Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A38: Quantum Criticality I: Kondo Lattice Systems |
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Sponsoring Units: DCMP Chair: Filip Ronning, Los Alamos National Laboratory Room: F149 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A38.00001: Entropy Accumulation, Divergent Gruneisen Ratio, and Crossover Energy Scales near Quantum Critical Points Jianda Wu , Lijun Zhu , Qimiao Si A quantum critical point (QCP) arises at the point of second order phase transition at zero temperature. General scaling arguments have been used to show that a thermodynamic ratio -- the Gruneisen ratio of thermal expansion to specific heat -- diverges at QCPs [1], and this divergence has been experimentally observed in heavy fermion metals [2]. An important consequence of this divergence is that entropy will be maximized in the quantum critical regime, and this has recently been directly observed in an elegant experiment on the field-induced QCP in Sr3Ru2O7 [3]. Here, we further address the relationship between the accumulation of entropy, the divergence as a function of both temperature (T) and control parameter (r) in the Gruneisen ratio, and the crossover energy scales in the T-r phase diagram. We consider these in some detail in the simplest examples of QCPs: the transverse-field Ising chain, and the transitions into itinerant magnets. We report the result of microscopic calculations of the entropy as a function of the control parameter r in both models. We show that, for the transverse-field Ising chain, there is an unusual contrast between the T- and r- dependence of the Gruneisen ratio.[1] L. Zhu et al, PRL 91, 066404 (2003);[2] P.Gegenwart et al, Nature Phys. 4, 186 (2008);[3] A.W. Rost et al, Science 325, 1360 (2009). [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A38.00002: Thermal expansion and Gr\"{u}neisen parameter in quantum Griffiths phases Thomas Vojta We consider the behavior of the Gr\"{u}neisen parameter, the ratio between thermal expansion and specific heat, at pressure-tuned infinite-randomness quantum-critical points and in the associated quantum Griffiths phases. We find that the Gr\"{u}neisen parameter diverges as $\ln(T_0/T)$ with vanishing temperature $T$ in the quantum Griffiths phases. At the infinite-randomness critical point itself, the Gr\"{u}neisen parameter behaves as $[\ln(T_0/T)]^{1+1/(\nu\psi)}$ where $\nu$ and $\psi$ are the correlation length and tunneling exponents. Analogous results hold for the magnetocaloric effect at magnetic-field tuned transitions. We contrast clean and dirty systems, we discuss subtle differences between Ising and Heisenberg symmetries, and we relate our findings to recent experiments on CePd$_{1-x}$Rh$_x$. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A38.00003: High-temperature signatures of quantum criticality in heavy-fermion systems J. Kroha , L. Borda , M. Klein , F. Reinert , P. Simon , O. Stockert , H. von L\"ohneysen We propose a new criterion for distinguishing the Hertz-Millis (HM) and local quantum critical (LQC) scenarios of magnetic quantum phase transitions (QPT) in heavy-fermion systems from their high-temperature behavior [1]. The criterion is based on our finding, using perturbative and numerical renormalization group techniques, that the complete screening of a single Kondo spin can be suppressed by the RKKY coupling to the surrounding magnetic ions even without magnetic ordering. As a consequence, the signature of Kondo breakdown can be observed in spectroscopic measurements above the lattice coherence and magnetic ordering temperatures, where fluctuations of the Fermi surface and quantum critical fluctuations do not play a role. We show that the predicted dependence of the screening scale $T_K$ on the RKKY coupling agrees in detail with recent scanning tunneling microscopy (STM) results on two-impurity Kondo systems. Applying the resulting high-temperature criterion to high-resolution photoemission measurements on CeCu$_{6-x}$Au$_{x}$ suggests that the QPT in this system is dominated by the LQC scenario. \\ \noindent [1] M. Klein {\it et al.}, PRL {\bf 101}, 266404 (2008); PRB {\bf 79}, 075111 (2009). [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A38.00004: Pressure-induced superconductivity and effective mass enhancement near antiferromagnetic quantum critical point in CePt$_{2}$In$_{7}$ Eric D. Bauer , H. O. Lee , V. A. Sidorov , N. Kurita , K. Gofryk , F. Ronning , Tuson Park , R. Movshovich , J. D. Thompson The discovery of the CeMIn$_{5}$ (M=Co, Rh, Ir) family of heavy fermion superconductors has been a watershed for the field of heavy fermion physics. These materials have not only provided an effective means to explore the rich interplay of magnetism and superconductivity (e.g., CeRhIn$_{5})$, the development of the heavy fermion state (e.g., Ce$_{1-x}$La$_{x}$CoIn$_{5})$, and quantum criticality (e.g., CeRhIn$_{5})$, but have also provided compelling evidence that structural tuning plays an essential role in enhancing their superconducting properties. I will present our discovery of superconductivity in a new, more two-dimensional member of this Ce$_{m}$M$_{n}$In$_{2m+3n}$ family, CePt$_{2}$In$_{7}$, which displays the coexistence of antiferromagnetism and superconductivity and an enhancement of the effective mass under pressure near an antiferromagnetic quantum critical point that is remarkably similar to CeRhIn$_{5}$. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A38.00005: Anomalous magnetic moment suppression in the superconducting and ferromagnetic coextistence region in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ P.-C. Ho , D.E. MacLaughlin , L. Shu , S. Zhao , J.M. Mackie , M.B. Maple , T. Yanagisawa A previous study [1] of the effect of the ferromagnetism (FM) on unconventional superconductivity (SC) in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ found that SC and FM are both suppressed toward a critical concentration $x_{cr,1} \sim 0.6$, and the $x$ dependence of the upper critical field $H_{c2}$ has a curvature break at $x_{cr,2} \sim 0.3$. The specific heat measurements indicate that FM extends into the SC region. In order to probe the FM in the SC-FM coexistence region, $\mu$-SR measurements are performed on the samples near $x_{cr,1}$ ($x =$ 0.55, 0.5, and 0.45). A small quasistatic field $\sim 40$\,Gauss was found in the field cooled state of these samples ($H = 10$\,Oe) and this field is corresponding to a frozen Nd moment of $\sim 0.1$\,$\mu_B$, which is much smaller than the CEF ground state moment of the Nd$^{3+}$ ion ($\sim 1.36 \mu_B$). The origin of the moment reduction in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ is unclear currently. The Kondo effect, which is usually involved in such a reduction, has never been observed in Nd-based materials. [1] Ho, et. al., 2009 APS March Meeting, A41.00005 (2009); manuscript in preparation (2009). [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A38.00006: Scaling and relaxational dynamics near Kondo-destroying quantum critical points Jedediah Pixley , Matthew Glossop , Stefan Kirchner , Qimiao Si We study the finite-temperature dynamical scaling in the vicinity of the Kondo-destroying quantum critical points in two quantum impurity models. For the pseudogap Anderson model, we use a combination of renormalization group, continuous time quantum Monte Carlo and large-N techniques to obtain the complete scaling functions of the local susceptibility and single-electron Green's function both in the coherent ( $\omega >$T) and relaxational ( $\omega <$T) regime [1]. We establish that the relaxation rate is linear in temperature for both quantities. The result for the Green's function is reminiscent of recent experimental findings in the Kondo-destroying quantum critical point of heavy fermion metals [2]. For the Bose-Fermi Kondo model, we report related results derived from the continuous time quantum Monte Carlo and large-N methods. \\[4pt] [1] M. T. Glossop, S. Kirchner, J. H. Pixley, and Q.Si, ``Critical Kondo destruction in a pseudogap Anderson model: scaling and relaxational dynamics,'' to be published (2009). \\[0pt] [2] S. Friedemann et al., to be published (2009). [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A38.00007: Berry phase effects in the quantum phase transitions of Kondo lattice systems Pallab Goswami , Qimiao Si There is considerable interest to determine the various quantum phases and phase transitions in the global phase diagram of the antiferromagnetic heavy fermion systems. Recent studies on the Kondo lattice model have used a quantum nonlinear sigma model representation. While these studies have captured the physics of the antiferromagnetic ordered phase with a Kondo destruction and, correspondingly, the small Fermi surface, how to access the phase transition from the antiferromagnetic side remains an open problem. It is to be expected that topological defects and the Berry's phase are important to the physics of Kondo singlet formation. Here, we consider the simpler case of one-dimensional Kondo-Heisenberg lattice model at and away from the half-filling. We demonstrate the crucial role that instanton configurations of the non-linear-sigma-model fields play in leading to Kondo-like properties. The case of Kondo lattice systems in higher dimensions is also briefly commented on. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A38.00008: BCS superconductivity in quantum critical metals Jian-Huang She , Jan Zaanen We consider the superconducting transition in fermionic quantum critical systems. Assuming the validity of Migdal theorem, the gap equation can be written in terms of the retarded pair susceptibility. Scale invariance leads to an algebraic gap equation, instead of the BCS exponential form. With reasonably small glue strength, we can get very large transition temperatures comparable to those observed in cuprates. We get nice dome structures around the QCP for a wide range of parameters. With a non-Lorentzian dynamical exponent, the upper critical field is greatly enhanced when approaching the critical point, while the transition temperature only changes modestly, in agreement with recent experiments on heavy fermions. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A38.00009: Competing states in the Kondo breakdown model Kangjun Seo , Catherine Pepin , Ilya Vekhter We investigate the emergence of ordered states around the Kondo breakdown quantum critical point in the slave boson description of the extended Kondo-Heisenberg and Anderson models. We consider the competition between magnetic, superconducting and spin gap states, and discuss the stability of the Kondo breakdown picture towards states competing with the simplest spin liquids considered so far. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A38.00010: Role of Phonons in Heavy Fermion Volume Collapse within the Periodic Anderson Model P. Reis , P. Zhang , M.A. Majidi , F. Assaad , T. Pruschke , J. Moreno , M. Jarrell , A.K. McMahan Recent X-ray and neutron diffraction studies indicate the involvement of phonons in the volume collapse of Cerium. Whether phonons are driven the volume change, or the effect has an electronic origin and phonons play a secondary role, is unknown. We address this problem within the Periodic Anderson Model using Continuous Time Quantum Monte Carlo as the cluster solver within DMFA and DCA. We consider several possibilities of Holstein phonons coupled with the local 4f electronic density, the d electron density, or the hybridization term. We calculate the temperature and volume dependence of the Debye frequency for these different electron-phonon couplings and evaluate the change in entropy during the transition. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A38.00011: Dynamics after a sweep through a quantum critical point Joel Moore , Frank Pollmann , Subroto Mukerjee , Andrew Green The coherent quantum evolution of a one-dimensional many-particle system after slowly sweeping the Hamiltonian through a critical point is studied using a generalized quantum Ising model containing both integrable and non-integrable regimes. It is known from previous work that universal power laws of the sweep rate appear in such quantities as the mean number of excitations created by the sweep. Several other phenomena are found that are not reflected by such averages: there are two scaling regimes of the entanglement entropy and a relaxation that is power-law in time rather than exponential. The final state of evolution after the quench is not characterized by any effective temperature, and the Loschmidt echo converges algebraically for long times, with cusplike singularities in the integrable case that are dynamically broadened by nonintegrable perturbations. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A38.00012: Correlations in a Band Insulator Michael Sentef , Jan Kunes , Arno P. Kampf , Philipp Werner Using DMFT we find a discontinuous band-to-Mott insulator transition upon an increase in the local Coulomb repulsion in a covalent band insulator [1,2], defined as a band insulator with partially filled local orbitals. The corresponding band gap is a hybridization gap arising from a particular pattern of hopping integrals. Similar characteristics apply to materials such as FeSi, FeSb$_{2}$ or CoTiSb [3], some of which exhibit temperature dependent magnetic and transport properties reminiscent of Kondo insulators. Both charge and spin gaps in the covalent band insulator shrink with increasing Coulomb repulsion. At moderate interaction strengths the gap renormalization is well described by a renormalization factor analogous to the quasiparticle weight in a Fermi liquid. \\[4pt] [1] M. Sentef, J. Kune\v{s}, P. Werner, and A.P. Kampf, Phys. Rev. B \textbf{80}, 155116 (2009) \\[0pt] [2] A.P. Kampf, M. Kollar, J. Kune\v{s}, M. Sentef, and D. Vollhardt, arXiv:0910.5126 [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A38.00013: Critical metal phase at the Anderson metal-insulator transition with Kondo impurities Eduardo Mucciolo , Stefan Kettemann , Imre Varga It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. When the exchange coupling $J$ is smaller than a certain value $J^\ast$, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states. The width of this critical region increases with a power of the concentration of magnetic impurities. [S. Kettemann, E. R. Mucciolo, and I. Varga, Phys. Rev. Lett. {\bf 103}, 126401 (2009).] [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A38.00014: DMRG study of the periodic Anderson model Pedro Bertussi , Marcello Silva Neto , Tatiana Rappoport , Raimundo dos Santos , Andre Malvezzi We study the ground state of the one-dimensional symmetric periodic Anderson model for various band fillings, $n$, and for several values of the on-site repulsion $U$, and of the hybridization $V$ between the $c$- and $f$-bands. Through the Density Matrix Renormalization Group (DMRG) method, we calculate magnetic correlation functions, and their structure factors, charge and pairing correlations, as well as inter-orbital correlations, such as $\langle c^\dagger f + h.c. \rangle $, $\langle {\bf S}^c\cdot {\bf S}^f\rangle $; we also obtain charge and spin gaps. The analysis of these quantities allows a thorough characterization of the system, which can be summarized in a phase diagram in the parameter space $(U,V,n)$. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A38.00015: Novel Electronic Order with Staggered Kondo and Crystalline Electric Field (CEF) Singlets Shintaro Hoshino , Jun'ya Otsuki , Yoshio Kuramoto With two localized f-electrons per site, the ground state of a solid can be a collection of CEF singlets plus the Fermi sea of conduction electrons. If the f-electrons interact strongly with conduction electrons, the ground state can also be a collective Kondo singlet. Competition between these two singlets may give rise to rich physics with exotic ordered phases. We propose a novel staggered order of these two kinds of singlets by using a model with CEF singlet-triplet states and conduction electrons, which are connected together by the Kondo coupling. We have performed highly accurate numerical calculations using the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. With one conduction electron per site, we have found a metal-insulator transition temperature below which the staggered singlet order emerges. The computed local density of states shows a peak below the Fermi level at a sublattice for Kondo singlet, and a vacant peak at another sublattice for CEF singlet. These two peaks add up to a double peaked structure as in Kondo insulators. We discuss possible relevance of the results to understanding actual systems such as PrFe$_{4}$P$_{12}$ and URu$_{2}$Si$_{2}$. [Preview Abstract] |
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