arXiv subjects

H. Beck

Publications and source records attributed to H. Beck.

At least 37 records · Page 2Linked to original sources

arXiv2010

Exciton condensation driving the periodic lattice distortion of 1T-TiSe2

We address the lattice instability of 1T-TiSe2 in the framework of the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase.

cond-mat.str-el↗

arXiv2010

Centrality dependence of proton and antiproton spectra in Pb+Pb collisions at 40A GeV and 158A GeV measured at the CERN SPS

The yields of (anti-)protons were measured by the NA49 Collaboration in centrality selected Pb+Pb collisions at 40A GeV and 158A GeV. Particle identification was obtained in the laboratory momentum range from 5 to 63 GeV/c by the measurement of the energy loss dE/dx in the TPC detector gas. The corresponding rapidity coverage extends 1.6 units from mid-rapidity into the forward hemisphere. Transverse mass spectra, the rapidity dependences of the average transverse mass, and rapidity density distributions were studied as a function of collision centrality. The values of the average transverse mass as well as the midrapidity yields of protons when normalized to the number of wounded nucleons show only modest centrality dependences. In contrast, the shape of the rapidity distribution changes significantly with collision centrality, especially at 40A GeV. The experimental results are compared to calculations of the HSD and UrQMD transport models.

nucl-ex↗

arXiv2009

Dramatic effective mass reduction driven by strong electronic correlations

We present angle-resolved photoemission experiments on 1T-TiSe2 at temperatures ranging from 13K to 288K. The data evidence a dramatic renormalization of the conduction band below 100K, whose origin can be explained with the exciton condensate phase model. The renormalization translates into a substantial effective mass reduction of the dominant charge carriers and can be directly related to the low temperature downturn of the resistivity of 1T-TiSe2. This observation is in opposition to the common belief that strong interactions produce heavier quasiparticles through an increased effective mass.

cond-mat.str-el↗

arXiv2009

Temperature dependent photoemission on 1T-TiSe2: Interpretation within the exciton condensate phase model

The charge density wave phase transition of 1T-TiSe2 is studied by angle-resolved photoemission over a wide temperature range. An important chemical potential shift which strongly evolves with temperature is evidenced. In the framework of the exciton condensate phase, the detailed temperature dependence of the associated order parameter is extracted. Having a mean-field-like behaviour at low temperature, it exhibits a non-zero value above the transition, interpreted as the signature of strong excitonic fluctuations, reminiscent of the pseudo-gap phase of high temperature superconductors. Integrated intensity around the Fermi level is found to display a trend similar to the measured resistivity and is discussed within the model.

cond-mat.str-el↗

arXiv2008

Temperature dependence of the excitonic insulator phase model in 1T-TiSe2

Recently, detailed calculations of the excitonic insulator phase model adapted to the case of 1\textit{T}-TiSe$_2$ have been presented. Through the spectral function theoretical photoemission intensity maps can be generated which are in very good agreement with experiment [Phys. Rev. Lett. {\bf 99}, (2007) 146403]. In this model, excitons condensate in a BCS-like manner and give rise to a charge density wave, characterized by an order parameter. Here, we assume an analytical form of the order parameter, allowing to perform temperature dependent calculations. The influence of this order parameter on the electronic spectral function, to be observed in photoemission spectra, is discussed. The resulting chemical potential shift and an estimation of the resistivity are also shown.

cond-mat.str-el↗

arXiv2008

Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach

Recently strong evidence has been found in favor of a BCS-like condensation of excitons in 1\textit{T}-TiSe$_2$. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the excitonic condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. Here, the calculations in the framework of this model are presented in detail. They represent an extension of the original excitonic insulator phase model of Jérome \textit{et al.} [Phys. Rev. {\bf 158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and further comparison with experiment are also discussed

cond-mat.str-el↗

arXiv2007

Evidence for an excitonic insulator phase in 1T-TiSe$_{2}$

We present a new high-resolution angle-resolved photoemission study of 1\textit{T}-TiSe$_{2}$ in both, its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalisations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A theoretical calculation of the spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favour of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1\textit{T}-TiSe$_{2}$.

cond-mat.str-el↗

arXiv2005

Charge order and phase segregation in overdoped bilayer manganites

There have been recent reports of charge ordering around $x=0.5$ in the bilayer manganites. At $x=0.5$, there appears to be a coexistence region of layered A-type antiferromagnetc and charge order. There are also reports of orbital order in this region without any Jahn-Teller effect. Based on physical grounds, this region is investigated from a model that incorporates the two $e_g$ orbitals at each Mn site and a near-neighbour Coulomb repulsion. It is shown that there indeed is both charge and orbital order close to the half-doped region coincident with a layered magnetic structure. Although the orbital order is known to drive the magnetic order, the layered magnetic structure is also favoured in this system by the lack of coherent transport across the planes and the reduced dimensionality of the lattice. The anisotropic hopping across the $e_g$ orbitals and the underlying layered structure largely determine the orbital arrangements in this region, while the charge order is primarily due to the long range interactions.

cond-mat.str-el↗

arXiv2004

Ward identity and optical-conductivity sum rule in the d-density wave state

We consider the role of the Ward identity in dealing with the transport properties of an interacting system forming a d-wave modulated charge-density wave or staggered flux phase. In particular, we address this issue from the point of view of the restricted optical-conductivity sum rule. Our aim is to provide a controlled approximation for the current-current correlation function which allows us also to determine analytically the corresponding sum rule. By analyzing the role of the vertex functions in both the microscopic interacting model and in the effective mean-field Hamiltonian, we propose a non-standard low-energy sum-rule for this system. We also discuss the possible applicability of these results for the description of cuprate superconductors in the pseudogap regime.

cond-mat.supr-con↗

arXiv2004

Effect of orbital currents on the restricted optical sum rule

We derive the restricted optical-conductivity sum rule for a model with circulating orbital currents. It is shown that an unusual coupling of the vector potential to the interaction term of the model Hamiltonian results in a non-standard form of the sum rule. As a consequence, the temperature dependence of the restricted spectral weight could be compatible with existing experimental data for high-$T_c$ cuprates above the critical temperature $T_c$. We extend our results to the superconducting state, and comment on the differences and analogies between these two symmetry-breaking phenomena.

cond-mat.str-el↗

arXiv2004

Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: precursor pairing vs the d-density-wave scenario

We derive the polarizability of an electron system in (i) the superconducting phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Such a calculation is motivated by the recent proposals that imaging the effects of an isolated impurity may distinguish between precursor pairing and dDW order in the pseudogap regime of the high-Tc superconductors. In all three cases, the wave-vector dependence of the polarizability is characterized by an azymuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit imperfect, due to a nonzero value of the hopping ratio t'/t in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also exhibited by the (q,omega) dependence of the retarded polarizability.

cond-mat.supr-con↗

arXiv2004

Magnetic oscillations in planar systems with the Dirac-like spectrum of quasiparticle excitations

The quantum magnetic oscillations are studied for planar condensed matter systems with a linear, Dirac-like spectrum of quasiparticle excitations. We derive analytical expressions for magnetic oscillations (de Haas - van Alphen effect) in the density of states, thermodynamic potential, magnetization and chemical potential both for zero and finite temperatures, and in the presence of scattering from impurities. We discuss also a possibility of using magnetic oscillations for detection of a gap that may open in the spectrum of quasiparticle excitations due to a nontrivial interaction between them.

cond-mat.str-el↗

arXiv2003

d-density wave state in an external magnetic field

We present a study of the electrical and thermal conductivities of the $d$-density wave (DDW) state in an external magnetic field $B$ in the low temperature regime and in the presence of impurities. We show that in the zero temperature limit, $T \to 0$, the Wiedemann-Franz (WF) law remains intact. For finite $T$ the WF law violation is possible and it is enhanced by the external field.

cond-mat.supr-con↗

arXiv2003

Transport properties in the d-density-wave state in an external magnetic field: The Wiedemann-Franz law

We derive the electrical and thermal conductivities of the $d$-density-wave state in an external magnetic field $B$ in the low-temperature regime and in the presence of impurities. We show that in the zero-temperature limit, $T \to 0$, the Wiedemann-Franz (WF) law remains intact irrespectively of the value of the applied field and chemical potential $μ$ as soon as there is a scattering from impurities. For finite $T \lesssim |μ|$ the WF law violation is possible and it is enhanced by the external field. Depending on the relative values of $B$ and $T$ the electrical conduction may dominate the heat conduction and vice versa. For $μ\gg T$ the WF is restored even in the presence of the external field.

cond-mat.str-el↗

arXiv2003

Internal structure of preformed Cooper pairs

In order to obtain information about the internal structure of the preformed pairs in the pseudogap state of high $T_c$ superconductors, we calculate the propagator of a singlet pair with center of mass coordinate $\mathbf{r}$, and relative distance $\pmbρ$, by solving the Bethe-Salpeter equation, representing the sum over repeated two-particle scattering events due to a distance dependent attraction. We define then a ``pair structure function'' $g_{P}(\mathbf{P},\pmbρ) $ that depends on the internal distance $\pmbρ$ between the partners and on the momentum $\mathbf{P}$ of the pair. We calculate this function both for a local potential and $s-$wave symmetry of the order parameter and for a separable potential and $d-$wave symmetry of the order parameter. The influence of the center of mass momentum, strenght of the interaction, temperature, density of particles and of the pseudogap in the one-electron spectrum is studied for both cases.

cond-mat.supr-con↗

arXiv2002

Effective action approach to the Leggett's mode in two-band superconductors

We investigate a collective excitation (Leggett's mode) corresponding to small fluctuations of the relative phase of two condensates in two-band superconductor using the effective ``phase only'' action. We consider the possibility of observing Leggett's mode in MgB$_2$ superconductor and conclude that for the known at present values of the two-band model parameters for MgB$_2$ Leggett's mode arises above the two-particle threshold.

cond-mat.supr-con↗

arXiv2002

Low temperature superfluid stiffness of d-wave superconductor in a magnetic field

The temperature and field dependence of the superfluid density $ρ_s$ in the vortex state of a d-wave superconductor are calculated using a microscopic model in the quasiclassical approximation. We show that at temperatures below T^{*} \varpropto \sqrt{H}$, the linear T dependence of rho_s crosses over to a T^2 dependence differently from the behavior of the effective penetration depth, lambda_eff^{-2}(T). We point out that the expected dependences could be probed by a mutual-inductance technique experiment.

cond-mat.supr-con↗

arXiv2002

Quantum Monte Carlo study of the 3D attractive Hubbard model

We study the three-dimensional (3D) attractive Hubbard model by means of the Determinant Quantum Monte Carlo method. This model is a prototype for the description of the smooth crossover between BCS superconductivity and Bose-Einstein condensation. By detailed finite-size scaling we extract the finite-temperature phase diagram of the model. In particular, we interpret the observed behavior according to a scenario of two fundamental temperature scales; T* associated with Cooper pair formation and Tc with condensation (giving rise to long-range superconducting order). Our results also indicate the presence of a recently conjectured phase transition hidden by the superconducting state. A comparison with the 2D case is briefly discussed, given its relevance for the physics of high-Tc cuprate superconductors.

cond-mat↗