tag:blogger.com,1999:blog-37307112853277591882017-02-08T20:50:17.521-08:00cond-mat.str-el - Strongly Correlated ElectronsSite for <a href="http://communitypeerreview.blogspot.com/">Community Peer Review</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.comBlogger5065125tag:blogger.com,1999:blog-3730711285327759188.post-50243122414169366742013-08-06T00:02:00.015-07:002013-08-06T00:02:07.509-07:001305.2888 (I. S. Burmistrov et al.)<h2 class="title"><a href="http://arxiv.org/abs/1305.2888">Multifractality at Anderson transitions with Coulomb interaction</a> [<a href="http://arxiv.org/pdf/1305.2888">PDF</a>]</h2>I. S. Burmistrov, I. V. Gornyi, A. D. Mirlin<a name='more'></a><blockquote class="abstract">We explore mesoscopic fluctuations and correlations of the local density of states (LDOS) near localization transition in a disordered interacting electronic system. It is shown that the LDOS multifractality survives in the presence of Coulomb interaction. We calculate the spectrum of multifractal dimensions in $2+\epsilon$ spatial dimensions and show that it differs from that in the absence of interaction. The multifractal character of fluctuations and correlations of the LDOS can be studied experimentally by scanning tunneling microscopy of two-dimensional and three-dimensional disordered structures.</blockquote>View original: <a href="http://arxiv.org/abs/1305.2888">http://arxiv.org/abs/1305.2888</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-19747784459934485672013-08-06T00:02:00.013-07:002013-08-06T00:02:06.283-07:001308.1014 (A. A. Zvyagin)<h2 class="title"><a href="http://arxiv.org/abs/1308.1014">New physics in frustrated magnets: Spin ices, monopoles, etc</a> [<a href="http://arxiv.org/pdf/1308.1014">PDF</a>]</h2>A. A. Zvyagin<a name='more'></a><blockquote class="abstract">During recent years the interest to frustrated magnets has grown considerably. Such systems reveal very peculiar properties which distinguish them from standard paramagnets, magnetically ordered regular systems (like ferro-, ferri-, and antiferromagnets), or spin glasses. In particular great amount of attention has been devoted to the so-called spin ices, in which magnetic frustration together with the large value of the single-ion magnetic anisotropy of a special kind, yield peculiar behavior. One of the most exciting features of spin ices is related to low-energy emergent excitations, which, from many viewpoints can be considered as analogies of Dirac's monopoles. In this article we review the main achievements of theory and experiment in this field of physics.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1014">http://arxiv.org/abs/1308.1014</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-24463508172345952792013-08-06T00:02:00.011-07:002013-08-06T00:02:05.242-07:001308.0603 (Johannes Bauer et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0603">Realizing a Kondo-correlated state with ultracold atoms</a> [<a href="http://arxiv.org/pdf/1308.0603">PDF</a>]</h2>Johannes Bauer, Christophe Salomon, Eugene Demler<a name='more'></a><blockquote class="abstract">We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of ${}^{40}$K and ${}^{23}$Na atoms can be used to generate a Kondo correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0603">http://arxiv.org/abs/1308.0603</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-21307962768789404822013-08-06T00:02:00.009-07:002013-08-06T00:02:04.311-07:001308.0638 (Enrique Muñoz et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0638">Reply to 'Comment on Universal out-of-equilibrium transport in<br /> Kondo-correlated quantum dots'</a> [<a href="http://arxiv.org/pdf/1308.0638">PDF</a>]</h2>Enrique Muñoz, C. J. Bolech, Stefan Kirchner<a name='more'></a><blockquote class="abstract">A recent comment on our work (Phys. Rev. Lett., vol. 110, 016601 (2013)) by A.A.Aligia claims that we "made mistakes in the evaluation of the lesser quantities". It is further claimed that the distribution function of the single-particle selfenergy of the interacting region in the Fermi liquid regime, e.g. at small bias voltage, low temperature, and small frequency, is continuous. These claims are based on a comparison of the particle-hole symmetric case with results obtained from the approach of A.A.Aligia. We disagree with these claims and show that the discrepancies that the comment alludes to originate from a violation of Ward identities by the method employed in the comment. A comparison of our approach with the numerical renormalization group shows perfect agreement for the symmetric case.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0638">http://arxiv.org/abs/1308.0638</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-10922336769208478562013-08-06T00:02:00.007-07:002013-08-06T00:02:03.515-07:001308.0664 (Tatsuya Kaneko et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0664">BCS-BEC crossover in the two-dimensional attractive Hubbard model:<br /> Variational cluster approach</a> [<a href="http://arxiv.org/pdf/1308.0664">PDF</a>]</h2>Tatsuya Kaneko, Yukinori Ohta<a name='more'></a><blockquote class="abstract">Variational cluster approximation is used to study the superconducting ground state in the two-dimensional attractive Hubbard model. We show that the calculated Bogoliubov quasiparticle spectra and condensation amplitude clearly exhibit the character of the Cooper pairs in momentum space and that the coherence length $\xi$ evaluated from the condensation amplitude demonstrates the smooth crossover from a weakly paired BCS state ($\xi\gg 1$) to a BEC state of tightly bound pairs ($\xi\ll 1$). The calculated kinetic and potential energies in the superconducting and normal ground states indicate that the superconducting state in the weak-coupling region is driven by the gain in potential energy, while in the strong-coupling region, it is driven by the gain in kinetic energy.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0664">http://arxiv.org/abs/1308.0664</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-60334600403395992692013-08-06T00:02:00.005-07:002013-08-06T00:02:02.529-07:001308.0705 (Krzysztof Rościszewski et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0705">A possibility of high spin hole states in doped CoO$_2$ layered systems</a> [<a href="http://arxiv.org/pdf/1308.0705">PDF</a>]</h2>Krzysztof Rościszewski, Andrzej M. Oleś<a name='more'></a><blockquote class="abstract">We introduce and investigate an effective five-band model for $t_{2g}$ and $e_g$ electrons to describe doped cobalt oxides with Co$^{3+}$ and Co$^{4+}$ ions in two-dimensional CoO$_2$ triangular lattice layers, as in Na$_{1-x}$CoO$_2$. The effective Hamiltonian includes anisotropic kinetic energy (due to both direct Co-Co and indirect Co-O-Co hoppings), on-site Coulomb interactions parameterized by intraorbital Hubbard repulsion $U$ and full Hund's exchange tensor, crystal-field terms and Jahn-Teller static distortions. We study it using correlated wave functions on $6\times 6$ clusters with periodic boundary conditions. The computations indicate low S=0 spin to high S=2 spin abrupt transition in the undoped systems when increasing strength of the crystal field, while intermediate S=1 spins are not found. Surprisingly, for the investigated realistic Hamiltonian parameters describing low spin states in CoO$_2$ planes, doping generates high $S=\frac{5}{2}$ spins at Co$^{4+}$ ions that are pairwise bound into singlets, seen here as pairs of up and down spins. It is found that such singlet pairs self-organize at higher doping into lines of spins with coexisting antiferromagnetic and ferromagnetic bonds, forming stripe-like structures. The ground states are insulating within the investigated range of doping because computed HOMO-LUMO gaps are never small enough.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0705">http://arxiv.org/abs/1308.0705</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-64535020736905183662013-08-06T00:02:00.003-07:002013-08-06T00:02:01.546-07:001308.0738 (Ioannis Rousochatzakis et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0738">Quantum dimer model for the spin-1/2 kagome Z2 spin liquid</a> [<a href="http://arxiv.org/pdf/1308.0738">PDF</a>]</h2>Ioannis Rousochatzakis, Yuan Wan, Frédéric Mila, Oleg Tchernyshyov<a name='more'></a><blockquote class="abstract">We revisit the old problem of the spin-1/2 Heisenberg antiferromagnet on kagome, a major candidate for a quantum spin liquid, focusing on the renormalization of nearest-neighbor valence-bond (NNVB) tunneling amplitudes by long-range singlet fluctuations outside the NNVB basis. We find that such virtual singlets have a dramatic influence on the dominant quantum dimer model (QDM) parameters, leading to qualitatively different physics within the NNVB framework. Based on recent numerical studies on the unconstrained QDM, the renormalized parameters favor a Z2 spin liquid, thus providing a plausible way to reconcile the NNVB description with density matrix renormalization group (DMRG) studies.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0738">http://arxiv.org/abs/1308.0738</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-14265820627199104392013-08-06T00:02:00.001-07:002013-08-06T00:02:00.566-07:001308.0812 (Dmitry Yudin et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0812">Fermi condensation near Van Hove singularities in the triangular lattice</a> [<a href="http://arxiv.org/pdf/1308.0812">PDF</a>]</h2>Dmitry Yudin, Daniel Hirschmeier, Hartmut Hafermann, Olle Eriksson, Alexander I. Lichtenstein, Mikhail I. Katsnelson<a name='more'></a><blockquote class="abstract">The proximity of the Fermi surface to Van Hove singularities drastically enhances interaction effects and leads to essentially new physics. In this work we address the formation of flat bands ("Fermi condensation") within the Hubbard model on the triangular lattice and provide a detailed analysis from an analytical and numerical perspective. To describe the effect we consider both weak-coupling and strong-coupling approaches, namely the renormalization group and dual fermion methods. It is shown that the band flattening is driven by correlations and is well pronounced even at sufficiently high temperatures, of the order of 0.1-0.2 of the hopping parameter. The effect can therefore be probed in experiments with ultracold fermions in optical lattices.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0812">http://arxiv.org/abs/1308.0812</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-35891136863561610262013-08-06T00:01:00.019-07:002013-08-06T00:01:56.998-07:001308.0823 (H. Schenck et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0823">Vector chiral phases in frustrated 2D XY model and quantum spin chains</a> [<a href="http://arxiv.org/pdf/1308.0823">PDF</a>]</h2>H. Schenck, V. L. Pokrovsky, T. Nattermann<a name='more'></a><blockquote class="abstract">The phase diagram of the frustrated 2D XY model is calculated analytically. The chiral (Ising) transition is described by three independent critical exponents which are calculated in $d=5/2-\epsilon$ dimensions. Vortex interaction is short range on small and logarithmic on large scales, if compared with the chiral correlation length $\xi$. The vortex unbinding transitions is triggered by the increase of $\xi$ and occurs before the chiral transition takes place. In a narrow region close to the Lifshitz point a reentrant quasi-ferromagnetic phase appears. Application to antiferromagnetic quantum spin chains and multiferroics are discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0823">http://arxiv.org/abs/1308.0823</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-60405126862353049002013-08-06T00:01:00.017-07:002013-08-06T00:01:55.938-07:001308.0839 (J. H. Pixley et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0839">Pairing Correlations Near a Kondo-Destruction Quantum Critical Point</a> [<a href="http://arxiv.org/pdf/1308.0839">PDF</a>]</h2>J. H. Pixley, Lili Deng, Kevin Ingersent, Qimiao Si<a name='more'></a><blockquote class="abstract">Motivated by the unconventional superconductivity observed in heavy-fermion metals, we investigate pairing susceptibilities near a continuous quantum phase transition of the Kondo-destruction type. We solve two-impurity Bose-Fermi Anderson models with Ising and Heisenberg forms of the interimpurity exchange interaction using continuous-time quantum Monte-Carlo and numerical renormalization-group methods. Each model exhibits a Kondo-destruction quantum critical point separating Kondo-screened and local-moment phases. For antiferromagnetic interimpurity exchange interactions, singlet pairing is found to be enhanced in the vicinity of the transition. Implications of this result for heavy-fermion superconductivity are discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0839">http://arxiv.org/abs/1308.0839</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-41701862834013591912013-08-06T00:01:00.015-07:002013-08-06T00:01:54.883-07:001308.0865 (Y. -J. Chen et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0865">Doping evolution of Zhang-Rice singlet spectral weight: a comprehensive<br /> examination by x-ray absorption spectroscopy</a> [<a href="http://arxiv.org/pdf/1308.0865">PDF</a>]</h2>Y. -J. Chen, M. G. Jiang, C. W. Luo, J. -Y. Lin, K. H. Wu, J. M. Lee, J. M. Chen, Y. K. Kuo, J. Y. Juang, Chung-Yu Mou<a name='more'></a><blockquote class="abstract">The total spectral weight \textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to mean field theory, we found that \textit{S} deviates from linear dependence on the doping level \textit{p}. The slope of \textit{S} versus \textit{p} changes continuously throughout the whole doping range with no sign of saturation up to \textit{p} = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSC's.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0865">http://arxiv.org/abs/1308.0865</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-40834866844664702912013-08-06T00:01:00.013-07:002013-08-06T00:01:54.008-07:001308.0923 (S. Fujiyama et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0923">Spin and Orbital Contributions to Magnetically Ordered Moments in 5d<br /> Layered Perovskite Sr2IrO4</a> [<a href="http://arxiv.org/pdf/1308.0923">PDF</a>]</h2>S. Fujiyama, H. Ohsumi, K. Ohashi, D. Hirai, B. J. Kim, T. Arima, M. Takata, H. Takagi<a name='more'></a><blockquote class="abstract">The ratio of orbital (L) and spin (S) contributions to the magnetically ordered moments of a 5d transition metal oxide, Sr2IrO4 was evaluated by non-resonant magnetic x-ray diffraction. We applied a new experimental setting to minimize the error in which we varied only the linear-polarization of incident x-ray at a fixed scattering angle. Strong polarization dependence of the intensity of magnetic diffraction was observed, from which we conclude that the ordered moments contain substantial contribution from the orbital degree of freedom with the ratio of <L>/<S> ~5.0, evidencing the pronounce effect of spin-orbit coupling. The obtained ratio is close to but slightly larger than the expected value for the ideal J_eff = 1/2 moment of a spin-orbital Mott insulator, |<J_1/2|L_z|J_1/2>|/|<J_1/2|S_z|J_1/2>| = 4, which cannot be accounted by the redistribution of orbital components within the t_2g bands associated with the elongation of the IrO6 octahedra.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0923">http://arxiv.org/abs/1308.0923</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-43616576850180799222013-08-06T00:01:00.011-07:002013-08-06T00:01:53.082-07:001308.0927 (Z. V. Pchelkina et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0927">Ab initio investigation of the exchange interactions in<br /> Bi$_2$Fe$_4$O$_9$: The Cairo pentagonal lattice compound</a> [<a href="http://arxiv.org/pdf/1308.0927">PDF</a>]</h2>Z. V. Pchelkina, S. V. Streltsov<a name='more'></a><blockquote class="abstract">We present the \emph{ab initio} calculation of the electronic structure and magnetic properties of Bi$_2$Fe$_4$O$_9$. This compound crystallizes in the orthorhombic crystal structure with the Fe$^{3+}$ ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9 within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K, J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi2Fe4O9 needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only two largest exchange integrals, then according to the ratio x\equiv J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal lattice, see. Ref.~1) the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0927">http://arxiv.org/abs/1308.0927</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-44295330653584575362013-08-06T00:01:00.009-07:002013-08-06T00:01:52.064-07:001308.0939 (S. Ramanan et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0939">BEC-BCS Crossover in Neutron Matter with Renormalization Group based<br /> Effective Interactions</a> [<a href="http://arxiv.org/pdf/1308.0939">PDF</a>]</h2>S. Ramanan, M. Urban<a name='more'></a><blockquote class="abstract">We study pure neutron matter in the BEC-BCS crossover regime using renormalization group based low-momentum interactions within the Nozi\`eres-Schmitt-Rink framework. This is an attempt to go beyond the mean field description for low-density matter. We work in the basis of so-called Weinberg eigenvectors where the operator $G_0V$ is diagonal, which proves to be an excellent choice that allows one to use non-local interactions in a very convenient way. We study the importance of correlations as a function of density. We notice that there is a significant reduction of the BCS critical temperature at low-densities as the neutron matter approaches the unitary limit.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0939">http://arxiv.org/abs/1308.0939</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-64695355945539250792013-08-06T00:01:00.007-07:002013-08-06T00:01:51.024-07:001308.0983 (Akira Oguri et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0983">Exact interacting Green's function for the Anderson impurity at high<br /> bias voltages</a> [<a href="http://arxiv.org/pdf/1308.0983">PDF</a>]</h2>Akira Oguri, Rui Sakano<a name='more'></a><blockquote class="abstract">We describe some exact high-energy properties of a single Anderson impurity connected to two noninteracting leads in a nonequilibrium steady state. In the limit of high bias voltages, and also in the high-temperature limit at thermal equilibrium, the model can be mapped on to an effective non-Hermitian Hamiltonian consisting of two sites, which correspond to the original impurity and its image that is defined in a doubled Hilbert space refereed to as Liouville-Fock space. For this, we provide a heuristic derivation using a path-integral representation of the Keldysh contour and the thermal field theory, in which the time evolution along the backward contour is replicated by extra degrees of freedom corresponding to the image. We find that the effective Hamiltonian can also be expressed in terms of charges and currents. From this, it can be deduced that the dynamic susceptibilities for the charges and the current fluctuations become independent of the Coulomb repulsion U in the high bias limit. Furthermore, the equation of motion for the Green's function and that for two other higher-order correlation functions consist a closed system. The exact solution obtained from the three coupled equations extends the atomic-limit solution such that the self-energy correctly captures the imaginary part caused by the relaxation processes at high energies. The spectral weights of the upper and lower Hubbard levels depend sensitively on the asymmetry in the tunneling couplings to the left and right leads.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0983">http://arxiv.org/abs/1308.0983</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-52487008772261672082013-08-06T00:01:00.005-07:002013-08-06T00:01:50.367-07:001308.0987 (Wei Chen et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0987">Implications of Resonant Inelastic X-ray Scattering Data for Theoretical<br /> Models of Cuprates</a> [<a href="http://arxiv.org/pdf/1308.0987">PDF</a>]</h2>Wei Chen, Oleg P. Sushkov<a name='more'></a><blockquote class="abstract">There are two commonly discussed points of view in theoretical description of cuprate superconductors, (i) Cuprates can be described by the modified t-J model. (ii) Overdoped cuprates are close to the regime of normal Fermi liquid (NFL). We argue that recent resonant inelastic X-ray scattering data challenge both points. While the modified t-J model describes well the strongly underdoped regime, it fails to describe high energy magnetic excitations when approaching optimal doping. This probably indicates failure of the Zhang-Rice singlet picture. In the overdoped regime the momentum-integrated spin structure factor S(\omega) has the same intensity and energy distribution as that in an undoped parent compound. This implies that the entire spin spectral sum rule is saturated at \omega ~ 2J, while in a NFL the spectral weight should saturate only at the total bandwidth which is much larger than 2J.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0987">http://arxiv.org/abs/1308.0987</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-26595258037757646702013-08-06T00:01:00.003-07:002013-08-06T00:01:49.301-07:001308.1072 (C. T. Wolowiec et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1072">Enhancement of superconductivity near the pressure-induced<br /> semiconductor-metal transition in BiS2-based compounds LnO(0.5)F(0.5)BiS2 (Ln<br /> = La, Ce, Pr, Nd)</a> [<a href="http://arxiv.org/pdf/1308.1072">PDF</a>]</h2>C. T. Wolowiec, B. D. White, I. Jeon, D. Yazici, K. Huang, M. B. Maple<a name='more'></a><blockquote class="abstract">Measurements of electrical resistivity were performed as a function of temperature between 3 and 300 K at various pressures up to 2.8 GPa on the LnO0.5F0.5BiS2 (Ln = Pr, Nd) compounds. At low pressure, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 exhibit superconductivity with Tc of 3.5 and 3.9 K, respectively. Both compounds display semiconducting behavior at low pressure in the normal state which is strongly suppressed with pressure. Both compounds undergo a pressure-induced transition from a low Tc superconducting phase to a high Tc superconducting phase in which Tc reaches 7.6 and 6.4 K for PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2, respectively. The pressure-induced transition is characterized by a rapid increase in Tc within a small pressure range of ~0.3 GPa for both compounds. For PrO0.5F0.5BiS2, the transition pressure correlates with the pressure where the rate of suppression of semiconducting behavior in the normal state saturates. For NdO0.5F0.5BiS2, the transition pressure between superconducting phases corresponds to the pressure that induces a semiconductor-metal transition in the normal state. The suppression of semiconducting behavior and evolution of Tc with pressure in the compounds LnO0.5F0.5BiS2 (Ln = Pr, Nd) is markedly similar to behavior recently reported for the BiS2-based compounds LnO0.5F0.5BiS2 (Ln = La, Ce). [arXiv 1307.4157 (2013), Phys. Rev. B accepted] A plot of the transition pressure versus lanthanide element (Ln = La, Ce, Pr, Nd) indicates that there is an increase in the value of transition pressure with decrease in atomic radius of the lanthanide element in LnO0.5F0.5BiS2. In addition, the "jump" in Tc between the two superconducting phases becomes smaller with decrease in atomic radius of lanthanide element (Ln = La, Ce, Pr, Nd). We suspect that this behavior is a general characteristic of the BiS2-based family of layered superconductors LnO0.5F0.5BiS2.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1072">http://arxiv.org/abs/1308.1072</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-49946713583795934232013-08-06T00:01:00.001-07:002013-08-06T00:01:48.221-07:001308.1085 (Michael M. Yee et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1085">Imaging the Kondo Insulating Gap on SmB6</a> [<a href="http://arxiv.org/pdf/1308.1085">PDF</a>]</h2>Michael M. Yee, Yang He, Anjan Soumyanarayanan, Dae-Jeong Kim, Zachary Fisk, Jennifer E. Hoffman<a name='more'></a><blockquote class="abstract">Topological insulators host spin-polarized surface states which robustly span the band gap and hold promise for novel applications. Recent theoretical predictions have suggested that topologically protected surface states may similarly span the hybridization gap in some strongly correlated heavy fermion materials, particularly SmB6. However, the process by which the Sm 4f electrons hybridize with the 5d electrons on the surface of SmB6, and the expected Fermi-level gap in the density of states out of which the predicted topological surface states must arise, have not been directly measured. We use scanning tunneling microscopy to conduct the first atomic resolution spectroscopic study of the cleaved surface of SmB6, and to reveal a robust hybridization gap which universally spans the Fermi level on four distinct surface morphologies despite dramatic shifts in the f band energy. Using a cotunneling model, we separate the density of states of the hybridized bands from which the predicted topological surface states must be disentangled. On all surfaces we observe residual spectral weight spanning the hybridization gap down to the lowest T, which may be consistent with a topological surface state.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1085">http://arxiv.org/abs/1308.1085</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-46183463333052826702013-08-05T00:01:00.013-07:002013-08-05T00:01:34.009-07:001308.0560 (A. Sherman et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0560">Strongly correlated electron system in the magnetic field</a> [<a href="http://arxiv.org/pdf/1308.0560">PDF</a>]</h2>A. Sherman, M. Schreiber<a name='more'></a><blockquote class="abstract">The energy spectrum of the two-dimensional t-J model in a perpendicular magnetic field is investigated. The density of states at the Fermi level as a function of the inverse magnetic field $\frac{1}{B}$ reveals oscillations in the range of hole concentrations $0.08<x<0.18$. In the used approximation zero-field Fermi surfaces are large for these $x$, and oscillation frequencies conform with such Fermi surfaces. However, the amplitude of these oscillations is modulated with a frequency which is smaller by an order of magnitude. The appearance of this modulation is related to van Hove singularities in the Landau subbands, which traverse the Fermi level with changing $B$. The singularities are connected with bending the Landau subbands due to strong electron correlations. The frequency of the modulation is of the same order of magnitude as the quantum oscillation frequency observed in underdoped cuprates.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0560">http://arxiv.org/abs/1308.0560</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-32025792903056234572013-08-05T00:01:00.011-07:002013-08-05T00:01:33.540-07:001308.0329 (Paul Chesler et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0329">Conformal field theories in a periodic potential: results from<br /> holography and field theory</a> [<a href="http://arxiv.org/pdf/1308.0329">PDF</a>]</h2>Paul Chesler, Andrew Lucas, Subir Sachdev<a name='more'></a><blockquote class="abstract">We study 2+1 dimensional conformal field theories (CFTs) with a globally conserved U(1) charge, placed in a chemical potential which is periodically modulated along the spatial direction x with zero average: \mu(x) = V cos(kx). The dynamics of such theories depends only on the dimensionless ratio V/k, and we expect that they flow in the infrared to new CFTs whose universality class changes as a function of V/k. We compute the frequency-dependent conductivity of strongly-coupled CFTs using holography of the Einstein-Maxwell theory in 4-dimensional anti-de Sitter space. We compare the results with the corresponding computation of weakly-coupled CFTs, perturbed away from the CFT of free, massless Dirac fermions (which describes graphene at low energies). We find that the results of the two computations have significant qualitative similarities. However, differences do appear in the vicinities of an infinite discrete set of values of V/k: the universality class of the infrared CFT changes at these values in the weakly-coupled theory, by the emergence of new zero modes of Dirac fermions which are remnants of local Fermi surfaces. The infrared theory changes continuously in holography, and the classical gravitational theory does not capture the physics of the discrete transition points between the infrared CFTs. We briefly note implications for a non-zero average chemical potential.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0329">http://arxiv.org/abs/1308.0329</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-87070833249345072062013-08-05T00:01:00.009-07:002013-08-05T00:01:32.430-07:001308.0343 (Emil J. Bergholtz et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0343">Topological Flat Band Models and Fractional Chern Insulators</a> [<a href="http://arxiv.org/pdf/1308.0343">PDF</a>]</h2>Emil J. Bergholtz, Zhao Liu<a name='more'></a><blockquote class="abstract">Topological insulators are accompanied by exotic edge states that are protected by a bulk single-particle band gap once the filled bands are characterized by non-trivial topological invariants. Interactions can have profound effects and lead to entirely new insulating phases, with an altogether much richer and less explored phenomenology, as is particularly clear in the case of partial filling of weakly dispersive bands. Most saliently, lattice generalizations of fractional quantum Hall states, dubbed fractional Chern insulators, have recently been predicted to be stabilized by interactions within nearly dispersionless bands with non-zero Chern number, $C$. Contrary to their continuum analogues, these states do not require an external magnetic field and may potentially persist at high temperatures, which make these systems very interesting in the context of applications such as topological quantum computation. This review recapitulates the basics of tight-binding models hosting nearly flat bands with non-trivial topology, $C\neq 0$, and summarizes the present understanding of interactions and strongly correlated phases within these bands. Emphasis is put on the analogy with continuum Landau level physics, as well as qualitatively new, lattice specific, aspects including Berry curvature fluctuations, competing instabilities as well as novel collective states of matter emerging in bands with $|C|>1$. Possible experimental realizations, including oxide interfaces and cold atom implementations as well as generalizations to flat bands characterized by other topological invariants are also discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0343">http://arxiv.org/abs/1308.0343</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-52141360283873808472013-08-05T00:01:00.007-07:002013-08-05T00:01:31.811-07:001308.0361 (S. Ejima et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0361">Order, criticality and excitations in the extended Falicov-Kimball model</a> [<a href="http://arxiv.org/pdf/1308.0361">PDF</a>]</h2>S. Ejima, T. Kaneko, Y. Ohta, H. Fehske<a name='more'></a><blockquote class="abstract">Using exact numerical techniques we investigate the nature of excitonic (electron-hole) bound states and the development of exciton coherence in the one-dimensional half-filled extended Falicov-Kimball model. The ground-state phase diagram of the model exhibits besides band insulator and staggered orbital ordered phases an excitonic insulator (EI) with power-law correlations. The criticality of the EI state shows up in the von Neuman entropy. The anomalous spectral function and condensation amplitude provide the binding energy and coherence length of the electron-hole pairs which, on their part, point towards a Coulomb interaction driven crossover from BCS-like electron-hole pairing fluctuations to tightly bound excitons. We show that while a mass imbalance between electrons and holes does not affect the location of the BCS-BEC crossover regime it favors staggered orbital ordering to the disadvantage of the EI.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0361">http://arxiv.org/abs/1308.0361</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-71847018567880530692013-08-05T00:01:00.005-07:002013-08-05T00:01:30.994-07:001308.0397 (Shuang Wu et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0397">Numerical study of magnetic and pairing correlation in bilayer<br /> triangular lattice</a> [<a href="http://arxiv.org/pdf/1308.0397">PDF</a>]</h2>Shuang Wu, Jinling Li, Pan Gao, Ying Liang, Tianxing Ma<a name='more'></a><blockquote class="abstract">By using the determinant Quantum Monte Carlo method, the magnetic and pairing correlation of the Na$_{x}$CoO$_{2}\cdot$yH$_{2}$O system are studied within the Hubbard model on a bilayer triangular lattice. The temperature dependence of spin correlation function and pairing susceptibility with several kinds of symmetries at different electron fillings and inter layer coupling terms are investigated. It is found that the system shows an antiferromagnetic correlation around the half filling, and the $fn$-wave pairing correlation dominates over other kinds of pairing symmetry in the low doping region. As the electron filling decreases away from the half filling, both the ferromagnetic correlation and the $f$-wave paring susceptibility are enhanced and tend to dominate. It is also shown that both the magnetic susceptibility and paring susceptibility decrease as the inter layer coupling increases.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0397">http://arxiv.org/abs/1308.0397</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-53600626011979224282013-08-05T00:01:00.003-07:002013-08-05T00:01:29.944-07:001308.0416 (M. Sekino et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0416">Temperature Dependence of Thermopower in Strongly Correlated<br /> Multiorbital Systems</a> [<a href="http://arxiv.org/pdf/1308.0416">PDF</a>]</h2>M. Sekino, S. Okamoto, W. Koshibae, M. Mori, S. Maekawa<a name='more'></a><blockquote class="abstract">Temperature dependence of thermopower in the multiorbital Hubbard model is studied by using the dynamical mean-field theory with the non-crossing approximation impurity solver. It is found that the Coulomb interaction, the Hund coupling, and the crystal filed splitting bring about non-monotonic temperature dependence of the thermopower, including its sign reversal. The implication of our theoretical results to some materials is discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0416">http://arxiv.org/abs/1308.0416</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-3730711285327759188.post-83637345258665402642013-08-05T00:01:00.001-07:002013-08-05T00:01:28.987-07:001308.0427 (Patrik Hlobil et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0427">Strong coupling behavior of the neutron resonance mode in unconventional<br /> superconductors</a> [<a href="http://arxiv.org/pdf/1308.0427">PDF</a>]</h2>Patrik Hlobil, Boris Narozhny, Jörg Schmalian<a name='more'></a><blockquote class="abstract">We analyze whether and how the neutron resonance mode in unconventional superconductors is affected by higher order corrections in the coupling between spin excitations and fermionic quasiparticles and find that in general such corrections cannot be ignored. In particular, we find that in two spatial dimensions (d=2) the corrections are of same order as the leading, weak coupling contributions demonstrating that the neutron resonance mode in unconventional superconductors is a strong coupling phenomenon. The origin of this behavior lies in the quantum-critical nature of the low energy spin dynamics in the superconducting state and the feedback of the resonance mode onto the fermionic excitations. While quantum critical fluctuations occur in any dimensionality smaller than the upper critical dimension d_{uc}=3, they can be analyzed in a controlled fashion by means of the \epsilon-expansion (\epsilon =3-d), such that the leading corrections to the resonance mode position are small. Regardless of the strong coupling nature of the resonance mode we show that it emerges only if the phase of the superconducting gap function varies on the Fermi surface, making it a powerful tool to investigate the microscopic structure of the pair condensate.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0427">http://arxiv.org/abs/1308.0427</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0