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The NMR relaxation rate and the static spin susceptibility in graphene are studied within a tight-binding description. At half filling, the NMR relaxation rate follows a power law as $T^2$ on the particle-hole symmetric side, while with a finite chemical potential $\mu$ and next-nearest neighbor $t'$, the $(\mu+3t')^2$ terms dominate at low excess charge $\delta$. The static spin susceptibility is linearly dependent on temperature $T$ at half filling when $t'=0$, while with a finite $\mu$ and $t'$, it should be dominated by $(\mu+3t')$ terms in low energy regime. These unusual phenomena are direct results of the low energy excitations of graphene, which behave as massless Dirac fermions. Furthermore, when $\delta$ is high enough, there is a pronounced crossover which divides the temperature dependence of the NMR relaxation rate and the static spin susceptibility into two temperature regimes: the NMR relaxation rate and the static spin susceptibility increase dramatically as temperature increases in the low temperature regime, and after the crossover, both decrease as temperature increases at high temperatures. This crossover is due to the well-known logarithmic Van Hove singularity in the density of states, and its position dependence of temperature is sensitive to $\delta$., Comment: 7 pages, 6 figures

Recent NMR experiments by Singer et al. [Singer et al. Phys. Rev. Lett. 95, 236403 (2005).] showed a deviation from Fermi-liquid behavior in carbon nanotubes with an energy gap evident at low temperatures. Here, a comprehensive theory for the magnetic field and temperature dependent NMR 13C spin-lattice relaxation is given in the framework of the Tomonaga-Luttinger liquid. The low temperature properties are governed by a gapped relaxation due to a spin gap (~ 30K), which crosses over smoothly to the Luttinger liquid behaviour with increasing temperature., Comment: 5 pages, 1 figure, 1 table

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., Comment: 8 pages, 6 figures

We present a mean-field theoretical study on the effect of a single non-magnetic impurity in quasi-one dimensional unconventional density wave. The local scattering potential is treated within the self-consistent $T$-matrix approximation. The local density of states around the impurity shows the presence of resonant states in the vicinity of the Fermi level, much the same way as in $d$-density waves or unconventional superconductors. The assumption for different forward and backscattering, characteristic to quasi-one dimensional systems in general, leads to a resonance state that is double peaked in the pseudogap. The Friedel oscillations around the impurity are also explored in great detail, both within and beyond the density wave coherence length $\xi_0$. Beyond $\xi_0$ we find power law behavior as opposed to the exponential decay of conventional density wave. The entropy and specific heat contribution of the impurity are also calculated for arbitrary scattering strengths., Comment: 13 pages, 4 figures

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., Comment: 10 pages, 6 figures

We have studied the interplay of a local phonon mode embedded in a metallic host (Holstein impurity model) using Abelian bosonization. The phonon frequency softens, which takes place in two steps: first, their frequency starts softening, and acquires finite lifetime. Then oscillations disappear from the response, and two distinct, finite dampings characterize them. Similar behaviour shows up in the spin-boson model. Due to phonons, the electrons experience an attractive, dynamic interaction. As a result, the electronic charge response enhances similarly to the spin response in the Kondo model. The local electronic density of states develops a dip around zero frequency. Thus the chance of charge-Kondo effect emerges., Comment: 8 pages, 3 figures, an erroneus "-" sign corrected, added corrections

We present a detailed theoretical study on the thermodynamic properties of impure quasi-one dimensional unconventional charge-, and spin-density waves in the framework of mean-field theory. The impurities are of the ordinary non-magnetic type. Making use of the full self-energy that takes into account all ladder-, and rainbow-type diagrams, we are able to calculate the relevant low temperature quantities for arbitrary impurity concentration and scattering rates. These are the density of states, specific heat and the shift in the chemical potential. Our results therefore cover the whole parameter space: they include both the self-consistent Born and the resonant unitary limits, and most importantly give exact results in between., Comment: 11 pages, 8 figures

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))., Comment: 4 pages, 3 figures

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., Comment: 20 pages, 12 figures

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., Comment: 4 pages, 3 figures

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., Comment: 11 figures, 13 pages, Revtex4 style

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., Comment: 7 pages, 6 figures

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., Comment: 7 pages, 6 figures

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., Comment: 7 pages, 4 figures

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., Comment: 7 pages, 6 figures

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., Comment: 8 pages, 7 figures

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., Comment: 4 pages, 7 figures

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., Comment: 18 pages, 12 figures

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., Comment: 9 pages, 7 figures

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., Comment: 4 pages, 4 figures

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., Comment: 11 pages, 5 figures

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 2\Delta. In the dc limit, universal electric conductivity is found., Comment: 9 pages, 5 figures

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., Comment: 7 pages, 9 figures

We have investigated the collective modes of unconventional charge and spin density waves (UCDW, USDW) in quasi-one dimensional systems in random phase approximation. The density correlator regains its normal state form due to the phase degree of freedom of the condensate. The possible effect of impurities is also discussed. From this, the current-current correlation function is evaluated through charge conservation. The spin susceptibility of USDW remains anisotropic an spite of the lack of any periodic modulation of the spin density. In UCDW, the spin response gets weaker in all three directions as the temperature is lowered., Comment: 7 pages, no figures

The low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 salt is known for its surprising angular dependent magnetoresistance (ADMR), which has been studied intensively in the last decade. However, the nature of the LTP has not been understood until now. Here we analyse theoretically ADMR in unconventional (or nodal) charge density wave (UCDW). In magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to spectacular ADMR. The present model accounts for many striking features of ADMR data in alpha-(BEDT-TTF)_2KHg(SCN)_4., Comment: 5 pages, 6 figures

We consider the effect of imperfect nesting in quasi-one dimensional unconventional density waves. The phase diagram is very close to those in a conventional DW. The linear and non-linear aspects of the electric conductivity are discussed. At T=0 the frequency dependent electric conductivity develops a small dip at low frequencies. The threshold electric field depends strongly on the imperfect nesting parameter, allowing us to describe the measured threshold electric field in the low temperature phase of the quasi-two dimensional organic conductor, alpha-(BEDT-TTF)_2KHg(SCN)_4 very well., Comment: 9 pages, 9 figures

In spite of extensive experimental studies of the angular dependent magnetoresistance (ADMR) of the low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 about a decade ago, the nature of LTP remains elusive. Here we present a new study of ADMR of LTP in alpha-(ET)_2 salts assuming that LTP is unconventional charge density wave (UCDW). In the presence of magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to striking ADMR in UCDW. The present model appears to account for many existing ADMR data of alpha-(BEDT-TTF)_2KHg(SCN)_4 remarkably well., Comment: 6 pages, 4 figures

We have investigated the effect of nonmagnetic impurities on the quasi-one-dimensional unconventional density wave (UDW) ground state. The thermodynamics were found to be close to those of a d-wave superconductor in the Born limit. Four different optical conductivity curves were found depending on the direction of the applied electric field and on the wavevector dependence of the gap., Comment: 14 pages, 9 figures

Many experiments suggest that the unidentified low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 is most likely unconventional charge density wave (UCDW). To further this identification we present our theoretical study of the threshold electric field of UCDW in a magnetic field. The magnetic field-temperature phase diagram is very similar to those in a d-wave superconductor. We find a rather strong field dependence of the threshold electric field, which should be readily accessible experimentally., Comment: 7 pages, 6 figures

We study theoretically the out of plane optical conductivity of d-wave superconductors in the presence of impurities at T=0K. Unlike the usual approach, we assume that the interlayer quasi-particle transport is due to coherent tunneling. The present model describes the T^2 dependence of the out of plane superfluid density observed in YBCO and Tl2201 for example. In the optical conductivity there is no Drude peak in agreement with experiment, and the interlayer Josephson tunneling is also assured in this model. In the unitary limit we predict a step like behaviour around omega=Delta in both the real and imaginary part of the optical conductivity., Comment: 7 pages, 7 figures

As it is well known most of charge density wave (CDW) and spin density wave (SDW) exhibit the nonlinear transport with well defined threshold electric field E_T. Here we study theoretically the threshold electric field of unconventional density waves. We find that the threshold field increases monotonically with temperature without divergent behaviour at T_c, unlike the one in conventional CDW. The present result in the 3D weak pinning limit appears to describe rather well the threshold electric field observed recently in the low-temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4., Comment: 4 pages, 2 figures

We study theoretically the optical conductivity of d-wave superconductors like in high temperature cuprates in the presence of impurities. We limit ourselves at T=0K and focus on the frequency dependence of both sigma_1(omega) and \omega*\sigma_2(omega) for omega<2Delta. When the impurity scattering is in the unitary limit, we find a peak in sigma_1(omega) with omega/Delta\simeq 0.1\sim 0.5$, which may account for the peak seen by Basov et al. in Zn-substituted YBCO., Comment: 7 pages, 9 figures, submitted to Europhys. Lett

We consider the possibility of formation of an unconventional spin density wave (USDW) in quasi-one dimensional electronic systems. In analogy with unconventional superconductivity, we develop a mean field theory of SDW allowing for the momentum dependent gap $\Delta({\bf k})$ on the Fermi surface. Conditions for the appearence of such a low temperature phase are investigated. The excitation spectrum and basic thermodynamic properties of the model are found to be very similar to those of d-wave superconductors in spite of the different topology of their Fermi surfaces. Several correlation functions are calculated, and the frequency dependent conductivity is evaluated for various gap functions. The latter is found to reflect the maximum gap value, however with no sharp onset for absorbtion., Comment: 13 pages, 11 figures, submitted to Phys. Rev. B

The quasiparticle contribution to the frequency dependent electric conductivity in the presence of randomly distributed impurities is calculated within mean field theory for spin density waves as formed in quasi one dimensional conductors like Bechgaard salts. Interchain hopping is taken into account and the effects of imperfect nesting are considered. In case of an electric field perpendicular to the chain direction there is no collective contribution to the conductivity and our results are directly applicable to recent measurements on both (TMTSF)2PF6 and quenched (TMTSF)2ClO4. The experimental data are well described by the theory in the clean limit, with somewhat larger scattering in the ClO4 salt., Comment: 7 pages (Eurotex), 2 eps figures, accepted by Europhysics Letters