arXiv subjects

I. Sega

Publications and source records attributed to I. Sega.

18 recordsLinked to original sources

arXiv2010

Anomalous normal-state properties of iron pnictides: phenomenological theory

We employ the phenomenological theory of the quasiparticle relaxation based on the simplified two-band description and the spin-fluctuation induced interband coupling to analyze recent normal-state transport data in electron-doped iron pnictides, in particular the Ba(Fe_1-x Co_x)_2As_2 family. Temperature dependence of the resistivity, thermopower and the Hall constant are evaluated. It is shown that their anomalous behavior emerging from experiments can be consistently described within the same framework assuming also non-Fermi-liquid spin fluctuations.

cond-mat.str-el↗

arXiv2009

Analysis of transport properties of iron pnictides: spin-fluctuation scenario

We present a phenomenological theory of quasiparticle scattering and transport relaxation in the normal state of iron pnictides based on the simplified two-band model coupled via spin fluctuations. In analogy with anomalous properties of cuprates it is shown that a large and anomalous normal-state resistivity and thermopower can be interpreted as the consequence of strong coupling to spin fluctuations. The generalization to the superconducting phase is also discussed.

cond-mat.str-el↗

arXiv2009

Freezing of spin dynamics in underdoped cuprates

The Mori's memory function approach to spin dynamics in doped antiferromagnetic insulator combined with the assumption of temperature independent static spin correlations and constant collective mode damping leads to w/T scaling in a broad range. The theory involving a nonuniversal scaling parameter is used to analyze recent inelastic neutron scattering results for underdoped cuprates. Adopting modified damping function also the emerging central peak in low-doped cuprates at low temperatures can be explained within the same framework.

cond-mat.str-el↗

arXiv2006

Magnetic collective mode in underdoped cuprates: a phenomenological analysis

The dynamical spin susceptibility as relevant for underdoped cuprates is analysed within the memory-function (MeF) approach. A phenomenological damping function combined with a $T$-independent sum rule is used to describe the anomalous normal state and the resonant peak in the superconducting state, in particular its position and its relative intensity to the normal state. The relation with the random-phase approximation is discussed. The MeF method is generalized to the bilayer system in order to compare with inelastic neutron scattering experiments on YBa$_2$Cu$_3$O$_{6+x}$ which alows also for a quantitative comparison. In this context the problem of missing integrated spectral intensity within the experimentally accessible energy window is also discussed.

cond-mat.str-el↗

arXiv2006

Entanglement of two delocalised electrons

Several convenient formulae for the entanglement of two indistinguishable delocalised spin-1/2 particles are introduced. This generalizes the standard formula for concurrence, valid only in the limit of localised or distinguishable particles. Several illustrative examples are given.

cond-mat.mes-hall↗

arXiv2006

Spin fluctuations in cuprates as the key to high $T_c$

Spin fluctuations represent the lowest established energy scale in cuprates and are crucial for the understanding of anomalous normal state properties and superconductivity in these materials. The memory-function approach to the spin response in the t-J model is described. Combined with numerical results for small systems it is able to explain the anomalous scaling at low doping and the crossover to the Fermi-liquid-like behavior in overdoped systems. Within the superconducting phase the theory reproduces the resonant peak and its peculiar double dispersion. Such spin fluctuations are then used as the input for the theory of superconductivity within the t-J model, where we show that an important role is played also by the next-nearest-neighbour hopping parameter $t'$.

cond-mat.str-el↗

arXiv2005

Dynamic Spin Structure Factor of SrCu2(BO3)2 at Finite Temperatures

Using finite temperature Lanczos technique on finite clusters we calculate dynamical spin structure factor of the quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of wavevector and temperature. Unusual temperature dependence of calculated spectra is in agreement with inelastic neutron scattering measurements. Normalized peak intensities of the single-triplet peak are q-independent, their unusual temperature dependence is analyzed in terms of thermodynamic quantities.

cond-mat.str-el↗

arXiv2005

Double dispersion of the magnetic resonant mode in cuprates

The magnetic excitation spectra in the vicinity of the resonant peak, as observed by inelastic neutron scattering in cuprates, are studied within the memory-function approach. It is shown that at intermediate doping the superconducting gap induces a double dispersion of the peak, with an anisotropy rotated between the downward and upward branch. Similar behavior, but with a spin-wave dispersion at higher energies, is obtained for the low-doping case assuming a large pairing pseudogap.

cond-mat.str-el↗

arXiv2004

Scaling of the magnetic response in doped antiferromagnets

A theory of the anomalous $ω/T$ scaling of the dynamic magnetic response in cuprates at low doping is presented. It is based on the memory function representation of the dynamical spin suceptibility in a doped antiferromagnet where the damping of the collective mode is constant and large, whereas the equal-time spin correlations saturate at low $T$. Exact diagonalization results within the t-J model are shown to support assumptions. Consequences, both for the scaling function and the normalization amplitude, are well in agreement with neutron scattering results.

cond-mat.str-el↗

arXiv2004

Magnetic response of nonmagnetic impurities in cuprates

A theory of the local magnetic response of a nonmagnetic impurity in a doped antiferromagnet, as relevant to the normal state in cuprates, is presented. It is based on the assumption of the overdamped collective mode in the bulk system and on the evidence, that equal-time spin correlations are only weakly renormalized in the vicinity of the impurity. The theory relates the Kondo-like behavior of the local susceptibility to the anomalous temperature dependence of the bulk magnetic susceptibility, where the observed increase of the Kondo temperature with doping reflects the crossover to the Fermi liquid regime and the spatial distribution of the magnetization is given by bulk antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2004

Crossover to non-Fermi-liquid spin dynamics in cuprates

The antiferromagnetic spin correlation function $S_{\bf Q}$, the staggered spin susceptibility $χ_{\bf Q}$ and the energy scale $ω_{FL}=S_{\bf Q}/χ_{\bf Q}$ are studied numerically within the t-J model and the Hubbard model, as relevant to cuprates. It is shown that $ω_{FL}$, related to the onset of the non-Fermi-liquid spin response at $T>ω_{FL}$, is very low in the regime below the 'optimum' hole doping $c_h < c_h^* \sim 0.16$, while it shows a steep increase in the overdoped regime. A quantitative analysis of NMR spin-spin relaxation-rate $1/T_{2G}$ for various cuprates reveals a similar behavior, indicating on a sharp, but continuous, crossover between a Fermi-liquid and a non-Fermi-liquid behavior as a function of doping.

cond-mat.str-el↗

arXiv2002

Magnetic fluctuations and resonant peak in cuprates: a microscopic theory

The theory for the dynamical spin susceptibility within the t-J model is developed, as relevant for the resonant magnetic peak and normal-state magnetic response in superconducting (SC) cuprates. The analysis is based on the equations of motion for spins and the memory-function presentation of magnetic response where the main damping of the low-energy spin collective mode comes from the decay into fermionic degrees of freedom. It is shown that the damping function at low doping is closely related to the c-axis optical conductivity. The analysis reproduces doping-dependent features of the resonant magnetic scattering.

cond-mat.str-el↗

arXiv2001

Spectral properties of underdoped cuprates

In the framework of the planar t-J model for cuprates we analyze the development of a pseudo gap in the density of states, which at low doping starts to emerge for temperatures T<J and persists up to the optimum doping. The analysis is based on numerical results for spectral functions obtained with the finite-temperature Lanczos method for finite two-dimensional clusters. Numerical results are additionally compared with the self consistent Born approximation (SCBA) results for hole-like (photoemission) and electron-like (inverse photoemission) spectra at T=0. The analysis is suggesting that the origin of the pseudo gap is in short-range antiferromagnetic (AFM) spin correlations and strong asymmetry between the hole and electron spectra in the underdoped regime. We analyze also the electron momentum distribution function (EMD). Our analytical results for a single hole in an AFM based on the SCBA indicate an anomalous momentum dependence of EMD showing "hole pockets" coexisting with a signature of an emerging large Fermi surface (FS). The position of the incipient FS and the structure of the EMD is determined by the momentum of the ground state. The main observation is the coexistence of two apparently contradicting FS scenarios. On the one hand, the delta-function like contributions at (pi/2,pi/2) indicate, that for finite doping a pocket-like small FS evolves from these points, provided provided that AFM long range order persists. On the other hand, the discontinuity which appears at the same momentum is more consistent with with infinitesimally short arc (point) of an emerging large FS.

cond-mat.str-el↗

arXiv2000

Electron momentum distribution in underdoped cuprates

We investigate the electron momentum distribution function (EMD) in a weakly doped two-dimensional quantum antiferromagnet (AFM) as described by the t-J model. Our analytical results for a single hole in an AFM based on the self-consistent Born approximation (SCBA) indicate an anomalous momentum dependence of EMD showing 'hole pockets' coexisting with a signature of an emerging large Fermi surface. The position of the incipient Fermi surface and the structure of the EMD is determined by the momentum of the ground state. Our analysis shows that this result remains robust in the presence of next-nearest neighbor hopping terms in the model. Exact diagonalization results for small clusters are with the SCBA reproduced quantitatively.

cond-mat.str-el↗

arXiv1999

Electron Momentum Distribution Function in the t-t'-J Model

We study the electron momentum distribution function (EMDF) for the two-dimensional t-t'-J model doped with one hole on finite clusters by the method of twisted boundary conditions. The results quantitatively agree with our analytical results for a single hole in the antiferromagnetic background, based on the self-consistent Born approximation (SCBA). Moreover, within the SCBA an anomalous momentum dependence of EMDF is found, pointing to an emerging large Fermi surface. The analysis shows that the presence of next-nearest-neighbor (NNN) hopping terms changes EMDF only quantitatively.

cond-mat.str-el↗

arXiv1999

c-axis Optical Conductivity in Cuprates

We investigate the c-axis optical conductivity and d.c. resistivity of cuprate superconductors in the normal state. Assuming that the interlayer hopping is incoherent we express the conductivity with planar spectral functions obtained (i) from angle-resolved photoemission experiments, (ii) using marginal Fermi liquid ansatz, and (iii) with the finite-temperature Lanczos method for finite two-dimensional systems described by the t-J model. Here in the low doping regime a pseudo-gap opening in the density of states appears to be responsible for a semimetallic-like behavior of the D.C. resistivity. In the optimally doped regime we find an anomalous relaxation rate. Analytically this result is reproduced with the use of the marginal Fermi liquid ansatz for the self energy with parameters obtained from the exact diagonalization results.

cond-mat.str-el↗

arXiv1998

c-axis conductivity in the normal state of cuprate superconductors

The c-axis optical conductivity and d.c. resistivity are calculated within the t-J model assuming that the interlayer hopping is incoherent. Use is made of numerical results for spectral functions recently obtained with the finite-temperature Lanczos method for finite two-dimensional systems. In the optimally doped regime we find an anomalous relaxation rate, suggesting a common relaxation mechanism for intra- and interlayer transport. At low doping a pseudo-gap opening in the density of states appears to be responsible for a semimetallic-like behavior of d.c. resistivity.

cond-mat.str-el↗

arXiv1998

Spin polaron in a magnetic field

The influence of the homogeneous magnetic field on a single mobile hole in a magnetic insulator, as represented by the two-dimensional t-J model, is investigated by considering the coupling of the field to the orbital current. The energy of the J=0 system is analysed via the high-temperature expansion and the small system diagonalization. The susceptibility is shown to be diamagnetic and diverging at low temperatures T. In contrast, in the antiferrmagnetic J>0 case small systems generically reveal a tendency towards a paramagnetic response in larger fields at low T. By employing at T=0 the cumulant expansion we study the ground state in arbitrary B, showing a behavior very sensitive to the character of the quasiparticle dispersion and the magnetic-field strength. At low B the perturbation and small-systems results are consistent with a pronounced diamagnetic susceptibility at T->0, but indicate on a suppressed contribution at intermediate T~J.

cond-mat.str-el↗