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A. Greco

Publications and source records attributed to A. Greco.

At least 55 records · Page 3Linked to original sources
arXiv2006

Electronic properties of correlated metals in the vicinity of a charge order transition: optical spectroscopy of $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl)

The infrared spectra of the quasi-two-dimensional organic conductors $α$-(BEDT-TTF)$_2$$M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl) were measured in the range from 50 to 7000 \cm down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependant reflectance of $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH$_4$ salt to Rb and further to Tl compounds. On reducing the temperature, the metallic character of the optical response in the NH$_4$ and Rb salts increases, and the effective mass diminishes. For the Tl compound, clear signatures of charge order are found albeit the metallic properties still dominate. From the temperature dependence of the electronic scattering rate the crossover temperature is estimated below which the coherent charge-carriers response sets in. The observations are in excellent agreement with recent theoretical predictions for a quarter-filled metallic system close to charge order.

cond-mat.str-el↗
arXiv2006

Charge order and phonon renormalizations: Possible implications for cobaltates

Several experimental and theoretical studies in cobaltates suggest the proximity of the system to charge ordering (CO). We show, qualitatively, in the frame of a $t-V$ model coupled to phonons that optical phonon modes at the $K$ and $M$ points of the Brillouin zone, which involves only $O$-ions displacement around a $Co$-ion, are good candidates to display anomalies due to the CO proximity. If by increasing of $H_2O$ content the system is pushed closer to CO, the mentioned phonon modes should show softening and broadening.

cond-mat.str-el↗
arXiv2006

Electron-phonon interaction dressed by electronic correlations near charge ordering as the origin for superconductivity in cobaltates

We consider possible routes to superconductivity in hydrated cobaltates Na_xCoO_2.yH_2O on the basis of the t-J-V model plus phonons on the triangular lattice. We studied the stability conditions for the homogeneous Fermi liquid (HFL) phase against different broken symmetry phases. Besides the sqrt(3)xsqrt(3)-CDW phase, triggered by the nearest-neighbour Coulomb interaction V, we have found that the HFL is unstable, at very low doping, against a bond-ordered phase due to J. We also discuss the occurrence of phase separation at low doping and V. The interplay between the electron-phonon interaction and correlations near the sqrt(3)xsqrt(3)-CDW leads to superconductivity in the unconventional next-nearest neighbour f-wave (NNN-f) channel with a dome shape for Tc around x ~ 0.35, and with values of a few Kelvin as seen in experiments. Near the bond-ordered phase at low doping we found tendencies to superconductivity with d-wave symmetry for finite J and x<0.15. Contact with experiments is given along the paper.

cond-mat.str-el↗
arXiv2006

Non-Fermi liquid behavior in nearly charge ordered layered metals

Non-Fermi liquid behavior is shown to occur in two-dimensional metals which are close to a charge ordering transition driven by the Coulomb repulsion. A linear temperature dependence of the scattering rate together with an increase of the electron effective mass occur above T*, a temperature scale much smaller than the Fermi temperature. It is shown that the anomalous temperature dependence of the optical conductivity of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2MHg(SCN)4, with M=NH4 and Rb, above T*=50-100 K, agrees qualitatively with our predictions for the electronic properties of nearly charge ordered two-dimensional metals.

cond-mat.str-el↗
arXiv2006

One-electron self energies and spectral functions for the t-J model in the large-N limit

Using a recently developed perturbative approach, which considers Hubbard operators as fundamental excitations, we have performed electronic self-energy and spectral function calculations for the $t-J$ model on the square lattice. We have found that the spectral functions along the Fermi surface are isotropic, even close to the critical doping where the $d$-density wave phase takes place. Fermi liquid behavior with scattering rate $\sim ω^2$ and a finite quasiparticle weight $Z$ was obtained. $Z$ decreases with decreasing doping taking low values for low doping. Results are compared with other ones, analytical and numerical like slave-boson and Lanczos diagonalization finding agreement. We discuss our results in the light of recent $ARPES$ experiments in cuprates.

cond-mat.str-el↗
arXiv2006

Collective excitations in unconventional charge-density wave systems

The excitation spectrum of the t-J model is studied on a square lattice in the large $N$ limit in a doping range where a $d$-$density$-$wave$ (DDW) forms below a transition temperature $T^\star$. Characteristic features of the DDW ground state are circulating currents which fluctuate above and condense into a staggered flux state below $T^\star$ and density fluctuations where the electron and the hole are localized at different sites. General expressions for the density response are given both above and below $T^\star$ and applied to Raman, X-ray, and neutron scattering. Numerical results show that the density response is mainly collective in nature consisting of broad, dispersive structures which transform into well-defined peaks mainly at small momentum transfers. One way to detect these excitations is by inelastic neutron scattering at small momentum transfers where the cross section (typically a few per cents of that for spin scattering) is substantially enhanced, exhibits a strong dependence on the direction of the transferred momentum and a well-pronounced peak somewhat below twice the DDW gap. Scattering from the DDW-induced Bragg peak is found to be weaker by two orders of magnitude compared with the momentum-integrated inelastic part.

cond-mat.str-el↗
arXiv2005

Signatures of electron-electron correlations in the optical spectra of $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ ($M$=NH$_4$ and Rb)

We interpret the optical spectra of $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (M=NH$_4$ and Rb) in terms of a 1/4 filled metallic system close to charge ordering and show that in the conductivity spectra of these compounds a fraction of the spectral weight is shifted from the Drude-peak to higher frequencies due to strong electronic correlations. Analyzing the temperature dependence of the electronic parameters, we distinguish between different aspects of the influence of electronic correlations on optical properties. We conclude, that the correlation effects are slightly weaker in the NH$_4$ compound compared to the Rb one.

cond-mat.str-el↗
arXiv2005

Cooperative effect of phonons and electronic correlations for superconductivity in cobaltates

We propose that unconventional superconductivity in hydrated sodium cobaltate $Na_xCoO_2$ results from an interplay of electronic correlations and electron-phonon interactions. On the basis of the $t-V$ model plus phonons we found evidences for a) unconventional superconductivity, b) realistic values of $T_c$ and c) the dome shape existing near $x \sim 0.35$. This picture is obtained for $V$ close to the critical Coulomb repulsion $V_c$ which separates the uniform Fermi liquid from $\sqrt{3} \times \sqrt{3}$ CDW ordered phase.

cond-mat.supr-con↗
arXiv2004

C-Axis Tunneling Spectra in High-T$_c$ Superconductors in the Presence of a d Charge-Density Wave

The optimally doped and underdoped region of the $t-J$ model at large N (N is the number of spin components) is governed by the competition of d-wave superconductivity (SC) and a d Charge-Density Wave (d-CDW).The partial destruction of the Fermi surface by the d-CDW and the resulting density of states are discussed. Furthermore, c-axis conductances for incoherent and coherent tunneling are calculated, considering both an isotropic and an anisotropic in-plane momentum dependence of the hopping matrix element between the planes. The influence of self-energy effects on the conductances is also considered using a model where the electrons interact with a dispersionless, low-lying branch of bosons. We show that available tunneling spectra from break-junctions are best explained by assuming that they result from incoherent tunneling with a strongly anisotropic hopping matrix element of the form suggested by band structure calculations. The conductance spectra are then characterized by one single peak which evolves continuously from the superconducting to the d-CDW state with decreasing doping. The intrinsic c-axis tunneling spectra are, on the other hand, best explained by coherent tunneling. Calculated spectra show at low temperatures two peaks due to SC and d-CDW. With increasing temperature the BCS-like peak moves to zero voltage and vanishes at T$_c$,exactly as in experiment.Our results thus can explain why break junction and intrinsic tunneling spectra are different from each other. Moreover, they support a scenario of two competing order parameters in the underdoped region of high-T$_c$ superconductors.

cond-mat.supr-con↗
arXiv2003

Low-Frequency Optical Properties of $β^{\prime\prime}$-(BEDO- TTF)$_5$[CsHg(SCN)$_4$]$_2$: Indications of Electronic Correlations in a 1/5-Filled Two-Dimensional Conductor

The polarized reflectivity of $β^{\prime\prime}$-(BEDO-TTF)$_5$[CsHg(SCN)$_4$]$_2$ is studied in the infrared range between 60 cm-1 and 6000 cm-1 from room temperature down to 10 K. Already at T=300 K a pseudogap in the optical conductivity is present of about 300 cm-1; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. In contrast to quarter-filled BEDT-TTF-based conductors of the $β^{\prime\prime}$-phase a robust Drude component in the conductivity spectra is observed which we ascribe to the larger fraction of charge carriers associated with the 1/5-filling of the conduction band. This observation is corroborated by exact diagonalization calculations on an extended Hubbard model on a square lattice for different fillings. A broad band at 4000 cm-1 which appears for the electric field polarized parallel to the stacks of the BEDO-TTF molecules is associated to structural modulations in the stacks; these modulations lead to a rise of the dc and microwave resistivity in 100 to 30 K temperature range.

cond-mat.str-el↗
arXiv2003

Dynamical properties of a strongly correlated model for quarter-filled layered organic molecular crystals

The dynamical properties of an extended Hubbard model, which is relevant to quarter-filled layered organic molecular crystals, are analyzed. We have computed the dynamical charge correlation function, spectral density, and optical conductivity using Lanczos diagonalization and large-N techniques. As the ratio of the nearest-neighbour Coulomb repulsion, V, to the hopping integral, t, increases there is a transition from a metallic phase to a charge ordered phase. Dynamical properties close to the ordering transition are found to differ from the ones expected in a conventional metal. Large-N calculations display an enhancement of spectral weight at low frequencies as the system is driven closer to the charge ordering transition in agreement with Lanczos calculations. As V is increased the charge correlation function displays a plasmon-like mode which, for wavevectors close to (pi,pi), increases in amplitude and softens as the charge ordering transition is approached. We propose that inelastic X-ray scattering be used to detect this mode. Large-N calculations predict superconductivity with dxy symmetry close to the ordering transition. We find that this is consistent with Lanczos diagonalization calculations, on lattices of 20 sites, which find that the binding energy of two holes becomes negative close to the charge ordering transition.

cond-mat.str-el↗
arXiv2002

Competition between superconductivity and the pseudogap phase in the t-J model

The t-J model in the large N limit (N denotes the number of spin components) yields a pseudogap phase in the underdoped region which is related to a d-wave charge density wave (d-CDW). We present results for the doping dependence of the superconducting and d-CDW order parameters as well as for collective excitations in the presence of these two order parameters. We argue that the electronic Raman spectrum with B$_{1g}$ symmetry probes the amplitude fluctuations of the d-CDW at zero momentum.

cond-mat.supr-con↗
arXiv2002

Influence of collective effects and the d-CDW on electronic Raman scattering in high-T$_c$ superconductors

Electronic Raman scattering in high-T$_c$ superconductors is studied within the t-J model. It is shown that the A$_{1g}$ and B$_{1g}$ spectra are dominated by amplitude fluctuations of the superconducting and the d-wave CDW order parameters, respectively. The B$_{2g}$ spectrum contains no collective effects and its broad peak reflects vaguely the doping dependence of T$_c$, similarly to the pronounced peak in the A$_{1g}$ spectrum. The agreement of our theory with the experiment supports the picture of two different, competing order parameters in the underdoped regime of high-T$_c$ superconductors.

cond-mat.supr-con↗
arXiv2001

Manifestation of marginal Fermi liquid and phonon excitations in photoemision experiments of cuprate superconductors

Recent ARPES experiments in cuprates superconductors show a kink in the electron dispersion near the Fermi energy. This kink coexists with a linear frequency dependence of the imaginary part of the electron self-energy. In this paper we show that both features could be accounted for if an electron-phonon interaction is included in a model where the electrons are described by a marginal Fermi liquid theory. Phonons provide the energy scale seen in the experiments but the quasiparticle weight at the Fermi level is zero. At high binding energy, in agreement with the experiment, the electron dispersion does not go to the one-electron band. We analyze the compatibility between the electron scattering rate extracted from ARPES experiment and the one extracted from transport properties. We conclude that the electron-phonon interaction relevant for transport properties is strongly screened respect to the one extracted from ARPES. This is in agreement with recent studies in the context of 1/N expansion on t-J model.

cond-mat.str-el↗
arXiv2001

Low-energy renormalization of the electron dispersion of high-T$_c$ superconductors

High-resolution ARPES studies in cuprates have detected low-energy changes in the dispersion and absorption of quasi-particles at low temperatures, in particular, in the superconducting state. Based on a new 1/N expansion of the t-J-Holstein model, which includes collective antiferromagnetic fluctuations already in leading order, we argue that the observed low-energy structures are mainly caused by phonons and not by spin fluctuations, at least, in the optimal and overdoped regime.

cond-mat.supr-con↗
arXiv2000

Influence of spin fluctuations on the superconducting transition temperature and resistivity in the t-J model at large N

Spin fluctuations enter the calculation of the superconducting transition temperature T$_c$ only in the next-to-leading order (i.e., in O(1/N$^2$) of the 1/N expansion of the t-J model. We have calculated these terms and show that they have only little influence on the value of T$_c$ obtained in the leading order O(1/N) in the optimal and overdoped region, i.e., for dopings larger than the instability towards a flux phase. This result disagrees with recent spin-fluctuation mediated pairing theories. The discrepancies can be traced back to the fact that in our case the coupling between electrons and spins is determined by the t-J model and not adjusted and that the spin susceptibility is rather broad and structureless and not strongly peaked at low energies as in spin-fluctuation models. Relating T$_c$ and transport we show that the effective interactions in the particle-particle and particle-hole channels are not simply related within the 1/N expansion by different Fermi surface averages of the same interactin as in the case of phonons or spin fluctuations. As a result, we find that large values for T$_c$ and rather small scattering rates in the normal state as found in the experiments can easily be reconciled with each other. We also show that correlation effects heavily suppress transport relaxation rates relative to quasiparticle relaxation rates in the case of phonons but not in the case of spin fluctuations.

cond-mat.supr-con↗
arXiv2000

Connection between the Slave-Particles and X-Operators Path-Integral Representations. a New Perturbative Approach

In the present work it is shown that the family of first-order Lagrangians for the t-J model and the corresponding correlation generating functional previously found can be exactly mapped into the slave-fermion decoupled representation. Next, by means of the Faddeev-Jackiw symplectic method, a different family of Lagrangians is constructed and it is shown how the corresponding correlation generating functional can be mapped into the slave-boson representation. Finally, in order to define the propagation of fermion modes we discuss two alternative ways to treat the fermionic sector in the path-integral formalism for the t-J model.

cond-mat.str-el↗