems in spherical semiconductor quantum dots is put to question. A sharper theoretical approach is suggested based on a new pseudo-potential method. In this new setting, analytical computations can be performed in most intermediate steps lending stronger support to the adopted physical assumptions. The resulting numerical values for physical quantities are found to be much closer to the experimental values than those existing so far in the literature.

cond-mat.mtrl-sci↗

arXiv2009

Graphene - a nearly perfect fluid

Hydrodynamics and collision dominated transport are crucial to understand the slow dynamics of many correlated quantum liquids. The ratio η/s of the shear viscosity ηto the entropy density s is uniquely suited to determine how strongly the excitations in a quantum fluid interact. We determine η/s in clean undoped graphene using a quantum kinetic theory. As a result of the quantum criticality of this system the ratio is smaller than in many other correlated quantum liquids and, interestingly, comes close to a lower bound conjectured in the context of the quark gluon plasma. We discuss possible consequences of the low viscosity, including pre-turbulent current flow.

cond-mat.mes-hall↗

arXiv2009

Magnetic ordering in electronically phase separated Sr/O co-doped La(2-x)Sr(x)CuO(4+y)

We present results of magnetic neutron diffraction experiments on the co-doped super-oxygenated La(2-x)Sr(x)CuO(4+y) (LSCO+O) system with x=0.09. The spin-density wave has been studied and we find long-range incommensurate antiferromagnetic order below T_N coinciding with the superconducting ordering temperature T_c=40 K. The incommensurability value is consistent with a hole-doping of n_h~1/8, but in contrast to non-superoxygenated La(2-x)Sr(x)CuO(4) with hole-doping close to n_h ~ 1/8 the magnetic order parameter is not field-dependent. We attribute this to the magnetic order being fully developed in LSCO+O as in the other striped lanthanum-cuprate systems.

cond-mat.supr-con↗

arXiv2009

Does a Fermi liquid on a half-filled Landau level have Pomeranchuk instabilities?

We present a theory of spontaneous Fermi surface deformations for half-filled Landau levels (filling factors of the form $ν=2n+1/2$). We assume the half-filled level to be in a compressible, Fermi liquid state with a circular Fermi surface. The Landau level projection is incorporated via a modified effective electron-electron interaction and the resulting band structure is described within the Hartree-Fock approximation. We regulate the infrared divergences in the theory and probe the intrinsic tendency of the Fermi surface to deform through Pomeranchuk instabilities. We find that the corresponding susceptibility never diverges, though the system is asymptotically unstable in the $n \to \infty$ limit.

cond-mat.mes-hall↗

arXiv2009

Magnetization and magneto-electric effect in La-doped BiFeO3

We report on magnetisation and magneto-capacitance measurements in the Bi1-xLaxFeO3 series for 0 < x < 0.15. We confirm that doping with La reduces the threshold magnetic field Hc for cancelling the magnetic spiral phase, and we show that Hc decreases as the La content increases up to x=0.15, which is the highest concentration for maintaining the non-centrosymmetric rhombohedral structure of BiFeO3. Measurements of the dielectric constant as a function of magnetic field in the series also show a maximum magneto-capacitance for x=0.15.

cond-mat.str-el↗

arXiv2009

Comment on 'Distorted perovskite with $e_g^1$ configuration as a frustrated spin system'

Interesting magnetic structure found experimentally in a series of manganites RMnO$_3$, where R is a rare earth, was explained, ostensibly, by Kimura et al, in terms of a classical Heisenberg model with an assumed set of exchange parameters, nearest-neighbor and next-nearest-neighbor interactions, $J_n$. They calculated a phase diagram as a function of these parameters for the ground state, in which the important "up-up-down-down" or E-phase occurs for finite ratios of the $J_n$. In this Comment we show that this state does not occur for such ratios; the error is traced to an incorrect method for minimizing the energy. The correct phase diagram is given. We also point out that the finite temperature phase diagram presented there is incorrect.

cond-mat.str-el↗

arXiv2009

Introduction to superconductivity in metals without inversion center

This Chapter gives a brief introduction to some basic aspects metals and superconductors in crystal without inversion symmetry. In a first part we analyze some normal state properties which arise through antisymmetric spin-orbit coupling existing in non-centrosymmetric materials and show its influence on the de Haas-van Alphen effect. For the superconducting phase we introduce a multi-band formulation which naturally arises due the spin splitting of the bands by spin-orbit coupling. It will then be shown how the states can be symmetry classified and their relation to the original classification in even-parity spin-singlet and odd-parity spin-triplet pairing states. The general Ginzburg-Landau functional will be derived and applied to the nucleation of superconductivity in a magnetic field. It will be shown that magneto-electric effects can modify the standard paramagnetic limiting behavior drastically.

cond-mat.supr-con↗

arXiv2012

Single step synthesis of Sr4V2O6Fe2As2: The blocking layer based potential future superconductor

We report synthesis, structural details and transport measurements on Sr4V2O6Fe2As2. Namely the stoichiometric amounts of V2O5 + SrO2 + Sr + 2FeAs are weighed mixed, ground thoroughly and palletized in rectangular form in a glove box in high purity Ar atmosphere. The pellet is further sealed in an evacuated (10-5 Torr) quartz tube and put for heat treatments at 750 and 1150 0C in a single step for 12 and 36 hours respectively. Finally the quartz ampoule is allowed to cool naturally to room temperature. The as synthesized sample is black in color. The compound crystallized in P4/nmm space group with lattice parameters a = b = 3.925A and c = 15.870A. Also seen are some small impurity lines. The compound did not exhibit superconductivity but instead a spin density wave (SDW) like metallic step at around 175 K is seen in R(T) measurements

cond-mat.supr-con↗

arXiv2009

Nonequilibrium transport through a singlet-triplet Anderson impurity

We study the Anderson model in which a configuration with a doublet is hybridized with another with a singlet and a triplet. We calculate the conductance through the system as a function of temperature and bias voltage, near the quantum critical line for which the system is exactly solvable. The results explain recent transport measurements in a single-molecule quantum dot.

cond-mat.str-el↗

arXiv2009

Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides

The possibility for an appreciable many-body contribution to the electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model where EPI is due to the electronic polarization of As- ions. The EPI-pol coupling ismuch larger than the one obtained in the LDA band structure calculations. It contributes significantly to the intra-band s-wave pairing and an appreciable positive As-isotope effect in the superconducting critical temperature is expected. In the Fe-breathing mode the linear (in the Fe-displacements) EPI-pol coupling vanishes, while the non-linear (quadratic) one is very strong. The part of the EPI-pol coupling, which is due to the "potential" energy (the Hubbard U) changes, is responsible for the giant magneto-elastic effects in MFe_{2}As_{2}, M=Ca, Sr, Ba since it gives much larger contribution to the magnetic pressure than the band structure effects do. This mechanism is contrary to the LDA prediction where the magneto-elastic effects are due to the "kinetic" energy effects, i.e. the changes in the density of states by the magneto-elastic effects. The proposed $EPI-pol is expected to be operative (and strong) in other Fe-based superconductors with electronically polarizable ions such as Se, Te, S etc., and in high-temperature superconductors due to the polarizability of the O-ions.

cond-mat.supr-con↗

arXiv2009

The emptiness formation probability correlation in homogeneous and dimerized XX chains

We review some known properties of the "emptiness formation probability" correlation which we have calculated numerically for spin-1/2 XX chains with constant (homogeneous) or alternating (dimerized) nearest-neighbor coupling and an external field (in $z$ direction) for arbitrary temperature. The long-distance asymptotic behavior of this correlation is known to be Gaussian at zero temperature and exponential at finite temperature for the homogeneous chain. By simple analytical arguments the exponential behavior at finite temperature extends to the dimerized system. Numerical results for the dimerized chain confirm the exponential decay at finite temperature and show Gaussian decay at zero temperature.

cond-mat.str-el↗

arXiv2009

Finite temperature topological order in 2D topological color codes

In this work the topological order at finite temperature in two-dimensional color code is studied. The topological entropy is used to measure the behavior of the topological order. Topological order in color code arises from the colored string-net structures. By imposing the hard constrained limit the exact solution of the entanglement entropy becomes possible. For finite size systems, by raising the temperature, one type of string-net structure is thermalized and the associative topological entropy vanishes. In the thermodynamic limit the underlying topological order is fragile even at very low temperatures. Taking first the thermodynamic limit and then the zero-temperature limit and vice versa does not commute, and their difference is related only to the topology of regions. The contribution of the colors and symmetry of the model in the topological entropy is also discussed. It is shown how the gauge symmetry of the color code underlies the topological entropy.

quant-ph↗

arXiv2009

Vortex core states in a minimal two-band model for iron-based superconductors

The pairing symmetry is one of the major issues in the study of iron-based superconductors. We adopt a minimal two-band tight-binding model with various channels of pairing interaction, and derive a set of two-band Bogoliubov-de Gennes (BdG) equations. The BdG equations are implemented in real space and then solved self-consistently via exact diagonalization. In the uniform case, we find that the $d_{x^2-y^2}$-wave pairing state is most favorable for a nearest-neighbor pairing interaction while the $s_{x^2y^2}$-wave pairing state is most favorable for a next-nearest-neighbor pairing interaction. The is consistent with that reported by Seo {\em et al.} [Phys. Rev. Lett. {\bf 101}, 206404 (2008)]. We then proceed to study the local electronic structure around a magnetic vortex core for both $d_{x^2-y^2}$-wave and $s_{x^2y^2}$-wave pairing symmetry in the mixed state. It is found from the local density of states (LDOS) spectra and its spatial variation that the resonance core states near the Fermi energy for the $d_{x^2-y^2}$-wave pairing symmetry are bound while those for the $s_{x^2y^2}$-wave pairing symmetry can evolve from the localized states into extended ones with varying electron filling factor. Furthermore, by including an effective exchange interaction, the emergent antiferromagnetic spin-density-wave (SDW) order can suppress the resonance core states, which provides one possible avenue to understand the absence of resonance peak as revealed by recent scanning tunneling microscopy experiment (STM) by Yin {\em et al.} [Phys. Rev. Lett. {\bf 102}, 097002 (2009)].

cond-mat.supr-con↗

arXiv2009

Unusual Fermi surface nesting in parent compounds of iron arsenic high temperature superconductors revealed by Angle Resolved Photoemission Spectroscopy

We use angle resolved photoemission spectroscopy (ARPES) to study the band structure of BaFe2As2 and CaFe2As2, two of the parent compounds of the iron arsenic high temperature superconductors. We find clear evidence for band back folding and hybridization demonstrating that conduction electrons are strongly affected by the emergence of magnetic order. Our high quality data revealed that although the Fermi surface is strongly three-dimensional, it does indeed have long parallel segments along the kz direction that can lead to the emergence of magnetic order. More interestingly, we find very unusual incommensurate nesting of the Fermi surface in the a-b plane that is present only at low temperatures. We speculate that this is a signature of a failed Charge Density Wave (CDW) state that was predicted by renormalization group studies.

cond-mat.supr-con↗

arXiv2009

Extra condition is necessary to have a unique cluster wave vectors set in the periodic cluster approximations

We added an extra condition, original lattice symmetry of chosen cluster around cluster central site, to the cluster approximation methods with periodic boundary condition such as dynamical cluster approximation (DCA), effective medium approximation (EMSCA) and nonlocal coherent potential approximation (NLCPA). For each cluster size, this condition leads to a unique cluster wave vectors set in the first Brillouin zone (FBZ) where they preserve full symmetry of first Brillouin zone around ${\bf K=0}$. In this case whole cluster wave vectors are restricted to the FBZ and when number of sites in the cluster is equal to the whole lattice sites, these approximations recover original lattice symmetry in real and k-spaces.

cond-mat.dis-nn↗

arXiv2009

Origin of the structural phase transition at 130 K in BaNi$_2$As$_2$: a combined study of optical spectroscopy and band structure calculations

BaNi$_2$As$_2$ exhibits a first order structural transition at 130 K. Understanding this structural transition is a crucial step towards understanding the electronic properties of the material. We present a combined optical spectroscopy and band structure calculation study on the compound across the transition. The study reveals that BaNi$_2$As$_2$ is a good metal with a rather high plasma frequency. The phase transition leads to a small reduction of conducting carriers. We identify that this reduction is caused by the removal of several small Fermi surface sheets contributed dominantly from the As-As bonding and Ni-As antibonding states.

cond-mat.supr-con↗

arXiv2009

Consequences of lattice imperfections and interchain couplings for the critical properties of spin-1/2 chain compounds

To allow for a comparison of theoretical predictions for spin chains with experimental data, it is often important to take impurity effects as well as interchain couplings into account. Here we present the field theory for finite spin chains at finite temperature and calculate experimentally measurable quantities like susceptibilities and nuclear magnetic resonance spectra. For the interchain couplings we concentrate on geometries relevant for cuprate spin chains like Sr$_2$CuO$_3$ and SrCuO$_2$. The field theoretical results are compared to experimental as well as numerical data obtained by the density matrix renormalization group.

cond-mat.str-el↗

arXiv2010

Exciting the Scissors Mode in crystals with strong spin-orbit coupling as in Bose-Einstein Condensates

In a recent study of the magnetic properties of rare-earth systems the two extreme situations have been considered in which the crystalline electrostatic field is large or small with respect to the spin-orbit interaction. In the first case the orbitals of localized electrons are firmly coupled to the lattice so that while an applied magnetic field rotates the spin, the charge profile remains fixed to the lattice. In the second case an applied magnetic field rotates both spin and density profile, even though through a very small angle. We show that in this second case we have, in addition to the use of photons or electrons, the possibility of exciting and observing the Scissors Mode in crystals in a way analogous to that used in Bose-Einstein Condensates.

cond-mat.str-el↗