1,721,111 research outputs found
Doping dependence of the vortex-core energy in ultra-thin films of cuprates
No full textDoping dependence of the vortex-core energy in bilayer films of cuprates BENFATTO, L., CASTELLANI, C., GIAMARCHI, Thierry The energy needed to create a vortex core is the basic ingredient to address the physics of thermal vortex fluctuations in underdoped cuprates. Here, we theoretically investigate its role in the occurrence of the Beresinskii-Kosterlitz-Thouless transition in a bilayer film with inhomogeneity. From the comparison with recent measurements of the penetration depth in two-unit-cell thin films, we can extract the value of the vortex-core energy μ and show that μ scales linearly with Tc at low doping. BENFATTO, L., CASTELLANI, C., GIAMARCHI, Thierry. Doping dependence of the vortex-core energy in bilayer films of cuprates. Physical Review. B, Condensed Matter, 2008, vol. 77, no. 1
Boltzmann electronic dc transport in multiorbital weakly disordered crystals
Full text linkMotivated by the increasing number of systems featuring multiple bands at low energy, we address the Boltzmann approach to transport in a multiband weakly disordered noninteracting crystal subject to a small electric field. In general, the multiband structure leads to a considerable complication of the Boltzmann equation. Indeed, even in the presence of elastic impurity scattering, one needs to compute for each band and momentum the dressed velocities, which account for scattering events. Here we provide a semianalytical solution to the Boltzmann equation that reduces such a challenging numerical task to the much simpler numerical computation of a small tensor whose dimension is set by the number of bands at the Fermi level. This approach further allows us to discuss the interplay of symmetry and disorder for different impurity types, including those originating from random-matrix Wigner ensembles. As an example of application, we consider the 2D isotropic Rashba metal and we discuss, in a full analytical fashion, how different types of disorders may break the exactness of the relaxation-time approximation and induce transport anisotropy, and may allow one to identify the presence of spin-orbit coupling as deviations of the conductivity from the Drude behavior
Spectroscopic signatures of massless gap opening in graphene
No full textGap opening in graphene is usually discussed in terms of a semiconductinglike spectrum, where the appearance of a finite gap at the Dirac point is accompanied by a finite mass for the fermions. In this paper we propose a gap scenario from graphene which preserves the massless characters of the carriers. This approach explains recent spectroscopic measurements carried out in epitaxially grown graphene, ranging from photoemission to optical transmission
Vertex renormalization in dc conductivity of doped chiral graphene
No full textThe remarkable transport properties of graphene follow not only from the Dirac-type energy dispersion, but also from the chiral nature of its excitations, which makes unclear the limits of applicability of the standard semiclassical Boltzmann approach. In this paper we provide a quantum derivation of the transport scattering time in graphene in the case of electron-phonon interaction. By using the Kubo formalism, we compute explicitly the vertex corrections to the dc conductivity by retaining the full chiral matrix structure of graphene. We show that at least in the regime of large chemical potential the Boltzmann picture is justified. This result is also robust against a small sublattice inequivalence, which partly spoils the role of chirality and leads to a doping dependence of the resistivity that can be relevant to recent transport experiments in doped graphene samples
Signature of the Leggett mode in the A(1g) Raman response: From MgB2 to iron-based superconductors
Full text linkThe Raman response in a superconductor is a powerful probe to investigate the symmetry of the superconducting gap. Here we show that in a multiband superconductor it also offers the unique opportunity to establish whether the driving pairing interaction has an intraband or interband character. In the model with one hole and one electron band the full gauge-invariant Raman response, obtained by accounting for the fluctuations of both the density and superconducting phase degrees of freedom, is always dominated by the Leggett mode, regardless its nature. However, while in the case of intraband-dominated pairing the Josephson-like phase fluctuations of the two condensates identify a well-defined peak, as observed in MgB2, for dominant interband pairing the Leggett resonance is pushed at twice the largest gap, resembling apparently a pair-breaking peak. The latter case is in very good agreement with experimental data in iron-based superconductors, suggesting that an interband pairing mechanism should be at play in these systems. These results have also interesting implications for the nonlinear optical response probed by means of intense THz fields
Nature and Raman signatures of the Higgs amplitude mode in the coexisting superconducting and charge-density-wave state
No full textWe investigate the behavior of the Higgs (amplitude) mode when superconductivity emerges on a preexisting charge-density-wave state. We show that the weak overdamped square-root singularity of the amplitude fluctuations in a standard BCS superconductor is converted in a sharp, undamped power-law divergence in the coexisting state, reminiscent of the Higgs behavior in Lorentz-invariant theories. This effect reflects in a strong superconducting resonance in the Raman spectra, both for an electronic and a phononic mechanism leading to the Raman visibility of the Higgs. In the latter case, our results are relevant to the interpretation of the Raman spectra measured experimentally in NbSe2
Broadening of the Beresinskii-Kosterlitz-Thouless superconducting transition by inhomogeneity and finite-size effects
No full textWe discuss the crucial role played by finite-size effects and inhomogeneity on the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional superconductors. In particular, we focus on the temperature dependence of the resistivity, that is dominated by superconducting fluctuations above the BKT transition temperature TBKT and by inhomogeneity below it. By means of a renormalization-group approach we establish a direct correspondence between the parameter values used to describe the BKT fluctuation regime and the distance between TBKT and the mean-field Ginzburg-Landau transition temperature. Below TBKT a resistive tail arises due to finite-size effects and inhomogeneity, that reflects also on the temperature dependence of the superfluid density. We apply our results to recent experimental data in superconducting LaAlO3/SrTiO3 heterostructures, and we extract several informations on the microscopic properties of the system from our BKT fitting parameters. Finally, we compare our approach to recent data analysis presented in the literature, where the physical meaning of the parameter values in the BKT formulas has been often overlooked
Infrared phonon activity and Fano interference in multilayer graphenes
No full textRecent optical measurements in bilayer graphene have reported a strong dependence on phonon peak intensity, as well on the asymmetric Fano lineshape, on the charge doping and on the bandgap, tuned by gate voltage. In this paper, we show how these features can be analyzed and predicted on a microscopic quantitative level using the charge-phonon theory applied to the specific case of graphene systems. We present a phase diagram where the infrared activity of both the symmetric (E-g) and antisymmetric (E-u) phonon modes is evaluated as a function of doping and gap. We also show how a switching mechanism between these two modes can occur, governing the dominance of the optical response of one mode with respect to the other. The theory presented here can be also generalized to bulk graphite and to multilayer systems with different stacking orders, providing a useful roadmap for the characterization of graphenic systems by optical infrared means
Sine-Gordon description of Berensinskii-Kosterlitz-Thouless physics at finite magnetic field
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