Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A14: Focus Session: Quantum Phase Transitions And Quantum Criticality |
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Sponsoring Units: DMP Chair: Vivien Zapf, Los Alamos National Laboratory Room: 008A |
Monday, March 2, 2015 8:00AM - 8:12AM |
A14.00001: Transport signatures of Majorana quantum criticality realized by dissipative resonant tunneling Huaixiu Zheng, Serge Florens, Harold Baranger We consider theoretically the transport properties of a spinless resonant electronic level coupled to strongly dissipative leads, in the regime of circuit impedance near the resistance quantum [1]. Using the Luttinger liquid analogy, one obtains an effective Hamiltonian expressed in terms of interacting Majorana fermions, in which all environmental degrees of freedom (leads and electromagnetic modes) are encapsulated in a single fermionic bath. A perturbative treatment of the Majorana interaction term yields the appearance of a marginal, linear dependence of the conductance on temperature when the system is tuned to its quantum critical point, in agreement with recent experimental observations [2]. \\[4pt] [1] H. Zheng, S. Florens, and H.U. Baranger, Phys. Rev. B 89, 235135 (2014).\\[0pt] [2] H. T. Mebrahtu, I. V. Borzenets, H. Zheng, Y. V. Bomze, A. I. Smirnov, S. Florens, H. U. Baranger, and G. Finkelstein, Nat. Phys. 9, 732 (2013). [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A14.00002: Metallic transport near a quantum critical point in organic superconductors from a renormalized Boltzmann theory Maryam Shahbazi, Claude Bourbonnais The electrical and thermal transport properties of the normal state of quasi-1D superconductors like Bechgaard salts are investigated by combining the linearised Boltzmann equation and the renormalisation group (RG) method. The collision integral operator is calculated using the Umklapp scattering amplitudes obtained by the RG method yielding the electrical resistivity($\rho$) and Seebeck coefficient($S$). The power law dependence, $\rho(T)\sim T^\alpha$, for resistivity is obtained by changing the antinesting parameter $t_\perp'$ simulating the pressure distance from the quantum critical point (QCP) between spin-density-wave (SDW) and d-wave SC (SCd) in the phase diagram. The resistivity evolves from a linear component ($\alpha\simeq 1$) at the QCP towards a Fermi liquid component ($\alpha\simeq2$) with increasing $t_\perp'$, which confirms an extended region of quantum criticality as a result of interference between SCd and SDW causing an anomalous growth of Umklapp scattering. Its anisotropy is also tied to the $k_\perp$-dependence of hot/cold scattering regions along the Fermi surface. Similar calculations for the Seebeck coefficient show deviations from the usual linear temperature dependence and also a change of sign near a SDW instability. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A14.00003: Magnetic and Ising quantum phase transitions in a model for isoelectronically tuned iron pnictides Jianda Wu, Qimiao Si, Elihu Abrahams Considerations of the bad-metal behavior led to an early proposal for a quantum critical point under a P for As doping in the iron pnictides [1], which has since been experimentally observed [2]. We study here an effective model for the isoelectronically tuned pnictides using a large-$N$ approach [3]. The model contains antiferromagnetic and Ising-nematic order parameters appropriate for $J_1$-$J_2$ exchange-coupled local moments on an Fe square lattice, and a damping caused by coherent itinerant electrons. The zero-temperature magnetic and Ising transitions are concurrent and essentially continuous. The order-parameter jumps are very small, and are further reduced by the inter-plane coupling; quantum criticality hence occurs over a wide dynamical range. Our results provide the basis for further studies on the quantum critical properties in the P-doped iron arsenides. [1] J. Dai, Q. Si, J-X Zhu, and E. Abrahams, PNAS, 106, 4118 (2009) [2] J. Analytis et al, Nature Phys. 10, 194 (2014) ; T. Shibauchi et al., Ann. Rev. Cond. Matt. Phys. 5, 113 (2014); C. de la Cruz, et al., Phys Rev Lett, 104, 017204 (2010). [3] J. Wu, Q. Si, and E. Abrahams, arXiv:1406.5136. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A14.00004: Higgs criticality in a two-dimensional metal Subir Sachdev, Debanjan Chowdhury We analyze a candidate theory for the strange metal near optimal hole-doping in the cuprate superconductors. The theory contains a quantum phase transition between metals with `large' and `small' Fermi surfaces, but the transition does not directly involve any broken global symmetries. The two metals have emergent SU(2) and U(1) gauge fields respectively, and the transition is driven by the condensation of a real Higgs field which carries an adjoint SU(2) charge. We propose a global phase diagram around this Higgs transition, and describe its relationship to a variety of recent experiments on the cuprate superconductors. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A14.00005: Quantum Criticality In Layered YFe$_{2}$Al$_{10}$ L.S. Wu, W.J. Gannon, K. Park, M.S. Kim, I.A. Zaliznyak, J.A. Rodriguez-Rivera, C. Broholm, A.M. Tsvelik, M.C. Aronson Most evidence for quantum criticality has been established in $f$-electron based heavy fermions. The observation and description of the quantum critical behaviors in magnetic systems driven by $d$ electrons has been very limited. YFe$_{2}$Al$_{10}$ is a rare $d$-electron compound where strong divergencies in magnetic susceptibility and specific heat were observed in zero field, although there is no evidence for long range order above 0.02 K We show that $\chi $ and C$_{\mathrm{M}}$/T exhibit T/B$^{0.6}$ scaling, and a scaling function of the singular part of the free energy was proposed that explains all the measured thermal and magnetic properties in a self-consistent way. Scaling analysis indicates that the spatial dimension d in YFe$_{2}$Al$_{10} $is equal to the dynamical exponent $z$. The validation of hyperscaling suggests the effective dimension may be in the range of 1\textless d$_{\mathrm{eff}}=$d$+$z$\le $4, which is in agreement with the observed anisotropy between the ac plane and the b axis Neutron scattering results on single crystals will be discussed. Our experiments suggest that YFe$_{2}$Al$_{10}$ is a novel 3d-electron system that is close to a T$=$0, B$=$0 ferromagnetic transition without the additional tuning. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A14.00006: Quantum critical point of Dirac fermions studied using efficient continuous-time projector quantum Monte Carlo methodÂ Lei Wang, Mauro Iazzi, Philippe Corboz, Matthias Troyer Quantum phase transition (QPT) of Dirac fermions is a fascinating topic both in condensed matter and in high energy physics. Besides its immediate connection to fundamental problems like mass generation and exotic phases of matter, it provides a common playground where state of the art numerical simulations can be crosschecked with various effective field theory predictions, thus deepen our understanding of both fields. The universality class of the QPT is fundamentally different from the usual bosonic field theory because of the coupling to the gapless fermionic mode at the critical point. We study lattice models with spinless and multi-flavor Dirac fermions using the newly developed efficient continuous-time projector quantum Monte Carlo method. Besides eliminating the Trotter error, the method also enables us to directly calculate derivative observables in a continuous range of interaction strengths, thus greatly enhancing the resolution of the quantum critical region. Compatible results are also obtained from infinite projected entangled-pair states calculations. We compare these numerical results with predictions of the Gross-Neveu theory and discuss their physical implications. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A14.00007: Steady-state dynamics and effective temperatures of quantum criticality in open systems Stefan Kirchner, Farzaneh Zamani, Pedro Ribeiro The interest in the dynamics of strongly correlated systems beyond the linear response regime has recently grown tremendously. The limitation in techniques presently available to tackle this regime should be contrasted with the richness of behavior expected for the non-linear regime. The situation may be simpler for steady states near quantum criticality, where concepts like scaling and universality apply. We present our results for the steady-state --both thermal and non-thermal-- scaling functions and steady-state dynamics in a quantum impurity model of local quantum criticality. Our model, the pseudogap Kondo model, allows us to study the concept of effective temperatures near fully interacting as well as trivial fixed points. In the vicinity of each fixed point we establish the existence of an effective temperature --different at each fixed point-- such that the equilibrium fluctuation-dissipation theorem is recovered. Interestingly, steady-state scaling functions in terms of the effective temperatures coincide with the equilibrium scaling functions in terms of the temperature. This result extends to higher correlation functions as is explicitly demonstrated for the Kondo singlet strength. We also study the non-linear charge transport in terms of the effective temperatures [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A14.00008: Quantum phase transitions in the Kondo-necklace model Nader Ghassemi, Shayan Hemmatiyan, Mahsa Rahimi Movassagh, Mahdi Kargarian, Ali T. Rezakhani, Abdollah Langari Kondo-necklace model can describe the magnetic low-energy limit of strongly correlated heavy fermion materials. There exist multiple energy scales in this model corresponding to each phase of the system. Here, we study quantum phase transitions between these different phases, and show the effect of anisotropies in terms of quantum information properties and vanishing energy gap. We employ the perturbative unitary transformations to calculate the energy gap and spin-spin correlations for the model one, two, and three spatial dimensions as well as for the spin ladders. In particular, we show that the method, although being perturbative, can predict the expected quantum critical point by imposing the spontaneous symmetry breaking, which is in good agreement with the results of numerical and Green's function analyses. We also use concurrence, a bipartite entanglement measure, to study the criticality of the model. Absence of singularities in the derivative of the concurrence in 2d and 3d in Kondo-necklace model shows this model has multipartite entanglement. We also discuss the crossover from the one-dimensional to the two-dimensional model via the ladder structure. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A14.00009: Emergent space-time supersymmetry in 3D Weyl semimetals and 2D Dirac semimetals Shao-Kai Jian, Yi-Fan Jiang, Hong Yao Supersymmetry (SUSY) interchanges bosons and fermions but no direct evidences of it have been revealed in nature yet. In this paper, we observe that fluctuating pair density waves (PDW) consist of two complex order parameters which can be superpartners of the unavoidably-doubled Weyl fermions in 3$+$1D lattice models. Using renormalization group, we theoretically show that SUSY emerges at PDW transitions in 3$+$1D Weyl semimetals (2$+$1D Dirac semimetals). We construct explicit fermionic lattice models featuring 3$+$1D Weyl fermions (2$+$1D Dirac fermions) and show that PDW is the leading instability as short-range interactions exceed a critical value and that the N$=$2 SUSY emerges at the continuous PDW transitions. We further discuss possible routes to realize these lattice models and experimental signatures of emergent SUSY at the PDW criticality. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A14.00010: iPEPS studies of emergent supersymmetry at the pair density wave transitions of Dirac fermions in 2+1D Yifan Jiang, Jiquan Pei, Shaokai Jian, Hong Yao We study the quantum phase transition between the Dirac semimetal and pairing density wave (PDW) phase of the spinless honeycomb model with nearest neighbor attractions by doing fermionic projected entangled pairing state (PEPS) algorithm for infinite lattices. It was recently shown by renormalization group (RG) analysis that space-time supersymmetry (SUSY) emerges in Dirac fermions at their PDW transition\footnote{Shao-Kai Jian, Yi-Fan Jiang, and Hong Yao, arXiv:1407.4497}. The connection of our present PEPS studies with the emergent space-time SUSY at the PDW transition shown by RG will be discussed. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A14.00011: How Ubiquitous is Total Electron Transmission through Nanostructures (Quantum Dragons)? Mark Novotny In transport through nanostructures connected to two semi-infinite leads, the transmission probability ${\cal T}(E)$ as a function of the energy $E$ of the incoming electron plays a central role in the Landauer calculation of the electrical conductance $G$. A quantum dragon nanostructure [1] is one which when connected to appropriate leads has total electron transmission for all energies, ${\cal T}(E)$$=$$1$. In two-lead measurements of single-channel quantum dragons, the quantum of conductance, $G_0$$=$$2e^2/h$, should be observed. A quantum dragon may have strong scattering. In [1] the disorder was along the axis of electron propagation, the $z$ axis. We show that quantum dragon nanostructures can be found for strong disorder perpendicular to the $z$ axis. In select types of nanostructures, we find the ratio of the dimension of the parameter space where quantum dragons exist to that of the complete parameter space. The results use the single-band tight-binding model, and are for the case with only one open channel and homogeneous leads. One type of nanostructure with ${\cal T}(E)$$=$$1$ has completely disordered slices perpendicular to the $z$ axis, but identical slices along the $z$ direction. \break [1] M.A. Novotny, Phys. Rev. B {\bf 90}, 165103 [14 pages] (2014) [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A14.00012: Single and Multi-channel Quantum Dragons from Rectangular Nanotubes Zhou Li, Mark Novotny Recently quantum dragons have been discovered theoretically [1]. Quantum dragons are nanostructures with correlated disorder that permit energy-independent total quantum transmission of electrons. Hence the electrical conductance $G$ in a two-terminal measurement should be the conductance quantum $G_0$$=$$2e^2/h$. The single-band tight banding model is used. An example of a single-channel quantum dragon is a rectangular nanotube with disorder along the direction $z$ of the electron propagation [1]. Quantum dragons are obtained by solving the time-independent Schr{\"o}dinger equation to obtain the electrical transmission ${\cal T}$ as a function of the incoming electron energy $E$. A quantum dragon has ${\cal T}(E)$$=$$1$ for all energies. This work generalizes the solution of the time-independent Schr{\"o}dinger equation to the case of more than one open channel, and applies the method to nanotubes formed from rectangular lattices. One can envision such single-walled rectangular nanotubes for iron starting from free-standing single-atom-thick Fe membranes which have recently been obtained experimentally [2]. \break [1] M.A.\ Novotny, Phys.\ Rev.\ B {\bf 90}, 165103 [14 pages] (2014). \break [2] J.\ Zhao, et al., Science {\bf 343}, 1228 (2014). [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A14.00013: Single and Multi-Channel Carbon-based Quantum Dragons Godfred Inkoom, Omadillo Abdurazakov, Mark Novotny In the coherent regime for electrical conductance measurements, two semi-infinite leads are connected to a finite nanostructure, and the nano-device conductance is calculated using the Landauer formula. Any channel $k$ that has transmission for electrons with energy $E$, ${\cal T}_k(E)$$=$$1$ contributes the conductance quantum $G_0$$=$$2e^2/h$. Any nano-device with at least one ${\cal T}_k(E)$$=$$1$ is called a quantum dragon [1]. The transmission probability ${\cal T}_{k}(E)$ can be obtained from the solution of the time-independent Schr{\"o}dinger equation. Uniform leads connected to armchair single-walled carbon nanotubes (SWCNTs) have ${\cal T}(E)$$=$$1$, while when connected to zigzag SWCNT the ${\cal T}(E)$ is less than unity. Appropriately dimerized leads connected to zigzag SWCNTs are quantum dragons, while when connected to armchair SWCNTs ${\cal T}(E)$ is less than unity [1]. We have generalized the matrix method and mapping methods of [1] in order to investigate SWCNTs that can be multi-channel quantum dragons. For example, one can use armchair SWCNT leads to connect to an armchair SWCNT to try to produce a multi-channel quantum dragon. \hfil\break [1] M.A.\ Novotny, Phys.\ Rev.\ B {\bf 90}, 165103 [14 pages] (2014). [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A14.00014: Quantum degradation of the second order phase transitions Sergei Stishov, Alla Petrova, Sergey Gavrilkin, Lubov Klinkova The specific heat, magnetization and thermal expansion of single crystals of antiferromagnetic insulator EuTe were measured at temperatures down to 2 K and in magnetic fields up to 90 kOe. The Neel temperature, being $\sim9.8$ K at H=0, decreases with magnetic field and tends to zero at $\sim76$ kOe, therefore forming the quantum critical point. The heat capacity and thermal expansion coefficient reveal $\lambda$-type anomalies at the second order magnetic phase transition at low magnetic fields, evolving to simple jumps at high magnetic fields and low temperatures, well described in a fluctuation free mean - field theory. The experimental data and the corresponding analysis favor the quantum concept of effective increasing space dimensionality at low temperatures that suppresses a fluctuation divergence at a second order phase transition. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A14.00015: Dynamic spin susceptibility of interacting electron systems Vladimir Zyuzin, Dmitrii Maslov We study the dynamic spin susceptibility of interacting electrons in spatial dimensions from one to three. In all cases, backscattering processes result in non-zero imaginary part of the spin susceptibility above the particle-hole continuum of non-interacting electrons. In one dimension, we employ the renormalization group to go beyond the second order and obtain a general expression for the spin susceptibility. In higher dimensions, we show that the imaginary part of the spin susceptibility arises from the same mechanism as non-analytic corrections to the Fermi-liquid theory. We relate the obtained results to the lifetime of collective spin modes. [Preview Abstract] |
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