Influence Of Rare Regions On Magnetic Quantum Phase Transitions, 2018 Missouri University of Science and Technology

#### Influence Of Rare Regions On Magnetic Quantum Phase Transitions, Rajesh S. Narayanan, Thomas Vojta, Dietrich Belitz, Theodore R. Kirkpatrick

*Thomas Vojta*

The effects of quenched disorder on the critical properties of itinerant quantum magnets are considered. Particular attention is paid to locally ordered rare regions that are formed in the presence of quenched disorder even when the bulk system is still in the nonmagnetic phase. It is shown that these local moments or instantons destroy the previously found critical fixed point in the case of antiferromagnets. In the case of itinerant ferromagnets, the critical behavior is unaffected by the rare regions due to an effective long-range interaction between the order parameter fluctuations.

Local Versus Nonlocal Order-Parameter Field Theories For Quantum Phase Transitions, 2018 Missouri University of Science and Technology

#### Local Versus Nonlocal Order-Parameter Field Theories For Quantum Phase Transitions, Dietrich Belitz, Theodore R. Kirkpatrick, Thomas Vojta

*Thomas Vojta*

General conditions are formulated that allow us to determine which quantum phase transitions in itinerant electron systems can be described by a local Landau-Ginzburg-Wilson (LGW) theory solely in terms of the order parameter. A crucial question is the degree to which the order parameter fluctuations couple to other soft modes. Three general classes of zero-wave-number order parameters, in the particle-hole spin-singlet and spin-triplet channels and in the particle-particle channel, respectively, are considered. It is shown that the particle-hole spin-singlet class does allow for a local LGW theory, while the other two classes do not. The implications of this result for ...

First Order Transitions And Multicritical Points In Weak Itinerant Ferromagnets, 2018 Missouri University of Science and Technology

#### First Order Transitions And Multicritical Points In Weak Itinerant Ferromagnets, Dietrich Belitz, Theodore R. Kirkpatrick, Thomas Vojta

*Thomas Vojta*

It is shown that the phase transition in low-Tc clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical end point. A semiquantitative discussion is given in terms of recent experiments on MnSi, and predictions for other experiments are made.

Generalization Of The Schwartz-Soffer Inequality For Correlated Random Fields, 2018 Missouri University of Science and Technology

#### Generalization Of The Schwartz-Soffer Inequality For Correlated Random Fields, Thomas Vojta, Michael Schreiber

*Thomas Vojta*

We investigate the influence of spatial correlations between the values of the random field on the critical behavior of random-field lattice models and derive a generalized version of the Schwartz-Soffer inequality for the averages of the susceptibility and its disconnected part. At the critical point this leads to a modification of the Schwartz-Soffer exponent inequality for the critical exponents η and η- describing the divergences of the susceptibility and its disconnected part, respectively. It now reads η- ≤ 2η-2y where 2y describes the divergence of the random-field correlation function in Fourier space. As an example we exactly calculate the susceptibility and ...

Generalized Contact Process With Two Symmetric Absorbing States In Two Dimensions, 2018 Missouri University of Science and Technology

#### Generalized Contact Process With Two Symmetric Absorbing States In Two Dimensions, Man Young Lee, Thomas Vojta

*Thomas Vojta*

We explore the two-dimensional generalized contact process with two absorbing states by means of large-scale Monte-Carlo simulations. In part of the phase diagram, an infinitesimal creation rate of active sites between inactive domains is sufficient to take the system from the inactive phase to the active phase. The system, therefore, displays two different nonequilibrium phase transitions. The critical behavior of the generic transition is compatible with the generalized voter universality class, implying that the symmetry-breaking and absorbing transitions coincide. In contrast, the transition at zero domain-boundary activation rate is not critical.

Evidence For Power-Law Griffiths Singularities In A Layered Heisenberg Magnet, 2018 Missouri University of Science and Technology

#### Evidence For Power-Law Griffiths Singularities In A Layered Heisenberg Magnet, Fawaz Hrahsheh, Hatem Barghathi, Priyanka Mohan, Rajesh Narayanan, Thomas Vojta

*Thomas Vojta*

We study the ferromagnetic phase transition in a randomly layered Heisenberg model. A recent strong-disorder renormalization group approach [Phys. Rev. B 81, 144407 (2010)] predicted that the critical point in this system is of exotic infinite-randomness type and is accompanied by strong power-law Griffiths singularities. Here, we report results of Monte-Carlo simulations that provide numerical evidence in support of these predictions. Specifically, we investigate the finite-size scaling behavior of the magnetic susceptibility which is characterized by a non-universal power-law divergence in the Griffiths phase. In addition, we calculate the time autocorrelation function of the spins. It features a very slow ...

Disorder Promotes Ferromagnetism: Rounding Of The Quantum Phase Transition In Sr1-XCaXRuo3, 2018 Missouri University of Science and Technology

#### Disorder Promotes Ferromagnetism: Rounding Of The Quantum Phase Transition In Sr1-XCaXRuo3, Laszlo Demko, Sandor Bordacs, Thomas Vojta, David Nozadze, Fawaz Hrahsheh, Christopher Svoboda, Balazs Dora, Hiroyuki Yamada, Masashi Kawasaki, Yoshinori Tokura, Istvan Kezsmarki

*Thomas Vojta*

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr1-xCaxRuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is ...

Disordered Bosons In One Dimension: From Weak- To Strong-Randomness Criticality, 2018 Missouri University of Science and Technology

#### Disordered Bosons In One Dimension: From Weak- To Strong-Randomness Criticality, Fawaz Hrahsheh, Thomas Vojta

*Thomas Vojta*

We investigate the superfluid-insulator quantum phase transition of one-dimensional bosons with off-diagonal disorder by means of large-scale Monte Carlo simulations. For weak disorder, we find the transition to be in the same universality class as the superfluid-Mott insulator transition of the clean system. The nature of the transition changes for stronger disorder. Beyond a critical disorder strength, we find nonuniversal, disorder-dependent critical behavior. We compare our results to recent perturbative and strong-disorder renormalization group predictions. We also discuss experimental implications as well as extensions of our results to other systems.

Dynamical Conductivity At The Dirty Superconductor-Metal Quantum Phase Transition, 2018 Missouri University of Science and Technology

#### Dynamical Conductivity At The Dirty Superconductor-Metal Quantum Phase Transition, Adrian Del Maestro, Bernd Rosenow, Jose A. Hoyos, Thomas Vojta

*Thomas Vojta*

We study the transport properties of ultrathin disordered nanowires in the neighborhood of the superconductor-metal quantum phase transition. To this end we combine numerical calculations with analytical strong-disorder renormalization group results. The quantum critical conductivity at zero temperature diverges logarithmically as a function of frequency. In the metallic phase, it obeys activated scaling associated with an infinite-randomness quantum critical point. We extend the scaling theory to higher dimensions and discuss implications for experiments.

Effect Of Rare Locally Ordered Regions On A Disordered Itinerant Quantum Antiferromagnet With Cubic Anisotropy, 2018 Missouri University of Science and Technology

#### Effect Of Rare Locally Ordered Regions On A Disordered Itinerant Quantum Antiferromagnet With Cubic Anisotropy, Rajesh S. Narayanan, Thomas Vojta

*Thomas Vojta*

We study the quantum phase transition of an itinerant antiferromagnet with cubic anisotropy in the presence of quenched disorder, paying particular attention to the locally ordered spatial regions that form in the Griffiths region. We derive an effective action where these rare regions are described in terms of static annealed disorder. A one-loop renormalization-group analysis of the effective action shows that for order-parameter dimensions p<4, the rare regions destroy the conventional critical behavior, and the renormalized disorder flows to infinity. For order-parameter dimensions p>4, the critical behavior is not influenced by the rare regions; it is described by the conventional dirty cubic fixed point. We also discuss the influence of the rare regions on the fluctuation-driven first-order transition ...

Do Interactions Increase Or Reduce The Conductance Of Disordered Electrons? It Depends!, 2018 Missouri University of Science and Technology

#### Do Interactions Increase Or Reduce The Conductance Of Disordered Electrons? It Depends!, Thomas Vojta, Frank Epperlein, Michael Schreiber

*Thomas Vojta*

We investigate the influence of electron-electron interactions on the conductance of two-dimensional disordered spinless electrons. We present an efficient numerical method based on diagonalization in a truncated basis of Hartree-Fock states to determine with high accuracy the low-energy properties in the entire parameter space. We find that weak interactions increase the dc conductance in the strongly localized regime while they decrease the dc conductance for weak disorder. Strong interactions always decrease the conductance. We also study the localization of single-particle excitations at the Fermi energy which turns out to be only weakly influenced by the interactions.

Disorder-Induced Rounding Of Certain Quantum Phase Transitions, 2018 Missouri University of Science and Technology

#### Disorder-Induced Rounding Of Certain Quantum Phase Transitions, Thomas Vojta

*Thomas Vojta*

We study the influence of quenched disorder on quantum phase transitions in systems with overdamped dynamics. For Ising order-parameter symmetry disorder destroys the sharp phase transition by rounding because a static order parameter can develop on rare spatial regions. This leads to an exponential dependence of the order parameter on the coupling constant. At finite temperatures the static order on the rare regions is destroyed. This restores the phase transition and leads to a double exponential relation between critical temperature and coupling strength. We discuss the behavior based on Lifshitz-tail arguments and illustrate the results by simulations of a model ...

Dissipation Effects In Random Transverse-Field Ising Chains, 2018 Missouri University of Science and Technology

#### Dissipation Effects In Random Transverse-Field Ising Chains, Jose A. Hoyos, Thomas Vojta

*Thomas Vojta*

We study the effects of Ohmic, super-Ohmic, and sub-Ohmic dissipation on the zero-temperature quantum phase transition in the random transverse-field Ising chain by means of an (asymptotically exact) analytical strong-disorder renormalization-group approach. We find that Ohmic damping destabilizes the infinite-randomness critical point and the associated quantum Griffiths singularities of the dissipationless system. The quantum dynamics of large magnetic clusters freezes completely, which destroys the sharp phase transition by smearing. The effects of sub-Ohmic dissipation are similar and also lead to a smeared transition. In contrast, super-Ohmic damping is an irrelevant perturbation; the critical behavior is thus identical to that of ...

Critical Behavior Of Disordered Quantum Magnets: The Relevance Of Rare Regions, 2018 Missouri University of Science and Technology

#### Critical Behavior Of Disordered Quantum Magnets: The Relevance Of Rare Regions, Rajesh S. Narayanan, Thomas Vojta, Dietrich Belitz, Theodore R. Kirkpatrick

*Thomas Vojta*

The effects of quenched disorder on the critical properties of itinerant quantum antiferromagnets and ferromagnets are considered. Particular attention is paid to locally ordered spatial regions that are formed in the presence of quenched disorder even when the bulk system is still in the paramagnetic phase. These rare regions or local moments are reflected in the existence of spatially inhomogeneous saddle points of the Landau-Ginzburg-Wilson functional. We derive an effective theory that takes into account small fluctuations around all of these saddle points. The resulting free energy functional contains a new term in addition to those obtained within the conventional ...

Critical Behavior Of A Quantum Spherical Model In A Random Field, 2018 Missouri University of Science and Technology

#### Critical Behavior Of A Quantum Spherical Model In A Random Field, Thomas Vojta, Michael Schreiber

*Thomas Vojta*

We consider the influence of a quenched random field on the critical behavior of a quantum spherical model at the zero-temperature quantum phase transition. We find a complete solution of the model for arbitrary translationally invariant pair interactions. It turns out that the critical behavior for zero as well as finite temperatures is dominated by static random field fluctuations rather than by quantum or thermal fluctuations. Therefore the critical behavior close to the zero-temperature quantum phase transition is identical to that close to a finite-temperature transition. The system does not show a crossover from quantum to classical behavior.

Comment On "Critical Behavior Of The Coulomb Glass", 2018 Missouri University of Science and Technology

#### Comment On "Critical Behavior Of The Coulomb Glass", Thomas Vojta, Michael Schreiber

*Thomas Vojta*

No abstract provided.

Breakdown Of Landau-Ginzburg-Wilson Theory For Certain Quantum Phase Transitions, 2018 Missouri University of Science and Technology

#### Breakdown Of Landau-Ginzburg-Wilson Theory For Certain Quantum Phase Transitions, Thomas Vojta, Dietrich Belitz, Rajesh S. Narayanan, Theodore R. Kirkpatrick

*Thomas Vojta*

The quantum ferromagnetic transition of itinerant electrons is considered. It is shown that the Landau-Ginzburg-Wilson theory described by Hertz and others breaks down due to a singular coupling between fluctuations of the conserved order parameter. This coupling induces an effective long-range interaction between the spins of the form 1/r^{2d-1}. It leads to unusual scaling behavior at the quantum critical point in 1 < d ≤ 3 dimensions, which is determined exactly.

Composition-Tuned Smeared Phase Transitions, 2018 Missouri University of Science and Technology

#### Composition-Tuned Smeared Phase Transitions, Fawaz Hrahsheh, David Nozadze, Thomas Vojta

*Thomas Vojta*

Phase transitions in random systems are smeared if individual spatial regions can order independently of the bulk system. In this paper, we study such smeared phase transitions (both classical and quantum) in substitutional alloys A 1-xB x that can be tuned from an ordered phase at composition x=0 to a disordered phase at x=1. We show that the ordered phase develops a pronounced tail that extends over all compositions x<1. Using optimal fluctuation theory, we derive the composition dependence of the order parameter and other quantities in the tail of the smeared phase transition. We also compare ...

Coulomb Gap At Finite Temperatures, 2018 Missouri University of Science and Technology

#### Coulomb Gap At Finite Temperatures, Masoud Sarvestani, Michael Schreiber, Thomas Vojta

*Thomas Vojta*

The Coulomb glass, a model of interacting localized electrons in a random potential, exhibits a soft gap, the Coulomb gap, in the single-particle density of states (DOS) g(ε,T) close to the chemical potential µ. In this paper we investigate the Coulomb gap at finite temperatures T by means of a Monte Carlo method. We find that the Coulomb gap fills with increasing temperature. In contrast to previous results the temperature dependence is, however, much stronger than g(µ,T)~TD-1 as predicted analytically. It can be described by power laws with the exponents 1.75 ± 0.1 for ...

Calculation Of The Residual Resistivity And The Thermoelectric Power Of Sp Impurities In Silver, 2018 Missouri University of Science and Technology

#### Calculation Of The Residual Resistivity And The Thermoelectric Power Of Sp Impurities In Silver, Thomas Vojta, Ingrid Mertig, Rudolf Zeller

*Thomas Vojta*

We present *ab initio* calculations for the residual resistivity and the thermopower of *sp* impurities in silver. The calculations are based on density-functional theory and the Korringa-Kohn-Rostoker Greens-function method. The linearized Boltzmann equation is solved by means of a Fermi-surface harmonic expansion to calculate the residual resistivity, taking into account the anisotropic electronic properties of the dilute alloy. The thermoelectric power was obtained from the energy dependence of the residual resistivity by means of Mott's formula. The results are in satisfying agreement with highly precise experimental results and confirm experimentally obtained deviations from a quadratic dependence on the valence ...