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I. Tifrea

Publications and source records attributed to I. Tifrea.

At least 19 recordsLinked to original sources
arXiv2017

Thermoelectric transport properties in graphene connected molecular junctions

We study the electronic contribution to the main thermoelectric properties of a molecular junction consisting of a single quantum dot coupled to graphene external leads. The system electrical conductivity (G), Seebeck coefficient ($S$), and the thermal conductivity ($κ$), are numerically calculated based on a Green's function formalism that includes contributions up to the Hartree-Fock level. We consider the system leads to be made either of pure or gapped-graphene. To describe the free electrons in the gapped-graphene electrodes we used two possible scenarios, the massive gap scenario, and the massless gap scenario, respectively. In all cases, the Fano effect is responsible for a strong violation of the Wiedemann-Franz law and we found a substantial increase of the system figure of merit $ZT$ due to a drastic reduction of the system thermal coefficient. In the case of gapped-graphene electrodes, the system figure of merit presents a maximum at an optimal value of the energy gap of the order of $Δ/D\sim$ 0.002 (massive gap scenario) and $Δ/D\sim$ 0.0026 (massless gap scenario). Additionally, for all cases, the system figure of merit is temperature dependent.

cond-mat.mes-hall↗
arXiv2016

NMR parameters in gapped graphene systems

We calculate the nuclear spin-lattice relaxation time and the Knight shift for the case of gapped graphene systems. Our calculations consider both the massive and massless gap scenarios. Both the spin-lattice relaxation time and the Knight shift depend on temperature, chemical potential, and the value of the electronic energy gap. In particular, at the Dirac point, the electronic energy gap has stronger effects on the system nuclear magnetic resonance parameters in the case of the massless gap scenario. Differently, at large values of the chemical potential, both gap scenarios behave in a similar way and the gapped graphene system approaches a Fermi gas from the nuclear magnetic resonance parameters point of view. Our results are important for nuclear magnetic resonance measurements that target the $^{13}$C active nuclei in graphene samples.

cond-mat.mtrl-sci↗
arXiv2014

Thermoelectric transport properties of a T-shaped double quantum dot system in the Coulomb blockade regime

We investigate the thermoelectric properties of a T-shaped double quantum dot system described by a generalized Anderson Hamiltonian. The system's electrical conduction (G) and the fundamental thermoelectric parameters such as the Seebeck coefficient ($S$) and the thermal conductivity ($κ$), along with the system's thermoelectric figure of merit (ZT) are numerically estimated based on a Green's function formalism that includes contributions up to the Hartree-Fock level. Our results account for finite onsite Coulomb interaction terms in both component quantum dots and discuss various ways leading to an enhanced thermoelectric figure of merit for the system. We demonstrate that the presence of Fano resonances in the Coulomb blockade regime is responsible for a strong violation of the Wiedemann-Franz law and a considerable enhancement of the system's figure of merit ($ZT$).

cond-mat.mes-hall↗
arXiv2011

Electronic Green's functions in a T-shaped multi-quantum dot system

We developed a set of equations to calculate the electronic Green's functions in a T-shaped multi-quantum dot system using the equation of motion method. We model the system using a generalized Anderson Hamiltonian which accounts for {\em finite} intradot on-site Coulomb interaction in all component dots as well as for the interdot electron tunneling between adjacent quantum dots. Our results are obtained within and beyond the Hartree-Fock approximation and provide a path to evaluate all the electronic correlations in the multi-quantum dot system in the Coulomb blockade regime. Both approximations provide information on the physical effects related to the finite intradot on-site Coulomb interaction. As a particular example for our generalized results, we considered the simplest T-shaped system consisting of two dots and proved that our approximation introduces important corrections in the detector and side dots Green's functions, and implicitly in the evaluation of the system's transport properties. The multi-quantum dot T-shaped setup may be of interest for the practical realization of qubit states in quantum dots systems.

cond-mat.mes-hall↗
arXiv2009

Transport and current noise characteristics of a T-shape double quantum dot system

We consider the transport and the noise characteristics for the case of a T-shape double quantum dot system using the equation of motion method. Our theoretical results, obtained in an approximation equivalent to the Hartree-Fock approximation, account for non-zero on-site Coulomb interaction in both the detector and side dots. The existence of a non-zero Coulomb interaction implies an additional two resonances in the detector's dot density of states and thereafter affects the electronic transport properties of the system. The system's conductance presents two Fano dips as function of the energy of the localized electronic level in the side dot. The Fano dips in the system's conductance can be observed both for strong (fast detector) and weak coupling (slow detector) between the detector dot and the external electrodes. Due to stronger electronic correlations the noise characteristics in the case of a slow detector are much higher. This setup may be of interest for the practical realization of qubit states in quantum dots systems.

cond-mat.mes-hall↗
arXiv2009

Universality of the conductance in quantum dot transport

We revisited the scaling behavior of the transport properties of a quantum dot system described by the spin-1/2 Anderson model using analytical methods. In the low temperature limit we show that the conductance has a universal behavior with universality between temperature and bias. We compare this result with the empirical formula used to fit the experimental data for conductance in the case of the equilibrium transport through a single channel quantum dot. In the high temperature limit the conductance obtained from the Anderson model is compared with previous results obtained from the Kondo model. The universal behavior is present also in the high temperature limit. These results are in good agreement with the Renormalization group calculations.

cond-mat.mes-hall↗
arXiv2008

Overhauser frequency shifts in semiconductor nanostructures

We calculate the Overhauser frequency shifts in semiconductor nanostructures resulting from the hyperfine interaction between nonequilibrium electronic spins and nuclear spins. The frequency shifts depend on the electronic local density of states and spin polarization as well as the electronic and nuclear spin relaxation mechanisms. Unlike previous calculations, our method accounts for the electron confinement in low dimensional semiconductor nanostructures, resulting in both nuclear spin polarizations and Overhauser shifts that are strongly dependent on position. Our results explain previously puzzling measurements of Overhauser shifts in an Al$_x$Ga$_{1-x}$As parabolic quantum well by showing the connection between the electron spin lifetime and the frequency shifts.

cond-mat.mtrl-sci↗
arXiv2007

Zero temperature conductance of parallel T-shape double quantum dots

We analyze the zero temperature conductance of a parallel T-shaped double quantum dot system. We present an analytical expression for the conductance of the system in terms of the total number of electrons in both quantum dots. Our results confirm that the system's conductance is strongly influenced by the dot which is not directly connected to the leads. We discuss our results in connection with similar results reported in the literature.

cond-mat.mes-hall↗
arXiv2005

Excitonic condensation in quasi-two-dimensional systems

We present a low energy model for the Bose-Einstein condensation in a quasi-two-dimensional excitonic gas. Using the flow equations of the Renormalization group and a $Φ^4$ model with the dynamical critical exponent $z=2$ we calculate the temperature dependence of the critical density, coherence length, magnetic susceptibility, and specific heat. The model can be relevant for the macroscopic coherence observed in GaAs/AlGaAs coupled quantum wells.

cond-mat.mtrl-sci↗
arXiv2005

Field-induced Bose-Einstein condensation of interacting dilute magnons in three-dimensional spin systems: A renormalization-group study

We use the Renormalization Group method to study the magnetic field influence on the Bose-Einstein condensation of interacting dilute magnons in three dimensional spin systems. We first considered a model with SU(2) symmetry (universality class $z=1$) and we obtain for the critical magnetic field a power law dependence on the critical temperature, $[H_c(T)-H_c(0)]\sim T^2$. In the case of U(1) symmetry (universality class $z=2$) the dependence is different, and the magnetic critical field depends linearly on the critical temperature, $[H_c(T)-H_c(0)]\sim T$. By considering a more relevant model, which includes also the system's anisotropy, we obtain for the same symmetry class a $T^{3/2}$ dependence of the magnetic critical field on the critical temperature. We discuss these theoretical predictions of the renormalization group in connection with experimental results reported in the literature.

cond-mat.mtrl-sci↗
arXiv2005

Evidence for a metallic--like state in the T=0 K phase diagram of a high temperature superconductor

We examine the effects of a phenomenological pseudogap on the T=0 K phase diagram of a high temperature superconductor within a self-consistent model which exhibits a d-wave pairing symmetry. At the mean-field level the presence of a pseudogap in the normal phase of the high temperature superconductor is proved to be essential for the existence of a metallic--like state in the density versus interaction phase diagram. In the small density limit, at high attractive interaction, bosonic--like degrees of freedom are likely to emerge. Our result should be relevant for underdoped high temperature superconductors, where there is a strong evidence for the presence of a pseudogap in the excitation spectrum of the normal state quasiparticles.

cond-mat.supr-con↗
arXiv2004

Bose-Einstein condensation of magnons

We use the Renormalization Group method to study the Bose-Einstein condensation of the interacting dilute magnons which appears in three dimensional spin systems in magnetic field. The obtained temperature dependence of the critical field $H_c(T)-H_c(0) \sim T^{2}$ is different from the recent self-consistent Hartree-Fock-Popov calculations (cond-mat/0405422) in which a $T^{3/2}$ dependence was reported . The origin of this difference is discussed in the framework of quantum critical phenomena.

cond-mat.supr-con↗
arXiv2004

Nuclear Spin Dynamics in Parabolic Quantum Wells

We present a detailed analytical and numerical analysis of the nuclear spin dynamics in parabolic quantum wells. The shallow potential of parabolic quantum wells permits substantial modification of the electronic wave function in small electric fields. The nuclear spin relaxation via the hyperfine interaction depends on the electronic local density of states, therefore the local nuclear relaxation time depends sensitively on the electric field. For an inhomogeneous nuclear magnetization, such as generated by dynamic nuclear polarization, the total nuclear magnetization dynamics can similarly be altered. We examine this effect quantitatively and the effect of temperature, field, well thickness, and nuclear spin diffusion.

cond-mat.mtrl-sci↗
arXiv2003

Fluctuation conductivity in layered d-wave superconductors near critical disorder

We consider the fluctuation conductivity in the critical region of a disorder induced quantum phase transition in layered d-wave superconductors. We specifically address the fluctuation contribution to the system's conductivity in the limit of large (quasi-two-dimensional system) and small (quasi-three-dimensional system) separation between adjacent layers of the system. Both in-plane and c-axis conductivities were discussed near the point of insulator-superconductor phase transition. The value of the dynamical critical exponent, $z=2$, permits a perturbative treatment of this quantum phase transition under the renormalization group approach. We discuss our results for the system conductivities in the critical region as function of temperature and disorder.

cond-mat↗
arXiv2003

Specific heat behavior of high temperature superconductors in the pseudogap regime

Experimental data obtained from thermodynamic measurements in underdoped high temperature superconductors show unusual anomalies in the temperature dependence of the electronic specific heat both in the normal state and at the critical point associated to the superconducting phase transition. The observed deviations from the standard behavior are probably linked with the opening of a pseudogap in the energy spectrum of the single-particle excitations associated with the normal state. Based on a phenomenological description of the pseudogap phase we perform analytical and numerical calculations for the temperature dependence of the specific heat for both the superconducting and normal state. The reduced specific heat jump at the transition point can be explained by a modified electronic single particle contribution to the specific heat in the presence of the normal state pseudogap. The hump observed in the normal state specific heat can be explained by the electronic pair contribution associated with strong fluctuations of the order parameter in the critical region. The obtained theoretical results are discussed in connection with experimental data for cuprates.

cond-mat.supr-con↗
arXiv2003

Non-Fermi liquid behavior of the electrical resistivity at the ferromagnetic quantum critical point

We propose a model for the non-Fermi behavior in the proximity of the quantum phase transition induced by the strong polarization of the electrons due to local magnetic moments. The self - consistent Renormalization - Group methods have been used to calculate the temperature dependence of the electrical resistivity and specific heat. The T^{5/3} dependence of resistivity and the T ln T dependence of the specific heat show that the magnetic impurities drive a ferromagnetic quantum phase transition and near the critical point the system present a non-Fermi liquid behavior. The model is in good agreement with the experimental data obtained for Ni_x Pd_{1-x} alloy.

cond-mat↗
arXiv2002

Ginzburg-Landau Expansion in Non-Fermi Liquid Superconductors: Effect of the Mass Renormalization Factor

We reconsider the Ginzburg-Landau expansion for the case of a non-Fermi liquid superconductor. We obtain analytical results for the Ginzburg-Landau functional in the critical region around the superconducting phase transition, T <= T_c, in two special limits of the model, i.e., the spin-charge separation case and the anomalous Fermi liquid case. For both cases, in the presence of a mass renormalization factor, we derived the form and the specific dependence of the coherence length, penetration depth, specific heat jump at the critical point, and the magnetic upper critical field. For both limits the obtained results reduce to the usual BCS results for a two dimensional s-wave superconductor. We compare our results with recent and relevant theoretical work. The results for a d--wave symmetry order parameter do not change qualitatively the results presented in this paper. Only numerical factors appear additionally in our expressions.

cond-mat.supr-con↗