arXiv subjects

P. Roura-Bas

Publications and source records attributed to P. Roura-Bas.

32 records · Page 2Linked to original sources

arXiv2015

Selfconsistent hybridization expansions for static properties of the Anderson impurity model

By means of a projector-operator formalism we derive an approximation based on a self consistent hybridization expansion to study the ground state properties of the Anderson Impurity model. We applied the approximation to the general case of finite Coulomb repulsion $U$, extending previous work with the same formalism in the infinite-$U$ case. The treatment provides a very accurate calculation of the ground state energy and their related zero temperature properties in the case in which $U$ is large enough, but still finite, as compared with the rest of energy scales involved in the model. The results for the valence of the impurity are compared with exact results that we obtain from equations derived using the Bethe ansatz and with a perturbative approach. The magnetization and magnetic susceptibility is also compared with Bethe ansatz results. In order to do this comparison, we also show how to regularize the Bethe ansatz integral equations necessary to calculate the impurity valence, for arbitrary values of the parameters.

cond-mat.str-el↗

arXiv2015

Restoring the SU(4) Kondo regime in a double quantum dot system

We calculate the spectral density and occupations of a system of two capacitively coupled quantum dots, each one connected to its own pair of conducting leads, in a regime of parameters in which the total coupling to the leads for each dot $Γ_i$ are different. The system has been used recently to perform pseudospin spectroscopy by controlling independently the voltages of the four leads. For an odd number of electrons in the system, $Γ_1=Γ_2$, equal dot levels $E_1=E_2$ and sufficiently large interdot repulsion $U_{12}$ the system lies in the SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime. In the more realistic case $Γ_1 \neq Γ_2$, pseudospin degeneracy is broken and the symmetry is reduced to SU(2). Nevertheless we find that the essential features of the SU(4) symmetric case are recovered by appropriately tuning the level difference $δ=E_2-E_1$. The system behaves as an SU(4) Kondo one at low energies. Our results are relevant for experiments which look for signatures of SU(4) symmetry in the Kondo regime of similar systems.

cond-mat.mes-hall↗

arXiv2015

Kondo physics in a Ni impurity embedded in O-doped Au chains

By means of ab initio calculations we study the effect of O-doping of Au chains containing a nanocontact represented by a Ni atom as a magnetic impurity. In contrast to pure Au chains, we find that with a minimun O-doping the $5d_{xz,yz}$ states of Au are pushed up, crossing the Fermi level. We also find that for certain O configurations, the Ni atom has two holes in the degenerate $3d_{xz,yz}$ orbitals, forming a spin $S=1$ due to a large Hund interaction. The coupling between the $5d_{xz,yz}$ Au bands and the $3d_{xz,yz}$ of Ni states leads to a possible realization of a two-channel $S=1$ Kondo effect. While this kind of Kondo effect is commonly found in bulk systems, it is rarely observed in low dimensions. The estimated Kondo scale of the system lies within the present achievable experimental resolution in transport measurements. Another possible scenario for certain atomic configurations is that one of the holes resides in a $3d_{z^2}$ orbital, leading to a two-stage Kondo effect, the second one with SU(4) symmetry.

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arXiv2014

Non-Fermi liquid behaviour in non-equilibrium transport through Co doped Au chains connected to four-fold symmetric leads

We calculate the differential conductance as a function of temperature and bias voltage, $G(T,V)$, through Au monoatomic chains with a substitutional Co atom as a magnetic impurity, connected to a four-fold symmetric lead. The system was recently proposed as a possible scenario for observation of the overscreened Kondo physics. Stretching the chain, the system could be tuned through a quantum critical point (QCP) with three different regimes, overscreened, underscreened and non Kondo phases. We present calculations of the impurity spectral function by using the numerical renormalization group (NRG) for the three different regimes characterizing the QCP. Non trivial behaviour of the spectral function is reported near the QCP. Comparison with results using the non crossing approximation (NCA), shows that the latter is reliable in the overscreened regime, when the anisotropy is larger than the Kondo temperature. For these parameters, which correspond to realistic previous estimates, $G(T,V)$ calculated within NCA exhibits clear signatures of the non-Fermi liquid behaviour within the overscreened regime.

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arXiv2013

Orbital Kondo spectroscopy in a double quantum dot system

We calculate the nonequilibrium conductance of a system of two capacitively coupled quantum dots, each one connected to its own pair of conducting leads. The system has been used recently to perform pseudospin spectroscopy by controlling independently the voltages of the four leads. The pseudospin is defined by the orbital occupation of one or the other dot. Starting from the SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime, for an odd number of electrons in the system, we show how the conductance through each dot varies as the symmetry is reduced to SU(2) by a pseudo-Zeeman splitting, and as bias voltages are applied to any of the dots. We analize the expected behavior of the system in general, and predict characteristic fingerprint features of the SU(4) to SU(2) crossover that have not been observed so far.

cond-mat.mes-hall↗

arXiv2013

Nonequilibrium transport through magnetic vibrating molecules

We calculate the nonequilibrium conductance through a molecule or a quantum dot in which the occupation of the relevant electronic level is coupled with intensity $λ$ to a phonon mode, and also to two conducting leads. The system is described by the Anderson-Holstein Hamiltonian. We solve the problem using the Keldysh formalism and the non-crossing approximation (NCA) for both, the electron-electron and the electron-phonon interactions. We obtain a moderate decrease of the Kondo temperature $T_K$ with $λ$ for fixed renormalized energy of the localized level $\tilde{E_d}$. The meaning and value of $\tilde{E_d}$ are discussed. The spectral density of localized electrons shows in addition to the Kondo peak of width $2 T_K$, satellites of this peak shifted by multiples of the phonon frequency $ ω_0$. The nonequilibrium conductance as a function of bias voltage $V_b$ at small temperatures, also displays peaks at multiples of $ω_0$ in addition to the central dominant Kondo peak near $V_b=0$.

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arXiv2013

Unusual Kondo physics in a Co impurity atom embedded in noble-metal chains

We analyze the conduction bands of the one dimensional noble-metal chains that contain a Co magnetic impurity by means of ab initio calculations. We compare the results obtained for Cu and Ag pure chains, as well as O doped Cu, Ag and Au chains with those previously found for Au pure chains. We find similar results in the case of Cu and Au hosts, whereas for Ag chains a different behavior is obtained. Differences and similarities among the different systems are analyzed by comparing the electronic structure of the three noble-metal hosts. The d-orbitals of Cu chains at the Fermi level have the same symmetry as in the case of Au chains. These orbitals hybridize with the corresponding ones of the Co impurity, giving rise to the possibility of exhibiting a two-channel Kondo physics.

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arXiv2013

Non-Fermi liquid behavior in transport through Co doped Au chains

We calculate the conductance as a function of temperature $G(T)$ through Au monoatomic chains containing one Co atom as a magnetic impurity, and connected to two conducting leads with a 4-fold symmetry axis. Using the information derived from {\it ab initio} calculations, we construct an effective model $\Heff$ that hybridizes a 3d$^7$ quadruplet at the Co site with two 3d$^8$ triplets through the hopping of 5d$_{xz}$ and 5d$_{yz}$ electrons of Au. The quadruplet is split by spin anisotropy due to spin-orbit coupling. Solving $\Heff$ with the numerical renormalization group (NRG) % Wb: reverted my own change we find that at low temperatures $G(T)=a-b \sqrt{T}$ and the ground state impurity entropy is $\ln(2)/2$, a behavior similar to the two-channel Kondo model. Stretching the chain leads to a non Kondo phase, with the physics of the underscreened Kondo model at the quantum critical point.

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arXiv2012

Thermopower of an SU(4) Kondo resonance under an SU(2) symmetry-breaking field

We calculate the thermopower of a quantum dot described by two doublets hybridized with two degenerate bands of two conducting leads, conserving orbital (band) and spin quantum numbers, as a function of the temperature $T$ and a splitting $δ$ of the quantum dot levels which breaks the SU(4) symmetry. The splitting can be regarded as a Zeeman (spin) or valley (orbital) splitting. We use the non-crossing approximation (NCA), the slave bosons in the mean-field approximation (SBMFA) and also the numerical renormalization group (NRG) for large $δ$. The model describes transport through clean C nanotubes %with weak disorder and in Si fin-type field effect transistors, under an applied magnetic field. The thermopower as a function of temperature $S(T)$ displays two dips that correspond to the energy scales given by the Kondo temperature $T_K$ and $δ$ and one peak when $k_BT$ reaches the charge-transfer energy. These features are much more pronounced than the corresponding ones in the conductance, indicating that the thermopower is a more sensitive probe of the electronic structure at intermediate or high energies. At low temperatures ($T \ll T_K$) $T_K S(T)/T$ is a constant that increases strongly near the degeneracy point $δ=0$. We find that the SBMFA fails to provide an accurate description of the thermopower for large $δ$. Instead, a combination of Fermi liquid relations with the quantum-dot occupations calculated within the NCA gives reliable results for $T \ll T_K$.

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arXiv2012

Nonequilibrium conductance through a benzene molecule in the Kondo regime

Starting from exact eigenstates for a symmetric ring, we derive a low-energy effective generalized Anderson Hamiltonian which contains two spin doublets with opposite momenta and a singlet for the neutral molecule. For benzene, the singlet (doublets) represent the ground state of the neutral (singly charged) molecule. We calculate the non-equilibrium conductance through a benzene molecule, doped with one electron or a hole (i.e. in the Kondo regime), and connected to two conducting leads at different positions. We solve the problem using the Keldysh formalism and the non-crossing approximation (NCA). When the leads are connected in the \emph{para} position (at 180 degrees), the model is equivalent to the ordinary impurity Anderson model and its known properties are recovered. For other positions, there is a partial destructive interference in the cotunneling processes involving the two doublets and as a consequence, the Kondo temperature and the height and width of the central peak (for bias voltage $V_b$ near zero) of the differential conductance $G=dI/dV_b$ (where $I$ is the current) are reduced. In addition, two peaks at finite $V_b$ appear. We study the position of these peaks, the temperature dependence of $G$ and the spectral densities. Our formalism can also be applied to carbon nanotube quantum dots with intervalley mixing.

cond-mat.mes-hall↗

arXiv2011

Interplay between quantum interference and Kondo effects in nonequilibrium transport through nanoscopic systems

We calculate the finite temperature and non-equilibrium electric current through systems described generically at low energy by a singlet and \emph{two} spin doublets for $N$ and $N \pm 1$ electrons respectively, coupled asymmetrically to two conducting leads, which allows for destructive interference in the conductance. The model is suitable for studying transport in a great variety of systems such us aromatic molecules, different geometries of quantum dots and rings with applied magnetic flux. As a consequence of the interplay between interference and Kondo effect, we find changes by several orders of magnitude in the values of the conductance and its temperature dependence as the doublet level splitting is changed by some external parameter. The differential conductance at finite bias is negative for some parameters.

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arXiv2010

Universal scaling in transport out of equilibrium through a single quantum dot using the noncrossing approximation

The universal scaling behavior is studied for nonequilibrium transport through a quantum dot. To describe the dot we use the standard Anderson impurity model and use the non-equilibrium non-crossing approximation in the limit of infinite Coulomb repulsion. After solving de hamiltonian, we calculate the conductance through the system as a function of temperature $T$ and bias voltage $V$ in the Kondo and in the mixed valent regime. We obtain a good scaling function in both regimes. In particular, in the mixed valent regime, we find excellent agreement with recent experiments and previous theoretical works.

cond-mat.str-el↗

arXiv2007

Kondo temperature and crystal field levels of Ce systems within a finite U Non-Crossing Approximation

We calculate the Kondo temperature ($T_K$) and crystal-field levels of strongly correlated multiorbital systems solving the Anderson Impurity Model with the finite U Non-Crossing Approximation (UNCA) in its simplest scheme, that is, considering the self energies at lowest order in the 1/N diagrammatic expansion. We introduced an approximation to the vertex function that includes the double energy dependence and investigate its effect on the values of $T_K$ for simple electronic models. We also analyze the competition between the two spin flip mechanisms, involving virtual transitions to empty and doubly occupied states, in the determination of the ground state symmetry by including an extra diagram of higher order in $1/N.$ We finally combine the resulting simple formalism with {\it ab initio} calculated electronic structures to obtain $T_K$'s, ground states, and crystal field splittings in excellent agreement with experimental results for two particular Ce compounds, namely CeIn$_3$ and CeSn$_3$.

cond-mat.str-el↗

arXiv2006

Crystal-field splittings in CeX (X= N, P, As, Sb, Bi) compounds

The unusual and interesting physical properties of rare earth intemetallic compounds have their origin in the combination of strongly correlated 4f states and their hybridization with the conduction electron sea, which gives rise to their complex low temperature Kondo behavior. In particular, Ce compounds are very sensitive to the crystalline and chemical environment, as compared to other rare earth systems. The interaction of the 4f state with the conduction band plays an important role in the determination of the different magnetic, structural and transport properties of these systems. Among the cerium compounds, those of the type CeX, which crystallize in the rock salt structure, exhibit extremely unusual magnetic properties. By making use of the mixed LDA-NCA calculation technique we analyse the crystal-field splittings of CeX compounds (X=N, P, As, Sb, Bi). The obtained ab-initio hybridization functions are taken as imputs to calculate the crystal-field splittings within NCA (non crossing approximation) and the tendencies are contrasted with experiments. KEY WORDS: Highly correlated systems, crystal fields, p-electron.

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