Showing papers on "Phase transition published in 2010"

Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures

Abstract: We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in semiconductor-superconductor heterostructures. The experimental system consists of one-dimensional semiconductor wire with strong spin-orbit Rashba interaction embedded into a superconducting quantum interference device. We show that the energy spectra of the Andreev bound states at the junction are qualitatively different in topologically trivial (i.e., not containing any Majorana) and nontrivial phases having an even and odd number of crossings at zero energy, respectively. The measurement of the supercurrent through the junction allows one to discern topologically distinct phases and observe a topological phase transition by simply changing the in-plane magnetic field or the gate voltage. The observation of this phase transition will be a direct demonstration of the existence of Majorana particles.

Many-body localization phase transition

Abstract: We use exact diagonalization to explore the many-body localization transition in a random-field spin-1/2 chain. We examine the correlations within each many-body eigenstate, looking at all high-energy states and thus effectively working at infinite temperature. For weak random field the eigenstates are thermal, as expected in this nonlocalized, ``ergodic'' phase. For strong random field the eigenstates are localized with only short-range entanglement. We roughly locate the localization transition and examine some of its finite-size scaling, finding that this quantum phase transition at nonzero temperature might be showing infinite-randomness scaling with a dynamic critical exponent $z\ensuremath{\rightarrow}\ensuremath{\infty}$.

Dicke quantum phase transition with a superfluid gas in an optical cavity

Abstract: A phase transition describes the sudden change of state of a physical system, such as melting or freezing. Quantum gases provide the opportunity to establish a direct link between experiments and generic models that capture the underlying physics. The Dicke model describes a collective matter-light interaction and has been predicted to show an intriguing quantum phase transition. Here we realize the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and observe the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-range interactions between the condensed atoms, induced by two-photon processes involving the cavity mode and a pump field. We show that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. Our results should facilitate studies of quantum gases with long-range interactions and provide access to novel quantum phases.

Interdependent networks: reducing the coupling strength leads to a change from a first to second order percolation transition.

Abstract: We study a system composed from two interdependent networks A and B, where a fraction of the nodes in network A depends on nodes of network B and a fraction of the nodes in network B depends on nodes of network A. Because of the coupling between the networks, when nodes in one network fail they cause dependent nodes in the other network to also fail. This invokes an iterative cascade of failures in both networks. When a critical fraction of nodes fail, the iterative process results in a percolation phase transition that completely fragments both networks. We show both analytically and numerically that reducing the coupling between the networks leads to a change from a first order percolation phase transition to a second order percolation transition at a critical point. The scaling of the percolation order parameter near the critical point is characterized by the critical exponent � ¼ 1.

A morphotropic phase boundary system based on polarization rotation and polarization extension

Abstract: Many ferroelectric solid solutions exhibit enhanced electromechanical properties at the morphotropic boundary separating two phases with different orientations of polarization. The mechanism of properties enhancement is associated with easy paths for polarization rotation in anisotropically flattened free energy profile. Another mechanism of properties enhancement related to free energy flattening is polarization extension. It is best known at temperature-driven ferroelectric-paraelectric phase transitions and may lead to exceedingly large properties. Its disadvantage is temperature instability of the enhancement. In this paper a temperature-composition phase diagram is proposed that exhibits compositionally driven-phase transitions with easy paths for both polarization rotation and polarization extension.

Effects of interactions on the topological classification of free fermion systems

Abstract: We describe in detail a counterexample to the topological classification of free fermion systems. We deal with a one-dimensional chain of Majorana fermions with an unusual T symmetry. The topological invariant for the free fermion classification lies in Z, but with the introduction of interactions the Z is broken to Z_8. We illustrate this in the microscopic model of the Majorana chain by constructing an explicit path between two distinct phases whose topological invariants are equal modulo 8, along which the system remains gapped. The path goes through a strongly interacting region. We also find the field-theory interpretation of this phenomenon. There is a second-order phase transition between the two phases in the free theory, which can be avoided by going through the strongly interacting region. We show that this transition is in the two-dimensional Ising universality class, where a first-order phase transition line, terminating at a second-order transition, can be avoided by going through the analog of a high-temperature paramagnetic phase. In fact, we construct the full phase diagram of the system as a function of the thermal operator (i.e., the mass term that tunes between the two phases in the free theory) and two quartic operators, obtaining a first-order Peierls transition region, a second-order transition region, and a region with no transition.

Mechanics and Physics of Porous Solids

Abstract: Foreword 1 The Strange World of Porous Solids 2 Fluid Mixtures 2.1 Chemical potential 2.2 Gibbs-Duhem Equality 2.3 Ideal Mixtures 2.4 Regular Solutions 3 The Deformable Porous Solid 3.1 Strain 3.2 Stress 3.3 Strain Work 3.4 From Solids to Porous Solids 4 The Saturated Poroelastic Solid 4.1 The Poroelastic Solid 4.2 Filling the Porous Solid 4.3 The Thermoporoelastic Solid 4.4 The Poroviscoelastic Solid 5 Fluid Transport and Deformation 5.1 Transport Laws 5.2 Coupling the Deformation and the Flow 5.3 Consolidation of a Soil Layer 6 Surface Energy and Capillarity 6.1 Physics and Mechanics of Interfaces 6.2 Capillarity in Porous Solids 6.3 Transport in Unsaturated Porous Solids 7 The Unsaturated Poroelastic Solid 7.1 Interface Stress as a Pre-Stress 7.2 Energy Balance for the Unsaturated Porous Solid 7.3 The Linear Unsaturated Poroelastic Solid 7.4 Extending Linear Unsaturated Poroelasticity 8 Uncon.ned Phase Transition 8.1 Chemical Potential and Phase Transition 8.2 Liquid-Vapor Transition 8.3 Liquid-Solid Transition 8.4 Gas bubble formation 8.5 Surface Energy and Phase Transition 9 Phase Transition in Porous Solids 9.1 In-Pore Phase Transition 9.2 Kinetics and Mechanics of Drying 9.3 Mechanics of Con.ned Crystallization 10 The Poroplastic Solid 10.1 Basic Concepts of Plasticity 10.2 From Plasticity to Poroplasticity 10.3 From material to structure 11 By Way of Conclusion

Universal Behavior and Electric-Field-Induced Structural Transition in Rare-Earth-Substituted BiFeO3

Abstract: The discovery of a universal behavior in rare-earth (RE)-substituted perovskite BiFe0 3 is reported. The structural transition from the ferroelectric rhombohedral phase to an orthorhombic phase exhibiting a double-polarization hysteresis loop and substantially enhanced electromechanical properties is found to occur independent of the RE dopant species. The structural transition can be universally achieved by controlling the average ionic radius of the A-site cation. Using calculations based on first principles, the energy landscape of BiFe0 3 is explored, and it is proposed that the origin of the double hysteresis loop and the concomitant enhancement in the piezoelectric coefficient is an electric-field-induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase.

Suppression of Octahedral Tilts and Associated Changes in Electronic Properties at Epitaxial Oxide Heterostructure Interfaces

Abstract: Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition near the interface in BiFeO3 and elucidate associated changes in electronic properties in a thin layer directly adjacent to the interface.

Dicke-model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity.

Abstract: We show that the motion of a laser-driven Bose-Einstein condensate in a high-finesse optical cavity realizes the spin-boson Dicke model. The quantum phase transition of the Dicke model from the normal to the superradiant phase corresponds to the self-organization of atoms from the homogeneous into a periodically patterned distribution above a critical driving strength. The fragility of the ground state due to photon measurement induced backaction is calculated.

Giant magnetocaloric effects by tailoring the phase transitions

Abstract: The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications

Universal behavior for magnetic entropy change in magnetocaloric materials: An analysis on the nature of phase transitions

Abstract: A universal curve for the change in the magnetic entropy has been recently proposed for materials with second-order phase transitions. In this work we have studied the universal behavior of the magnetocaloric effect in the family of cobalt Laves phases, RCo2, and mixed manganites, La2/3CaxSr1�x1/3MnO3, which exhibit first- and second-order phase transitions. The rescaled magnetic entropy change curves for different applied fields collapse onto a single curve for materials with second-order phase transition as opposed to the first-order phase transition compounds, for which this collapse does not hold. This result suggests that the universal curve may be used as a further criterion to distinguish the order of the phase transition.

Dynamical Phase Transitions and Instabilities in Open Atomic Many-Body Systems

Abstract: We discuss an open driven-dissipative many-body system, in which the competition of unitary Hamiltonian and dissipative Liouvillian dynamics leads to a nonequilibrium phase transition. It shares features of a quantum phase transition in that it is interaction driven, and of a classical phase transition, in that the ordered phase is continuously connected to a thermal state. We characterize the phase diagram and the critical behavior at the phase transition approached as a function of time. We find a novel fluctuation induced dynamical instability, which occurs at long wavelength as a consequence of a subtle dissipative renormalization effect on the speed of sound.

Solvent-Induced Structural Transition of Self-Assembled Dipeptide: From Organogels to Microcrystals

Abstract: Organogels that are self-assembled from simple peptide molecules are an interesting class of nano- and mesoscale soft matter with simplicity and functionality. Investigating the precise roles of the organic solvents and their effects on stabilization of the formed organogel is an important topic for the development of low-molecular-weight gelators. We report the structural transition of an organogel self-assembled from a single dipeptide building block, diphenylalanine (L-Phe-L-Phe, FF), in toluene into a flower-like microcrystal merely by introducing ethanol as a co-solvent; this provides deeper insights into the phase transition between mesostable gels and thermodynamically stable microcrystals. Multiple characterization techniques were used to reveal the transitions. The results indicate that there are different molecular-packing modes formed in the gels and in the microcrystals. Further studies show that the co-solvent, ethanol, which has a higher polarity than toluene, might be involved in the formation of hydrogen bonds during molecular self-assembly of the dipeptide in mixed solvents, thus leading to the transition of organogels into microcrystals. The structural transformation modulated by the co-solvent might have a potential implication in controllable molecular self-assembly.

The Classical Theory

Abstract: Publisher Summary This chapter outlines the theoretical framework for the most commonly used theory of nucleation—the classical theory. Phase transformations in the region of metastability are initiated within the original phase by the nucleation of the small regions of the new phase, which then grow to macroscopic dimensions. The nucleation barrier arises from the energy penalty for creating an interface between the cluster and the original phase. If the phase transition is thermodynamically favored, sufficiently large clusters of the new phase must have a free energy lower than the same atoms retaining the configuration of the original phase. However, the atoms in the region of the interface between the original and new phases are in a higher energy state than they would have in the two macroscopic phases. Fluctuations are not limited to the metastable region; they also occur in systems in equilibrium. However, only under metastable conditions there is an extended range of stability for fluctuations combined with a characteristic energy barrier to give the distinctive phenomena of nucleation.

Phase transitions in confined water nanofilms

Abstract: Phase transitions in water are normally classified as first or second order. But in confined quasi-one-dimensional films of water, simulations show that the solid–liquid transition can take place by means of a first-order transition or a continuous one without a distinction between solid and liquid.

Computational Studies of Quantum Spin Systems

Abstract: These lecture notes introduce quantum spin systems and several computational methods for studying their ground‐state and finite‐temperature properties. Symmetry‐breaking and critical phenomena are first discussed in the simpler setting of Monte Carlo studies of classical spin systems, to illustrate finite‐size scaling at continuous and first‐order phase transitions. Exact diagonalization and quantum Monte Carlo (stochastic series expansion) algorithms and their computer implementations are then discussed in detail. Applications of the methods are illustrated by results for some of the most essential models in quantum magnetism, such as the S = 1/2 Heisenberg antiferromagnet in one and two dimensions, as well as extended models useful for studying quantum phase transitions between antiferromagnetic and magnetically disordered states.

Multichannel generalization of Kitaev's Majorana end states and a practical route to realize them in thin films.

Abstract: The ends of one-dimensional $p+ip$ superconductors have long been predicted to possess localized Majorana fermion modes [A. Kitaev, arXiv:cond-mat/0010440]. We show that Majorana end states are robust beyond the strict 1D single-channel limit, so long as the sample width is not much larger than the superconducting coherence length, and they exist when an odd number of transverse quantization channels are occupied. Consequently, the system undergoes a sequence of topological phase transitions driven by changing the chemical potential. These observations make it feasible to implement quasi-1D $p+ip$ superconductors in metallic thin-film microstructures, which offer 3--4 orders of magnitude larger energy scales than semiconductor-based schemes. Some promising candidate materials are described.

QCD phase transition in a strong magnetic background

Abstract: We investigate the properties of the deconfining/chiral restoring transition for two flavor QCD in the presence of a uniform background magnetic field. We adopt standard staggered fermions and a lattice spacing of the order of 0.3 fm. We explore different values of the bare quark mass, corresponding to pion masses in the range 200--480 MeV, and magnetic fields up to $|e|B\ensuremath{\sim}0.75\text{ }\text{ }{\mathrm{GeV}}^{2}$. The deconfinement and chiral symmetry restoration temperatures remain compatible with each other and rise very slightly ($l2%$ for our largest magnetic field) as a function of the magnetic field. On the other hand, the transition seems to become sharper as the magnetic field increases.

Explosive percolation transition is actually continuous.

Abstract: Recently a discontinuous percolation transition was reported in a new "explosive percolation" problem for irreversible systems [D. Achlioptas, R. M. D'Souza, and J. Spencer, Science 323, 1453 (2009)] in striking contrast to ordinary percolation. We consider a representative model which shows that the explosive percolation transition is actually a continuous, second order phase transition though with a uniquely small critical exponent of the percolation cluster size. We describe the unusual scaling properties of this transition and find its critical exponents and dimensions.

Evidence for a first-order liquid-liquid transition in high-pressure hydrogen from ab initio simulations

Abstract: Using quantum simulation techniques based on either density functional theory or quantum Monte Carlo, we find clear evidence of a first-order transition in liquid hydrogen, between a low conductivity molecular state and a high conductivity atomic state. Using the temperature dependence of the discontinuity in the electronic conductivity, we estimate the critical point of the transition at temperatures near 2,000 K and pressures near 120 GPa. Furthermore, we have determined the melting curve of molecular hydrogen up to pressures of 200 GPa, finding a reentrant melting line. The melting line crosses the metalization line at 700 K and 220 GPa using density functional energetics and at 550 K and 290 GPa using quantum Monte Carlo energetics.

Imaging the Fano lattice to ‘hidden order’ transition in URu 2 Si 2

Abstract: Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called ‘heavy fermions’. In URu2Si2 these effects begin at temperatures around 55 K but they are suddenly altered by an unidentified electronic phase transition at To = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu2Si2 electronic structure simultaneously in r-space and k-space. Above To, the ‘Fano lattice’ electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below To, a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below To of a light k-space band into two new heavy fermion bands. Thus, the URu2Si2 ‘hidden order’ state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states. A long-standing mystery in condensed matter physics is that of the appearance of a 'hidden order' state in URu2Si2 at low temperature, an unexpected phase change that is accompanied by a sharp change in bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state (already at a higher temperature) where electron-like charge carriers propagate through the solid with an effective mass thousands of times larger than that of a free electron. Schmidt et al. have now used scanning tunnelling microscopy and spectroscopy to visualize the electronic structure of URu2Si2 with subatomic resolution. In the process, they observe the electronic structure associated with a magnetic 'Kondo' lattice, which was assumed to cause heavy fermion effects, but never observed directly. Further, the spectroscopic findings show how the hidden order state evolves with decreasing temperature from this lattice. A longstanding mystery in condensed-matter physics involves the appearance of a 'hidden order' state in URu2Si2 at low temperature — an unexpected phase change that is accompanied by a sharp change in the bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state. Here, scanning tunnelling microscopy and spectroscopy have been used to image the electronic structure of URu2Si2 at sub-atomic resolution, revealing how the hidden order state evolves with decreasing temperature.

Analytic holographic superconductor

Abstract: We investigate a holographic superconductor that admits an analytic treatment near the phase transition. In the dual $3+1$-dimensional field theory, the phase transition occurs when a scalar operator of scaling dimension two gets a vacuum expectation value. We calculate current-current correlation functions along with the speed of second sound near the critical temperature. We also make some remarks about critical exponents. An analytic treatment is possible because an underlying Heun equation describing the zero mode of the phase transition has a polynomial solution. Amusingly, the treatment here may generalize for an order parameter with any integer spin, and we propose a Lagrangian for a spin-two holographic superconductor.

Reversible Phase Transitions in Self-Assembled Monolayers at the Liquid−Solid Interface: Temperature-Controlled Opening and Closing of Nanopores

Abstract: We present a variable-temperature study of monolayer self-assembly at the liquid-solid interface By means of in situ scanning tunneling microscopy (STM), reversible phase transitions from a nanoporous low-temperature phase to a more densely packed high-temperature phase are observed The occurrence of the phase transition and the respective transition temperature were found to depend on the type of solvent and solute concentration Estimates of the entropic cost and enthalpic gain upon monolayer self-assembly suggest that coadsorption of solvent molecules within the cavities of the nanoporous structure renders this polymorph thermodynamically stable at low temperatures At elevated temperatures, however, desorption of these relatively weakly bound solvent molecules destabilizes the nanoporous polymorph, and the densely packed polymorph becomes thermodynamically favored Interestingly, the structural phase transition provides external control over the monolayer morphology and, for the system under discussion, results in an effective opening and closing of supramolecular nanopores in a two-dimensional molecular monolayer

Visualizing Critical Correlations Near the Metal-Insulator Transition in Ga1-xMnxAs

Abstract: Electronic states in disordered conductors on the verge of localization are predicted to exhibit critical spatial characteristics indicative of the proximity to a metal-insulator phase transition. We used scanning tunneling microscopy to visualize electronic states in Ga(1-x)Mn(x)As samples close to this transition. Our measurements show that doping-induced disorder produces strong spatial variations in the local tunneling conductance across a wide range of energies. Near the Fermi energy, where spectroscopic signatures of electron-electron interaction are the most prominent, the electronic states exhibit a diverging spatial correlation length. Power-law decay of the spatial correlations is accompanied by log-normal distributions of the local density of states and multifractal spatial characteristics.

Extended mapping and exploration of the vanadium dioxide stress-temperature phase diagram.

Abstract: Single-crystal micro- and nanomaterials often exhibit higher yield strength than their bulk counterparts. This enhancement is widely recognized in structural materials but is rarely exploited to probe fundamental physics of electronic materials. Vanadium dioxide exhibits coupled electronic and structural phase transitions that involve different structures existing at different strain states. Full understanding of the driving mechanism of these coupled transitions necessitates concurrent structural and electrical measurements over a wide phase space. Taking advantages of the superior mechanical property of micro/nanocrystals of VO2, we map and explore its stress-temperature phase diagram over a phase space that is more than an order of magnitude broader than previously attained. New structural and electronic aspects were observed crossing phase boundaries at high-strain states. Our work shows that the actively tuning strain in micro/nanoscale electronic materials provides an effective route to investigate ...

The ``Coulomb Phase'' in Frustrated Systems

Abstract: The “Coulomb phase” is an emergent state for lattice models (particularly highly frustrated antiferromagnets), which have local constraints that can be mapped to a divergence-free “flux.” The coarse-grained versions of this flux or polarization behave analogously to electric or magnetic fields; in particular, defects at which the local constraint is violated behave as effective charges with Coulomb interactions. I survey the derivation of the characteristic power-law correlation functions and the pinch points in reciprocal space plots of diffuse scattering, as well as applications to magnetic relaxation, quantum-mechanical generalizations, phase transitions to long-range-ordered states, and the effects of disorder.

Nematic Liquid Crystals: From Maier-Saupe to a Continuum Theory

Abstract: We define a continuum energy functional that effectively interpolates between the mean-field Maier-Saupe energy and the continuum Landau-de Gennes energy functional and can describe both spatially homogeneous and inhomogeneous systems. In the mean-field approach the main macroscopic variable, the Q-tensor order parameter, is defined in terms of the second moment of a probability distribution function. This definition imposes certain constraints on the eigenvalues of the Q-tensor order parameter, which may be interpreted as physical constraints. We define a thermotropic bulk potential which blows up whenever the eigenvalues of the Q-tensor order parameter approach physically unrealistic values. As a consequence, the minimizers of this continuum energy functional have physically realistic order parameters in all temperature regimes. We study the asymptotics of this bulk potential and show that this model also predicts a first-order nematic-isotropic phase transition, whilst respecting the physical constrain...

Quantum phase transitions in holographic models of magnetism and superconductors

Abstract: We study a holographic model realizing an ``antiferromagnetic'' phase in which a global $SU(2)$ symmetry representing spin is broken down to a $U(1)$ by the presence of a finite electric charge density. This involves the condensation of a neutral scalar field in a charged anti--de Sitter black hole. We observe that the phase transition for both neutral and charged (as in the standard holographic superconductor) order parameters can be driven to zero temperature by a tuning of the UV conformal dimension of the order parameter, resulting in a quantum phase transition of the Berezinskii-Kosterlitz-Thouless--type. We also characterize the antiferromagnetic phase and an externally forced ferromagnetic phase by showing that they contain the expected spin waves with linear and quadratic dispersions, respectively.