104 research outputs found
Explaining the specific heat of liquids based on instantaneous normal modes
No full textThe successful prediction of the specific heat of solids is a milestone in the kinetic theory of matter due to Debye. No such success, however, has ever been obtained for the specific heat of liquids, which has remained a mystery for over a century. A theory of specific heat of liquids is derived here using a recently proposed analytical form of the vibrational density of states of liquids, which takes into account saddle points in the liquid energy landscape via the so-called instantaneous normal modes (INMs), corresponding to negative eigenvalues (imaginary frequencies) of the Hessian matrix. The theory is able to explain the typical monotonic decrease in specific heat with temperature observed in liquids in terms of the average INM excitation lifetime decreasing with T (in accordance with the Arrehnius law) and provides an excellent single-parameter fitting to several sets of experimental data for atomic and molecular liquids. It also correlates the height of the liquid energy barrier with the slope of the specific heat in the function of temperature in accordance with the available data. These findings demonstrate that the specific heat of liquids is controlled by the instantaneous normal modes, i.e., by localized unstable (exponentially decaying) vibrational excitations and provide the missing connection among anharmonicity, saddle points in the energy landscape, and the thermodynamics of liquids
Chasing the cuprates with dilatonic dyons
No full textAbstract: Magnetic field and momentum dissipation are key ingredients in describing condensed matter systems. We include them in gauge/gravity and systematically explore the bottom-up panorama of holographic IR effective field theories based on bulk EinsteinMaxwell Lagrangians plus scalars. The class of solutions here examined appears insufficient to capture the phenomenology of charge transport in the cuprates. We analyze in particular the temperature scaling of the resistivity and of the Hall angle. Keeping an open attitude, we illustrate weak and strong points of the approach
Deformations, relaxation, and broken symmetries in liquids, solids, and glasses: A unified topological field theory
No full textWe combine hydrodynamic and field theoretic methods to develop a general theory of phonons as Goldstone bosons in crystals, glasses, and liquids based on nonaffine displacements and the consequent Goldstone phase relaxation. We relate the conservation, or lack thereof, of specific higher-form currents with properties of the underlying deformation field-nonaffinity-which dictates how molecules move under an applied stress or deformation. In particular, the single-valuedness of the deformation field is associated with conservation of higher-form charges that count the number of topological defects. Our formalism predicts, from first principles, the presence of propagating shear waves above a critical wave vector in liquids, thus giving a formal derivation of the phenomenon in terms of fundamental symmetries. The same picture provides also a theoretical explanation of the corresponding "positive sound dispersion" phenomenon for longitudinal sound. Importantly, accordingly to our theory, the main collective relaxation timescale of a liquid or a glass (known as the a relaxation for the latter) is given by the phase relaxation time, which is not necessarily related to the Maxwell time. Finally, we build a non-equilibrium effective action using the in-in formalism defined on the Schwinger-Keldysh contour, that further supports the emerging picture. In summary, our work suggests that the fundamental difference between solids, fluids, and glasses has to be identified with the associated generalized higher-form global symmetries and their topological structure, and that the Burgers vector for the displacement fields serves as a suitable topological order parameter distinguishing the solid (ordered) phase and the amorphous ones (fluids, glasses)
Anharmonic theory of superconductivity and its applications to emerging quantum materials
No full textThe role of anharmonicity on superconductivity has often been disregarded in the past. Recently, it has been recognized that anharmonic decoherence could play a fundamental role in determining the superconducting properties (electron-phonon coupling, critical temperature, etc) of a large class of materials, including systems close to structural soft-mode instabilities, amorphous solids and metals under extreme high-pressure conditions. Here, we review recent theoretical progress on the role of anharmonic effects, and in particular certain universal properties of anharmonic damping, on superconductivity. Our focus regards the combination of microscopic-agnostic effective theories for bosonic mediators with the well-established BCS theory and Migdal-Eliashberg theory for superconductivity. We discuss in detail the theoretical frameworks, their possible implementation within first-principles methods, and the experimental probes for anharmonic decoherence. Finally, we present several concrete applications to emerging quantum materials, including hydrides, ferroelectrics and systems with charge density wave instabilities
Correlation between optical phonon softening and superconducting in YBaCuO within -wave Eliashberg theory
No full textWe provide a mathematical description, based on d-wave Eliashberg theory, of the strong correlation between the experimentally observed softening of Raman modes associated with in-plane oxygen motions and the corresponding superconducting critical temperature , as a function of oxygen doping , in YBaCuO. The theoretical model provides a direct link between physical trends of soft optical (in-plane) oxygen modes, the level of oxygen doping , and the superconducting . Different regimes observed in the trend of vs doping can be related to corresponding regimes of optical phonon softening in the Raman spectra. These results provide further evidence related to the physical origin of high-temperature superconductivity in rare-earth cuprate oxides and to the significant role of electron-phonon coupling therein.Accepted for publication in J. Phys. Material
Anharmonic phonon damping enhances the Tc of BCS-type superconductors
No full textA theory of superconductivity is presented where the effect of anharmonicity, as entailed in the acoustic, or optical, phonon damping, is explicitly considered in the pairing mechanism. The gap equation is solved including diffusive Akhiezer damping for longitudinal acoustic phonons or Klemens damping for optical phonons, with a damping coefficient which, in either case, can be directly related to the Gruneisen parameter and hence to the anharmonic coefficients in the interatomic potential. The results show that the increase of anharmonicity has a strikingly nonmonotonic effect on the critical temperature T-c. The optimal damping coefficient yielding maximum T-c is set by the velocity of the bosonic mediator. This theory may open up unprecedented opportunities for material design where T-c may be tuned via the anharmonicity of the interatomic potential, and presents implications for the superconductivity in the recently discovered hydrides, where anharmonicity is very strong and for which the anharmonic damping is especially relevant
Experimental identification of topological defects in 2D colloidal glass
No full textTopological defects are singularities in the order parameter space that are
mathematically described by topological invariants and cannot be removed by
continuous transformations. These defects play a significant role in various
fields, ranging from cosmology to solid-state physics and biological matter.
The definition of these irregularities requires an ordered reference
configuration, leading to decades of debate about the existence of topological
defects in disordered systems, such as glasses. Recently, it has been proposed
that well-defined topological defects might emerge in the dynamical properties
of glasses under deformation, potentially relating to their plastic behavior.
In this study, we investigate a two-dimensional colloidal glass system composed
of particles interacting by an effective magnetic repulsion. We reveal the
presence of topological defects in the eigenspace of the vibrational
frequencies of this experimental 2D amorphous solid. The vibrational density of
states of this 2D amorphous material exhibits distinct glassy properties such
as the presence of a boson peak anomaly. Through extensive numerical and
theoretical quantitative analysis, we establish a robust positive correlation
between the vibrational characteristics and the total number of topological
defects. We furthermore show that defects of opposite charge tend to pair
together and prove their local statistical correlation with the "soft spots",
the regions more prone to plastic flow. This work provides the experimental
confirmation for the existence of topological defects in disordered systems
revealing a complex interplay between topology, disorder, and vibrational
behavior
Physics of Phonon-Polaritons in Amorphous Materials
No full textThe nature of bosonic excitations in disordered materials has remained elusive due to the difficulties in defining key concepts such as quasi-particles in the presence of disorder. We report on an experimental observation of phonon-polaritons in glasses, including a prominent boson peak (BP), i.e., excess of THz modes over the Debye law. A theoretical framework based on the concept of diffusons is developed to describe the broadening linewidth of the polariton due to disorder-induced scattering. It is shown here for the first time that the BP frequency and the Ioffe-Regel (IR) crossover frequency of the polariton collapse onto the same power-law decay with the diffusivity of the bosonic excitation. This analysis dismisses the hypothesis of the BP being caused by a relic of the van Hove singularity. The presented framework establishes a new methodology to analyze bosonic excitations in amorphous media, well beyond the traditional case of acoustic phonons, and establishes the IR crossover as the fundamental physical mechanism behind the BP
Possible enhancement of the superconducting due to sharp Kohn-like soft phonon anomalies
No full textPhonon softening is a ubiquitous phenomenon in condensed matter systems which
is often associated with charge density wave (CDW) instabilities and
anharmonicity. The interplay between phonon softening, CDW and
superconductivity is a topic of intense debate. In this work, the effects of
anomalous soft phonon instabilities on superconductivity are studied based on a
recently developed theoretical framework that accounts for phonon damping and
softening within the Migdal-Eliashberg theory. Model calculations show that the
phonon softening in the form of a sharp dip in the phonon dispersion relation,
either acoustic or optical (including the case of Kohn-type anomalies typically
associated with CDW), can cause a manifold increase of the electron-phonon
coupling constant . This, under certain conditions, which are
consistent with the concept of optimal frequency introduced by Bergmann and
Rainer, can produce a large increase of the superconducting transition
temperature . In summary, our results suggest the possibility of reaching
high-temperature superconductivity by exploiting soft phonon anomalies
restricted in momentum space.Comment: v3: matching the published versio
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