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At least 343 records · Page 19Linked to original sources

arXiv2010

Magnetic, magnetocaloric and neutron diffraction studies on TbNi5-xMx (M = Co and Fe) compounds

The effect of substitution of Co and Fe for Ni in TbNi5 on the structural, magnetic and magneto-thermal properties has been investigated. Considerable enhancement of Curie temperature is observed with Fe substitution, whereas the increase is nominal in the case of Co. Neutron diffraction measurements reveal the redistribution of moments and site preference of substitutional ions in Ni 2c and 3g sites. In TbNi4Fe, both Ni and Fe as well as Tb are found to carry moment while in the case of TbNi4Co, mainly Tb carries the moment. Magnetocaloric behavior has been investigated from the magnetization and the heat capacity measurements. The magnetic and magnetocaloric properties are found to be strongly correlated in these compounds.

cond-mat.str-el↗

arXiv2011

Strength of the phonon-coupling mode in $La_{2-x}Sr_{x}CuO_{4}$, $Bi_{2}Sr_{2}CaCu_{2}O_{8+x}$ and $YBa_{2}Cu_{3}O_{6+x}$: An estimation from the ARPES-nodal measurements

Despite the intensive efforts for determining the mechanism that causes high-temperature superconductivity in copper oxide materials, no consensus on the pairing mechanism has been reached. Recent advances in high resolution angle-resolved photoemission spectroscopies have suggested that a sizeable electron-phonon coupling exists as the principal cause for kinks in the dispersion relations (energy versus wave vector) of the electronic states. Here, we report on a systematic study about the influence of the electron-phonon coupling parameter "$λ$" in the electronic quasiparticle dispersions along the nodal direction for $La_{2-x}Sr_{x}CuO_{4}$, $Bi_{2}Sr_{2}CaCu_{2}O_{8+x}$ and $YBa_{2}Cu_{3}O_{6+x}$. Information about the dressing of the charge carriers, i.e., on the enhancement of the effective mass and the strength of the coupling mode, is obtained as a function of the doping concentration, temperature, momentum and energy from the kink dispersion in the (0-0)-$(π,π)$ direction of momentum-space avoiding the complications of the \textit{d-}wave superconducting gap. Our analysis shows a remarkable agreement between theory and experiment for different samples and at different doping levels. This includes our recently introduced theoretical model to adjust the experimental data of the fermionic band dispersion, emphasising the necessary distinction between the general electron mass-enhancement parameter $λ^{*}$ and the conventional electron-phonon coupling parameter $λ$.

cond-mat.supr-con↗

arXiv2010

Signatures of a gap in the conductivity of graphene

The electrical conductivity of suspended graphene has recently been measured for the first time, and found to behave as σ~ \sqrt{|n|} as expected for Dirac quasiparticles at large carrier density. The charge inhomogeneity is strongly reduced in suspended samples, which revealed an unexpected insulating trend in σ(T). Above a transitional density n*, the temperature dependence was found to revert to metallic. We show that these features of the DC conductivity are consistent with a simple model of gapped Dirac quasiparticles, with specific signatures of a gap in the vicinity of the charge-neutrality point. Our findings are reminiscent of the conductivity profile in semiconducting materials, exhibiting a thermal activation for T \geq \tilde T and a weakly T-dependent background for T \leq \tilde T, where \tilde T is given by the saturation density \tilde n associated with the residual charge inhomogeneity. We discuss possible origins of a bandgap in graphene as well as alternative scenarios.

cond-mat.str-el↗

arXiv2010

Invisibility in one dimensional systems: a possible way to achieve it

The aim of this paper is to provide the mathematical framework for analyzing the possibility of rendering one dimensional objects visible with difficulty and ultimately invisible,by changing the experimental conditions to which they are subdued. The calculation was performed using the Hubbard model,and it will be based on a recent expression for a particular class of Q1D organic salts. Two sets of material parameters for which the reflectivity of a 1D object tends to zero have been determined.

cond-mat.str-el↗

arXiv2011

Magnetism in graphite oxide: The role of epoxy groups

We investigate the magnetism in graphite by controlled oxidation. Our approach renders graphite an insulator while maintaining its structure. Fourier transform infrared spectroscopy and X-ray absorption near edge structure spectra reveal that graphite oxide has epoxy groups on its surface and it is not thermally stable. Magnetic susceptibility data exhibit negative Curie temperature, field irreversibility, and slow relaxation. The magnetic properties diminish after the epoxy groups are destroyed. The overall results indicate the unexpected magnetism is associated with the presence of epoxy groups.

cond-mat.str-el↗

arXiv2010

Ideas by S.V. Vonsovsky and Modern Model Treatment of Magnetism

A review of fundamental works by Shubin and Vonsovsky on the formulation of the polar and s-d(f) exchange models is given. Their ideas are compared with subsequent developments in the theory of magnetism in d- and f-metals and their compounds. Modern approaches including different slave-boson and slave-fermion representations, formation of exotic quasiparticles etc. are discussed. Internal connections between various many-electron models (the Heisenberg, Hubbard, t-J, Anderson Hamiltonians) are presented. Description of anomalous rare-earth and actinide compounds (Kondo lattices, systems with heavy fermions and non-Fermi-liquid behavior) within the framework of the s-d(f) exchange model and related models is considered.

cond-mat.str-el↗

arXiv2010

The effect of spin-orbit interaction and attractive Coulomb potential on the magnetic properties of Ga$_{1-x}$Mn$_{x}$As

We employ the dynamical mean-field approximation to study the magnetic properties of a model relevant for the dilute magnetic semiconductors. Our model includes the spin-orbit coupling on the hole bands, the exchange interaction, and the attractive Coulomb potential between the negatively charged magnetic ions and the itinerant holes. The inclusion of the Coulomb potential significantly renormalizes the exchange coupling and enhances the ferromagnetic transition temperature for a wide range of couplings. We also explore the effect of the spin-orbit interaction by using two different values of the ratio of the effective masses of the heavy and light holes. We show that in the regime of small $J_{c}$-$V$ the spin-orbit interaction enhances $T_{c}$, while for large enough values of $J_{c}$-$V$ magnetic frustration reduces $T_c$ to values comparable to the previously calculated strong coupling limit.

cond-mat.str-el↗

arXiv2011

Effect of pressure on magnetization and magnetostriction jumps in the manganite Eu$_{0.58}$Sr$_{0.42}$MnO$_{3}$

We have demonstrated the effect of pressure on magnetization jump and its associated magnetostriction in Eu$_{0.58}$Sr$_{0.42}$MnO$_{3}$. The critical field lowered by the applied pressure is explained by a suppression of the metastable blocked state, which is inherent to the phase separated manganite system exhibiting the magnetic avalanche. To reduce a factor of the bad thermal coupling in the bulk sample, and to further examine the nature of the magnetization jump, we have performed the influence of pressure on the isothermal magnetization of the powder sample. There is no discrepancy in the critical field between the bulk and powder samples although the $MH$ curve of the latter is not saturated at high fields. These findings indicate a crucial role of the frozen phase separated state in the abrupt transition.

cond-mat.str-el↗

arXiv2010

Possible evidence for two-gap superconductivity in TiSe_2Cu_x

We report a comprehensive TF-muSR study of TiSe_2Cu_2. The magnetic penetration depth was found to saturate at low temperature as expected in an s-wave SC. As x is increased we find that the superfluid density increases and the size of the superconducting gap, calculated from the temperature dependence of the superfluid density, is approaching the BCS value. However, for low values of x, the gap is smaller than the weak-coupling BCS prediction suggesting that two superconducting gaps are present in the sample.

cond-mat.supr-con↗

arXiv2010

Correlations between two Anderson impurities

We study the intersite correlation effects on the Kondo dynamics of two Anderson impurities. Two characteristic energy scales are present associated with the splitting of the uniform and staggered spin susceptibilities. The high energy scale has similar properties as the coherence scale in heavy fermion materials while the low energy scale characterizes the competing orders between Kondo singlet state and inter-impurity singlet state. A singlet superconductivity instability is found near the quantum critical point and is associated with the singular staggered spin susceptibility.

cond-mat.str-el↗

arXiv2010

Spin chirality ordering and anomalous Hall effect in the ferromagnetic Kondo lattice model on the triangular lattice

We investigate the ground-state phase diagram for the ferromagnetic Kondo lattice model on a triangular lattice by a variational calculation for various spin orderings up to a four-site unit cell. We find that a noncoplanar four-sublattice ordering with a finite scalar spin chirality emerges at and around 1/4 filling, in addition to the 3/4-filled case, which was predicted to be induced by the perfect nesting of the Fermi surface [I. Martin and C. D. Batista: Phys. Rev. Lett. {\bf 101} (2008) 156402]. The 1/4-filling phase is stable in a wider range of parameters than the 3/4-filling one, and includes a large region of gapped insulating state characterized by a Chern number. We also compute the Hall conductivity in the chiral-ordered phases.

cond-mat.str-el↗

arXiv2010

Particle-hole instability in the $AdS_4$ holography

We show that particle-hole pairing is realized in the background of a charged black hole in magnetic field. The pairing instability occurs for sufficiently large fermion charges, which correspond to the Fermi liquid regime. The critical temperature for Fermi liquids is proportinal to the magnetic field and vanishes as we approach the non-Fermi liquid state. The pairing order parameter leads to a relative shift of the Fermi surfaces corresponding to the bulk fermions with spin up and down. The value of the shift in Fermi momentum $k_F$ and the critical temperature $T_c$ are proportional to the effective density of states at the Fermi surface. Our one-loop calculations provide a dual description of the magnetic catalysis for the lowest Landau level in graphene. This analyses may be relevant for the antiferromagnetic behavior in the cuprate superconductors and for the chiral spirals in the chiral magnetic effect. We also discuss thermodynamic and transport properties of a system at the boundary at zero magnetic field. The scaling behavior of the specific heat is $c\sim T$ for Fermi liquid and $c\sim T^{2ν}$ for non-Fermi liquid, while the behavior of the DC conductivity is the same $σ\sim T^{-2ν}$ in both cases. While it can be difficult to extract transport and hydrodynamic from the lattice, the $AdS/CFT$ approach provides a robust frame for nonperturbative calculation of these properties.

hep-th↗

arXiv2010

Green's function technique for a two-electrode mesoscopic system under bias

We present a Green's function technique for studying the nonlinear conductance of a nanocontact system with two electrodes at different chemical potentials. The retarded Green's function for a single-impurity Anderson model with two reservoirs is obtained in terms of a $5\times 5$ matrix in which the effect of bias is contained. A complete set of basis vectors for the single-impurity Anderson model has been provided before formulating the Green's function. Finally, we present a self-consistent method to fix the undetermined quantities existing in the matrix elements for the retarded Green's function.

cond-mat.mes-hall↗

arXiv2012

A fermionic superfluid state for many spinful species - II

In a previous report (arxiv:0901.2487), we introduced a new fermionic variational wavefunction, suitable for interacting multi-species systems and sustaining superfluidity. This wavefunction contains a new quantum index. Here we introduce a spin triplet version of this wavefunction, with parallel spin pairs only. We also present a single fermion species wavefunction, which may be relevant for the problem of the BCS to BEC transition.

cond-mat.supr-con↗

arXiv2012

Magnetic orderings and phase separations in the zero-bandwidth limit of the extended Hubbard model with intersite magnetic interactions

A simple effective model for a description of magnetically ordered insulators is analysed. The tight binding Hamiltonian consists of the effective on-site interaction (U) and intersite magnetic exchange interactions (Jz, Jxy) between nearest-neighbours. The phase diagrams of this model have been determined within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. We show that, depending on the values of interaction parameters and the electron concentration, the system can exhibit not only homogeneous phases: (anti-)ferromagnetic (F) and nonordered (NO), but also phase separated states (PS: F-NO).

cond-mat.str-el↗

arXiv2010

Anisotropic superconductivity in layered LaSb$_2$: the role of phase fluctuations

We present magnetic susceptibility and electrical transport measurements of the highly anisotropic compound LaSb$_2$ observing a very broad transition into a clean, consistent with type-I, superconducting state with distinct features of 2 dimensionality. Application of hydrostatic pressure induces a 2- to 3-dimensional crossover evidenced by a reduced anisotropy and transition width. The superconducting transition appears phase fluctuation limited at ambient pressure with fluctuations observed for temperatures greater than 8 times the superconducting critical temperature.

cond-mat.supr-con↗

arXiv2011

Optical probing of correlation driven liquid-to-insulator transition in 2D electron gas

We study the quantum Hall liquid and the metal-insulator transition in a high mobility two dimensional electron gas, by means of photoluminescence and magneto-transport. In the integer and fractional regime at nu > 1/3, analyzing the emission energy dispersion we probe the magneto-Coulomb screening and the hidden symmetry of the electron liquid. In the fractional regime above above nu =1/3 the system undergoes the metal-to-insulator transition, and in the insulating phase the dispersion becomes linear with evidence of an increased renormalized mass.

cond-mat.str-el↗

arXiv2010

Composite fermion dynamics in half-filled Landau levels of graphene

We report on exact-diagonalization studies of correlated many-electron states in the half-filled Landau levels of graphene, including pseudospin (valley) degeneracy. We demonstrate that the polarized Fermi sea of non-interacting composite fermions remains stable against a pairing transition in the lowest two Landau levels. However, it undergoes spontaneous depolarization, which is unprotected owing to the lack of single-particle pseudospin splitting. These results suggest the absence of the Pfaffian phase in graphene.

cond-mat.str-el↗