arXiv subjects

J. Lorenzana

Publications and source records attributed to J. Lorenzana.

At least 37 records · Page 2Linked to original sources

arXiv2013

Universal scaling of the order-parameter distribution in strongly disordered superconductors

We investigate theoretically and experimentally the statistical properties of the inhomogeneous order-parameter distribution (OPD) at the verge of the superconductor-insulator transition (SIT). We find within two prototype fermionic and bosonic models for disordered superconductors that one can identify a universal rescaling of the OPD. By performing scanning-tunneling microscopy experiments in three samples of NbN with increasing disorder we show that such a rescaling describes also with an excellent accuracy the experimental data. These results can provide a breakthrough in our understanding of the SIT.

cond-mat.supr-con↗

arXiv2013

Coupling of a high-energy excitation to superconducting quasiparticles in a cuprate from Coherent Charge Fluctuation Spectroscopy

Dynamical information on spin degrees of freedom of proteins or solids can be obtained by Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR). A technique with similar versatility for charge degrees of freedom and their ultrafast correlations could move forward the understanding of systems like unconventional superconductors. By perturbing the superconducting state in a high-Tc cuprate using a femtosecond laser pulse, we generate coherent oscillations of the Cooper pair condensate which can be described by an NMR/ESR formalism. The oscillations are detected by transient broad-band reflectivity and found to resonate at the typical scale of Mott physics (2.6 eV), suggesting the existence of a non-retarded contribution to the pairing interaction, as in unconventional (non Migdal-Eliashberg) theories.

cond-mat.supr-con↗

arXiv2012

Short Range Smectic and Long Range Nematic Order in the Pseudogap Phase of Cuprates

We present a model for the combined nematic and `smectic' or stripe-like orders seen in recent scanning tunneling microscopy (STM) experiments in cuprates. We model the stripe order as an electronic charge density wave with associated Peierls distortion -- a `Pomeranchuk wave'. Disorder restricts this primary order to nanoscale domains, while secondary coupling to strain generates nematic order with considerably longer range.

cond-mat.supr-con↗

arXiv2012

Gutzwiller Magnetic Phase Diagram of the Cuprates

A general constructive procedure is presented for analyzing magnetic instabilities in two-dimensional materials, in terms of [predominantly] double nesting, and applied to Hartree-Fock HF+RPA and Gutzwiller approximation GA+RPA calculations of the Hubbard model. Applied to the cuprates, it is found that competing magnetic interactions are present only for hole doping, between half filling and the Van Hove singularity. While HF+RPA instabilities are present at all dopings (for sufficiently large Hubbard U), in a Gutzwiller approximation they are restricted to a doping range close to the range of relevance for the physical cuprates. The same model would hold for charge instabilities, except that the interaction is more likely to be q-dependent.

cond-mat.supr-con↗

arXiv2012

Gutzwiller Magnetic Phase Diagram of the Undoped t-t'-U Hubbard Model

We calculate the magnetic phase diagram of the half-filled t-t'-U Hubbard model as a function of t' and U, within the Gutzwiller approximation RPA (GA+RPA). As U increases, the system first crosses over to one of a wide variety of incommensurate phases, whose origin is clarified in terms of double nesting. We evaluate the stability regime of the incommensurate phases by allowing for symmetry-breaking with regard to the formation of spin spirals, and find a crossover to commensurate phases as U increases and a full gap opens. The results are compared with a variety of other recent calculations, and in general good agreement is found. For parameters appropriate to the cuprates, double occupancy should be only mildly suppressed in the absence of magnetic order, inconsistent with a strong coupling scenario.

cond-mat.supr-con↗

arXiv2012

Hidden Ferronematic Order in Underdoped Cuprates

We study a model for low doped cuprates where holes aggregate into oriented stripe segments which have a vortex and an antivortex fixed to the extremes. We argue that due to the interaction between segments a state with macroscopic polarization is stabilized, which we call a ferronematic. This state can be characterized as a charge nematic which, due to the net polarization, breaks inversion symmetry and also exhibits an incommensurate spin modulation. Our calculation can reproduce the doping dependent spin structure factor of lanthanum cuprates in excellent agreement with experiment and allows to rationalize experiments in which the incommensurability has an order parameter-like temperature dependence.

cond-mat.supr-con↗

arXiv2012

Density functional theory with adaptive pair density

We propose a density functional to find the ground state energy and density of interacting particles, where both the density and the pair density can adjust in the presence of an inhomogeneous potential. As a proof of principle we formulate an a priori exact functional for the inhomogeneous Hubbard model. The functional has the same form as the Gutzwiller approximation but with an unknown kinetic energy reduction factor. An approximation to the functional based on the exact solution of the uniform problem leads to a substantial improvement over the local density approximation.

cond-mat.str-el↗

arXiv2012

Stripes in cuprate superconductors: Excitations and dynamic dichotomy

We present a short account of the present experimental situation of stripes in cuprates followed by a review of our present understanding of their ground state and excited state properties. Collective modes, the dynamical structure factor, and the optical conductivity of stripes are computed using the time-dependent Gutzwiller approximation applied to realistic one band and three band Hubbard models, and are found to be in excellent agreement with experiment. On the other hand, experiments like angle-resolved photoemission and scanning tunneling microscopy show the coexistence of stripes at high energies with Fermi liquid quasiparticles at low energies. We show that a phenomenological model going beyond mean-field can reconcile this dynamic dichotomy.

cond-mat.str-el↗

arXiv2011

Superfluid density and phase relaxation in superconductors with strong disorder

As a prototype of a disordered superconductor we consider the attractive Hubbard model with on-site disorder. We solve the Bogoljubov-de-Gennes equations on two-dimensional finite clusters at zero temperature and evaluate the electromagnetic response to a vector potential. We find that the standard decoupling between transverse and longitudinal response does not apply in the presence of disorder. Moreover the superfluid density is strongly reduced by the relaxation of the phase of the order parameter already at mean-field level when disorder is large. We also find that the anharmonicity of the phase fluctuations is strongly enhanced by disorder. Beyond mean-field, this provides an enhancement of quantum fluctuations inducing a zero-temperature transition to a non-superconducting phase of disordered preformed pairs. Finally, the connection of our findings with the glassy physics for extreme dirty superconductors is discussed.

cond-mat.supr-con↗

arXiv2011

Nematic phase without Heisenberg physics in FeAs planes

We use Monte Carlo simulations and analytical arguments to analyze a frustrated Ising model with nearest neighbour antiferromagnetic coupling $J_1$ and next nearest neighbour coupling $J_2$. The model is inspired on the physics of pnictide superconductors and to some extent we argue that it can be more representative of this physics than the Heisenberg counterpart. Parameters are chosen such that the ground state is a columnar or striped state, as observed experimentally, but is close to the transition to the simple Neel ordered antiferromagnetic state $R = J_2/|J_1|\gtrsim 0.5$. We find that a nematic phase is induced close to $R = 0.5$ by finite size effects and argue that this explains experiments in imperfect samples which find a more robust nematic state as the quality of the sample decreases [A. Jesche et al., Phys. Rev. B 81, 134525 (2010)]. Including the effect of a weak coupling with the lattice we find that a structural transition occurs associated with a nematic phase, with a magnetic transition occurring at a lower temperature. These two transitions merge into a single structural and magnetic transition with a stronger first-order character for larger spin-lattice couplings. These two situations are in agreement with the different phenomenologies found in different families of pnictides.

cond-mat.supr-con↗

arXiv2011

Influence of correlations on transitive electron-phonon couplings in cuprate superconductors

We investigate a model for the CuO_2 plane of high-T_c superconductors where the charge carriers are coupled to A_{1g} and B_{1g} symmetric out-of plane vibrations of the oxygen atoms in the presence of local Hubbard correlations. The coupling is implemented via a modulation of the hopping integral and we calculate the renormalization of vertex and pairing scattering functions based on the time-dependent Gutziller approximation. Contrary to local electron-phonon couplings we find that the transitive coupling can even be enhanced by correlations for certain momenta and symmetries of the vibrations. While this effect may be important for certain properties, we find that, with regard to superconductivity, electron-electron correlations still generically lead to a suppression of the pairing correlations. Our results allow for an estimate of correlation effects on the electron-phonon induced pair scattering from weak electron-electron interactions up to the Mott regime. For onsite repulsions relevant to cuprate superconductors our calculations reveal a significant contribution of B_{1g} phonons to d-wave superconductivity.

cond-mat.str-el↗

arXiv2010

Spin canting as a result of the competition between stripes and spirals in cuprates

Based on the extended Hubbard model we calculate the energy of stripe and spiral ground states. We find that uniform spirals get favored by a large $t'/t$ ratio but are unstable at small doping towards stripes and checkerboard textures with spin canting. The structure of these inhomogeneities also depends on t'/t and the associated spin currents may induce a small lattice distortion associated with local dipole moments. We discuss a new kind of stripe which appears as a domain wall of the antiferromagnetic (AF) order parameter with a fractional change of the phase of the AF order. For large |t'/t| spirals can be stabilized under certain conditions in the overdoped regime which may explain the elastic incommensurate magnetic response recently observed in iron-codoped Bi2201 materials.

cond-mat.str-el↗

arXiv2010

Dynamical charge and spin density wave scattering in cuprate superconductor

We show that a variety of spectral features in high-T_c cuprates can be understood from the coupling of charge carriers to some kind of dynamical order which we exemplify in terms of fluctuating charge and spin density waves. Two theoretical models are investigated which capture different aspects of such dynamical scattering. The first approach leaves the ground state in the disordered phase but couples the electrons to bosonic degrees of freedom, corresponding to the quasi singular scattering associated with the closeness to an ordered phase. The second, more phenomological approach starts from the construction of a frequency dependent order parameter which vanishes for small energies. Both theories capture scanning tunneling microscopy and angle-resoved photoemission experiments which suggest the protection of quasiparticles close to the Fermi energy but the manifestation of long-range order at higher frequencies.

cond-mat.supr-con↗

arXiv2010

Proximity of Iron Pnictide Superconductors to a Quantum Tricritical Point

We determine the nature of the magnetic quantum critical point in the doped LaFeAsO using a set of constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The system turns out to be remarkably close to a quantum tricritical point, where the nature of the phase transition changes from first to second order. We compare with the effective field theory and discuss the experimental consequences.

cond-mat.supr-con↗

arXiv2010

Real time observation of cuprates structural dynamics by Ultrafast Electron Crystallography

The phonon-mediated attractive interaction between carriers leads to the Cooper pair formation in conventional superconductors. Despite decades of research, the glue holding Cooper pairs in high-temperature superconducting cuprates is still controversial, and the same is true as for the relative involvement of structural and electronic degrees of freedom. Ultrafast electron crystallography (UEC) offers, through observation of spatio-temporally resolved diffraction, the means for determining structural dynamics and the possible role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse excites the charge carriers, which relax through electron-electron and electron-phonon coupling, and the consequential structural distortion is followed diffracting fs electron pulses. In this review, the recent findings obtained on cuprates are summarized. In particular, we discuss the strength and symmetry of the directional electron-phonon coupling in Bi2Sr2CaCu2O8+δ(BSCCO), as well as the c-axis structural instability induced by near-infrared pulses in La2CuO4 (LCO). The theoretical implications of these results are discussed with focus on the possibility of charge stripes being significant in accounting for the polarization anisotropy of BSCCO, and cohesion energy (Madelung) calculations being descriptive of the c-axis instability in LCO.

cond-mat.supr-con↗

arXiv2009

Infrared optical absorption spectra of CuO single crystals: Fermion-spinon band and dimensional crossover of the antiferromagnetic order

We have obtained mid-infrared optical absorption spectra of the S=1/2 quasi one-dimensional CuO using polarized transmission measurement and interpreted the spectra in terms of phonon assisted magnetic excitations. When the electric field is parallel to the main antiferromagnetic direction a Delta shaped peak is observed with the maximum at 0.23eV which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at 0.16eV which is attributed to two-magnon absorption but the spinon peak remains. This behavior is interpreted as due to a dimensional crossover where the low temperature three-dimensional magnetic phase keeps short range characteristics of a one-dimensional magnet.

cond-mat.str-el↗

arXiv2009

Phonon renormalization from local and transitive electron-lattice couplings in strongly correlated systems

Within the time-dependent Gutzwiller approximation (TDGA) applied to Holstein- and SSH-Hubbard models we study the influence of electron correlations on the phonon self-energy. For the local Holstein coupling we find that the phonon frequency renormalization gets weakened upon increasing the onsite interaction $U$ for all momenta. In contrast, correlations can enhance the phonon frequency shift for small wave-vectors in the SSH-Hubbard model. Moreover the TDGA applied to the latter model provides a mechanism which leads to phonon frequency corrections at intermediate momenta due to the coupling with double occupancy fluctuations. Both models display a shift of the nesting-induced to a $q=0$ instability when the onsite interaction becomes sufficiently strong and thus establishing phase separation as a generic phenomenon of strongly correlated electron-phonon coupled systems.

cond-mat.str-el↗

arXiv2009

Diagonal stripes in the spin glass phase of cuprates

Based on the unrestricted Gutzwiller approximation we study the possibility that the diagonal incommensurate spin scattering in the spin glass phase of lanthanum cuprates originates from stripe formation. Similar to the metallic phase two types of diagonal stripe structures appear to be stable: (a) site-centered textures which have one hole per site along the stripe and (b) ferromagnetic stair-case structures which are the diagonal equivalent to bond-centered stripes in the metallic phase and which on average have a filling of 3/4 holes per stripe site. We give a detailed analysis of the stability of both diagonal textures with regard to the vertical ones.

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