arXiv subjects

Balázs Dóra

Publications and source records attributed to Balázs Dóra.

At least 127 records · Page 7Linked to original sources

arXiv2007

Reentrant Kondo effect in Landau quantized graphene

We have studied the interplay of an Anderson impurity in Landau quantized graphene, with special emphasis on the influence of the chemical potential. Within the slave-boson mean-field theory, we found reentrant Kondo behaviour by varying the chemical potential or gate voltage. Between Landau levels, the density of states is suppressed, and by changing the graphene's Fermi energy, we cross from metallic to semiconducting regions. Hence, the corresponding Kondo behaviour is also influenced. The f-level spectral function reveals both the presence of Landau levels in the conduction band and the Kondo resonance.

cond-mat.str-el↗

arXiv2007

Magnetotransport and thermoelectricity in disordered graphene

We have studied the electric and thermal response of two-dimensional Dirac-fermions in a quantizing magnetic field in the presence of localized disorder. The electric and heat current operators in the presence of magnetic field are derived. The self-energy due to impurities is calculated self-consistently, and depends strongly on the frequency and field strength, resulting in asymmetric peaks in the density of states at the Landau level energies, and small islands connecting them. The Shubnikov-de Haas oscillations remain periodic in 1/B, in spite of the distinct quantization of quasiparticle orbits compared to normal metals. The Seebeck coefficient depends strongly on the field strength and orientation. For finite field and chemical potential, the Wiedemann-Franz law can be violated.

cond-mat.str-el↗

arXiv2006

Pseudogap enhancement due to magnetic impurities in d-density waves

We study the effect of quantum magnetic impurities on d-wave spin density waves (d-SDW). The impurity spins are aligned coherently according to the spin space anisotropy of the condensate. Both the order parameter and transition temperature increases due to the coherent interplay between magnetic scatterers and d-SDW. This can explain the recent experimental data on the pseudogap enhancement of Ni substituted NdBa_2{Cu_{1-y}Ni_y}O_6.8 from Pimenov et al. (Phys. Rev. Lett. 94, 227003 (2005)).

cond-mat.str-el↗

arXiv2006

Unconventional Density Waves in Organic Conductors and in Superconductors

Unconventional density waves (UDW) are one of the ground states in metallic crystalline solids and have been speculated already in 1968. However, more focused studies on UDW started only recently, perhaps after the identification of the low temperature phase in alpha-(BEDT-TTF)_2KHg(SCN)_4 as unconventional charge density wave (UCDW) in 2002. More recently, the metallic phase of Bechgaard salts (TMTSF)_2X with X=PF_6 and ReO_4 under both pressure and magnetic field appears to be unconventional spin density wave (USDW). The pseudogap regime of high T_c superconductors LSCO, YBCO, Bi2212 and the one in CeCoIn_5 belong to d-wave density waves (d-DW). In these identifications, the angular dependent magnetoresistance and the giant Nernst effect have played the crucial role. These are the simplest manifestations of the Landau quantization of quasiparticle energy in UDW in the presence of magnetic field (the Nersesyan effect). Also we speculate that UDW will be most likely found in alpha$-(BEDT-TTF)_2I_3, alpha-(BEDT-TTF)_2I_2Br, kappa-(BEDT-TTF)_2Cu(NCS)_2, kappa-(BEDT-TTF)_2Cu(CN)_2Br, lambda-(BEDT)_2GaCl_4 and in many other organic compounds.

cond-mat.str-el↗

arXiv2006

The Wiedemann-Franz law in the SU(N) Wolff model

We study the electrical and thermal transport through the SU(N) Wolff model with the use of bosonization. The Wilson ratio reaches unity as N grows to infinity. The electric conductance is dominated by the charge channel, and decreases monotonically with increasing interaction. The thermal conductivity enhances in the presence of local Hubbard U. The Wiedemann-Franz law is violated, the Lorentz number depends strongly on the interaction parameter, which can be regarded as a manifestation of spin-charge separation.

cond-mat.str-el↗

arXiv2005

Unconventional charge density wave driven by electron-phonon coupling

We report our study on unconventional charge density waves (UCDW) (i.e. a charge density wave with wavevector dependent gap) in pure quasi-one dimensional conductors. We develop a new possible mechanism of establishment of such a low temperature phase, in which the driving force of the phase transition is the electron-phonon interaction with coupling depending on both the momentum transfer q and the momentum of the scattered electron k. Mean field treatment is applied to obtain the excitation spectrum, correlation functions such as the density correlator and the optical conductivity, and the effective mass of the phase excitation. The fluctuation of the order parameter leads to the sliding of the UCDW as a whole. In the absence of impurities, we calculated the effect of this fluctuation on the optical properties. The inclusion of the collective mode significantly alters the optical conductivity, and leads to an effective mass which is nonmonotonic in temperature as opposed to conventional CDWs.

cond-mat.str-el↗

arXiv2005

The pseudogap phase in (TaSe_4)_2I

We have developed the mean-field theory of coexisting charge-density waves (CDW) and unconventional charge-density waves (UCDW). The double phase transition manifests itself in the thermodynamic quantities and in the magnetic response, such as spin susceptibility and spin-lattice relaxation rate. Our theory applies to quasi-one dimensional (TaSe_4)_2I, where above the CDW transition, thermal fluctuations die out rapidly, but robust pseudogap behaviour is still detected. We argue, that the fluctuations are suppressed due to UCDW, which partially gaps the Fermi surface, and causes non-Fermi-liquid (pseudogap) behaviour.

cond-mat.str-el↗

arXiv2005

Magnetotransport in d-wave density waves

Angle dependent magnetoresistance (ADMR) and giant Nernst effect are hallmarks of unconventional density waves (UDW). Here these transport properties for d-wave density wave (d-DW) are computed for quasi-two-dimensional systems. The present theory describes ADMR observed in the pseudogap phase of Y_0.68Pr_0.32Ba_2Cu_3O_7 and CeCoIn_5 single crystals very satisfactorily.

cond-mat.str-el↗

arXiv2005

The Fermi edge singularity in the SU(N) Wolff model

The low temperature properties of the SU(N) Wolff impurity model are studied via Abelian bosonization. The path integral treatment of the problem allows for an exact evaluation of low temperature properties of the model. The single particle Green's function enhances due to the presence of local correlation. The basic correlation function such as the charge or spin correlator are also influenced by the presence of impurity, and show local Fermi liquid behaviour. The X-ray absorption is affected by the presence of local Hubbard interaction. The exponent is decreased (increased) for repulsive (attractive) interactions.

cond-mat.str-el↗

arXiv2005

Boundary effect on CDW: Friedel oscillations, STM image

We study the effect of open boundary condition on charge density waves (CDW). The electron density oscillates rapidly close to the boundary, and additional non-oscillating terms (~ln(r)) appear. The Friedel oscillations survive beyond the CDW coherence length (v_F/Delta), but their amplitude gets heavily suppressed. The scanning tunneling microscopy image (STM) of CDW shows clear features of the boundary. The local tunneling conductance becomes asymmetric with respect to the Fermi energy, and considerable amount of spectral weight is transferred to the lower gap edge. Also it exhibits additional zeros reflecting the influence of the boundary.

cond-mat.str-el↗

arXiv2004

Gapped optical excitations from gapless phases: imperfect nesting in unconventional density waves

We consider the effect of imperfect nesting in quasi-one-dimensional unconventional density waves in the case, when the imperfect nesting and the gap depends on the same wavevector component. The phase diagram is very similar to that in a conventional density wave. The density of states is highly asymmetric with respect to the Fermi energy. The optical conductivity at T=0 remains unchanged for small deviations from perfect nesting. For higher imperfect nesting parameter, an optical gap opens, and considerable amount of spectral weight is transferred to higher frequencies. This makes the optical response of our system very similar to that of a conventional density wave. Qualitatively similar results are expected in d-density waves.

cond-mat.str-el↗

arXiv2004

Unconventional density wave in CeCoIn_5?

Very recently large Nernst effect and Seebeck effect were observed above the superconducting transition temperature 2.3K in a heavy fermion superconductor CeCoIn_5. We shall interpret this large Nernst effect in terms of unconventional density wave (UDW), which appears around T=18K. Also the temperature dependence of the Seebeck coefficient below T=18K is described in terms of UDW. Another hallmark for UDW is the angular dependent magnetoresistance, which should be readily accessible experimentally.

cond-mat.str-el↗

arXiv2004

Recent Advances in Unconventional Density Waves

Unconventional density wave (UDW) has been speculated as a possible electronic ground state in excitonic insulator in 1968. Recent surge of interest in UDW is partly due to the proposal that the pseudogap phase in high T_c cuprate superconductors is d-wave density wave (d-DW). Here we review our recent works on UDW within the framework of mean field theory. In particular we have shown that many properties of the low temperature phase (LTP) in alpha-(BEDT-TTF)_2MHg(SCN)_4 with M=K, Rb and Tl are well characterized in terms of unconventional charge density wave (UCDW). In this identification the Landau quantization of the quasiparticle motion in a magnetic field (the Nersesyan effect) plays the crucial role. Indeed the angular dependent magnetoresistance and the negative giant Nernst effect are two hallmarks of UDW.

cond-mat.str-el↗

arXiv2004

Large-N limit of a magnetic impurity in unconventional density waves

We investigate the effect of unconventional density wave (UDW) condensate on an Anderson impurity using large-N technique at T=0. In accordance with previous treatments of a Kondo impurity in pseudogap phases, we find that Kondo effect occurs only in a certain range of parameters. The f-electron density of states reflects the influence of UDW at low energies and around the maximum of the density wave gap. The static spin susceptibility diverges at the critical coupling, indicating the transition from strong to weak coupling. In the dynamic spin susceptibility an additional peak appears showing the presence the UDW gap. Predictions concerning non-linear density of states are made. Our results apply to other unconventional condensates such as d-wave superconductors and d-density waves as well.

cond-mat.str-el↗

arXiv2003

Magnetothermopower and Nernst effect in unconventional charge density waves

Recently we have shown that the striking angular dependent magnetoresistance in the low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 is consistently described in terms of unconventional charge density wave (UCDW). Here we investigate theoretically the thermoelectric power and the Nernst effect in UDW. The present results account consistently for the recent data of magnetothermopower in alpha-(BEDT-TTF)_2KHg(SCN)_4 obtained by Choi et al. (Phys. Rev. B, 65, 205119 (2002)). This confirms further our identification of LTP in this salt as UCDW. We propose also that the Nernst effect provides a clear signature of UDW.

cond-mat.str-el↗

arXiv2003

Aspects of unconventional density waves

Recently many people discuss unconventional density waves (i.e. unconventional charge density waves (UCDW) and unconventional spin density waves (USDW)). Unlike in conventional density waves, the quasiparticle spectrum in these systems is gapless. Also these systems remain metallic. Indeed it appears that there are many candidates for UDW. The low temperature phase of alpha-(BEDT-TTF)_2KHg(SCN)_4, the antiferromagnetic phase in URu_2Si_2, the CDW in transition metal dichalcogenite NbSe_2, the pseudogap phase in high T_c cuprate superconductors, the glassy phase in organic superconductor kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br. After a brief introduction on UCDW and USDW, we shall discuss some of the above systems, where we believe we have evidence for unconventional density waves.

cond-mat.str-el↗

arXiv2003

Impurity effects in unconventional density waves in the unitary limit

We investigate the effect of strong, nonmagnetic impurities on quasi-one-dimensional conventional and unconventional density waves (DW and UDW). The conventional case remains unaffected similarly to s-wave superconductors in the presence of weak, nonmagnetic impurities. The thermodynamic properties of UDW were found to be identical to those of a d-wave superconductor in the unitary limit. The real and imaginary part of the optical conductivity is determined for electric fields applied in the perpendicular directions. A new structure can be present corresponding to excitations from the bound state at the Fermi energy to the gap maximum in addition to the usual peak at 2Δ. In the dc limit, universal electric conductivity is found.

cond-mat.str-el↗

arXiv2003

Optical conductivity of superconducting Sr_2RuO_4

We compute the optical conductivity of 2D f-wave superconductors and also of the multigap model proposed recently by Zhitomirsky and Rice at T=0K in the Born limit. The presence of interband impurity scattering was found to play an important role: the contributions from the two bands mix up, and new structures are seen in the tunneling density of states and in the optical spectrum as well, corresponding to interband transitions. This will provide a sensitive test in selecting the competing models for the triplet superconductivity in Sr_2RuO_4.

cond-mat.supr-con↗