Low-temperature Properties Of Glasses Research Articles

Quantum sensors for microscopic tunneling systems

The anomalous low-temperature properties of glasses arise from intrinsic excitable entities, so-called tunneling Two-Level-Systems (TLS), whose microscopic nature has been baffling solid-state physicists for decades. TLS have become particularly important for micro-fabricated quantum devices such as superconducting qubits, where they are a major source of decoherence. Here, we present a method to characterize individual TLS in virtually arbitrary materials deposited as thin films. The material is used as the dielectric in a capacitor that shunts the Josephson junction of a superconducting qubit. In such a hybrid quantum system the qubit serves as an interface to detect and control individual TLS. We demonstrate spectroscopic measurements of TLS resonances, evaluate their coupling to applied strain and DC-electric fields, and find evidence of strong interaction between coherent TLS in the sample material. Our approach opens avenues for quantum material spectroscopy to investigate the structure of tunneling defects and to develop low-loss dielectrics that are urgently required for the advancement of superconducting quantum computers.

Are universal “anomalous” properties of glasses at low temperatures truly universal?

The specific heat Cp and other properties of glasses (ranging from amorphous solids to disordered crystals) at low temperatures are well known to be markedly different from those in fully-ordered crystals. For decades, this qualitative, and even quantitative, universal behavior of glasses has been thoroughly studied. However, a clear understanding of its origin and microscopic nature, needless to say, a closed theory, is still lacking. To shed light on this matter, I review the situation in this work, mainly by compiling and discussing measured low-temperature Cp data of many glasses and disordered crystals, as well as highlighting a few exceptions to that “universality rule”. Thus, one can see that, in contrast to other low-temperature properties of glasses, the magnitude of the “glassy” Cp excess at low temperature is far from being universal. Even worse, some molecular crystals without a clear sign of disorder exhibit linear coefficients in Cp larger than those found in many amorphous solids, whereas a few of the latter show negligible values.

Do two-level systems and boson peak persist or vanish in hyperaged geological glasses of amber?

In this work, we extend, review and jointly discuss earlier experiments conducted by us in hyperaged geological glasses, either in Dominican amber (20 million years old) or in Spanish amber from El Soplao (110 million years old). After characterization of their thermodynamic and elastic properties (using Differential Scanning Calorimetry around the glass-transition temperature, and measuring mass density and sound velocity), their specific heat was measured at low and very low temperatures. By directly comparing pristine amber samples (i.e. highly stabilized polymer glasses after ageing for millions of years) to the same samples after being totally or partially rejuvenated, we have found that the two most prominent universal “anomalous” low-temperature properties of glasses, namely the tunnelling two-level systems and the so-called “boson peak”, persist essentially unchanged in both types of hyperaged geological glasses. Therefore, non-Debye low-energy excitations of glasses appear to be robust, intrinsic properties of non-crystalline solids which do not vanish by accessing to very deep states in the potential energy landscape.

Inversion symmetric vs. asymmetric excitations and the low-temperature universal properties of Ar:N2 and Ar:N2:CO glasses

The bias energies of various two-level systems (TLSs) and their strengths of interactions with the strain are calculated for Ar:N2 glass. Unlike the case in KBr:CN, a distinct class of TLSs having weak interaction with the strain and untypically small bias energies is not found. The addition of CO molecules introduces CO flips which form such a class of weakly interacting TLSs, albeit at much lower coupling than that at which they are typically observed in solids. We conclude that because of the absence of a distinct class of weakly interacting TLSs, Ar:N2 is a non-universal glass, the first such system in three dimensions and in ambient pressure. Our results further suggest that Ar:N2:CO may show universal properties, but at temperatures lower than , much smaller than the typical temperature associated with universality, because of the untypical softness of this system. Our results thus shed light on two long-standing questions regarding the low-temperature properties of glasses: the necessary and sufficient conditions for quantitative universality of phonon attenuation, and what dictates the energy scale of ≈ 3 K below which universality is typically observed.

Calorimetric and thermodynamic study of glass-forming monohydroxy alcohols

In this work, we review and concurrently discuss earlier and recent experiments performed on simple aliphatic glass-forming monoalcohols at low temperatures. By doing this, we find an interesting model system to explore different relevant issues concerning molecular glass-forming liquids and even the glass transition phenomenon itself. They include the role played by the molecular aspect ratio in vitrification/crystallisation kinetics, the reported appearance of particular cases of polymorphism and polyamorphism, and the influence of chemical isomerism and the location of the hydrogen bond on lattice dynamics and, hence, on low-temperature properties of glasses.

Logarithmic Temperature Dependence of the Sound Speed in Amorphous Silica at Low Temperatures

The tunneling model has enjoyed considerable success in describing the low temperature properties of glasses. However, departures from the tunneling model have been noted in experiments at very low temperatures (below 100 mK). We have measured the change in the sound speed ΔV/V0 between 1 and 40 mK in an amorphous silica double paddle oscillator oscillating at 14 kHz. Most importantly, the sound speed displayed the logarithmic temperature dependence predicted by the tunneling model to a lower temperature than in previous experiments on amorphous silica. Below 3 mK the sound speed departed from the logarithmic temperature dependence and began to level off. The leveling off can be explained by either an intrinsic effect or thermal decoupling of the sample from the thermometer. By heating the oscillator with a gamma source to determine the thermal resistance of the oscillator, it was found that a relatively large stray heat input (9×10−4 nW) would be needed to cause the leveling off. Further work will be needed to determine whether the leveling off is due to stray heat or intrinsic physics.

Thermal relaxation calorimetry on ethanol at low temperatures

Ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal which by quenching becomes an orientationally-disordered crystal with glassy properties, and the ordinary amorphous glass. Therefore, it appeared as a good model system to investigate low-temperature properties of glasses, including the role played by orientational vs translational disorder. In the present work, we extend and improve previous measurements by implementing higher-accuracy calorimetric methods at low temperatures (especially, two complementary versions of the thermal relaxation method). We have employed these new measuring methods to study the possible effect of water impurities on the calorimetric and thermodynamic behavior of the different solid phases of ethanol.

Quantum effects of electromagnetic fluxes in cold glasses

The effects of electromagnetic fluxes on the low-temperature properties of glasses are discussed within a generalized tunneling model, considering the motion of a charged particle on a closed path in a double-well potential. All thermodynamic properties associated with this type of tunneling states should become periodic functions of the amount of enclosed flux. The oscillatory behaviour of the dielectric response induced by static magnetic and time-dependent electric fields demonstrates the observable action of the electromagnetic flux in glasses at ultra-low temperature.

Beyond the tunneling model: quantum phenomena in ultracold glasses

The low-temperature properties of amorphous solids are governed by atomic tunneling systems, the parameters of which are widely distributed due to the irregular structure of these materials. The standard tunneling model (STM) approximates such tunneling systems by particles moving in a double-well potential. Furthermore it assumes a constant distribution of the relevant parameters and in this way allows an understanding of many low-temperature properties of glasses on a phenomenological basis. However, a number of recent experiments show that the STM in its present form fails to explain some essential aspects of the physics of glasses at temperatures below about 100 mK . As examples we mention the rapid decay of spontaneous polarization echoes, the temperature dependence of the elastic and dielectric properties, the observation of various memory effects and the occurrence of an unexpected magnetic field dependence of the dielectric properties. A possible cause for the shortcomings of the STM might be the interaction between tunneling systems via their elastic and electric moments, which have not been considered in the original formulation of the STM. We will summarize the new findings and will discuss their implications.

Non-TLS relaxations in polymer glasses

Low temperature properties of glasses are successfully described by the two level system (TLS) model. As long as the TLS model is valid, TLS relaxations can be classified as equilibrium or nonequilibrium. A question, which is not clearly answered up to now is where the limit of TLS model applicability is and how the deviations from the TLS model manifest themselves in experiments. Measurements of nonequilibrium relaxations via optical hole burning spectroscopy can help to answer this question. It is shown that relaxations in polymer glasses at temperatures above some boundary, which is, for example, about 4 K for polymethylmethacrylate and polysrtyrene, requires a new model description.

Correlated Persistent Tunneling Currents in Glasses

Low temperature properties of glasses are derived within a generalized tunneling model, considering the motion of charged particles on a closed path in a double-well potential. The presence of a magnetic induction field $\mathit{B}$ violates the time reversal invariance due to the Aharonov-Bohm phase, and leads to flux periodic energy levels. At low temperature, this effect is shown to be strongly enhanced by dipole-dipole and elastic interactions between tunneling systems and becomes measurable. Thus, the recently observed strong sensitivity of the electric permittivity to weak magnetic fields can be explained. In addition, superimposed oscillations as a function of the magnetic field are predicted.

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature ( K) specific heat and heat release of small amorphous and crystalline SiO2 particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P0. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P0 in agreement with the behavior we found for small SiO2 particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses.

Density of Tunneling States and Potential Energy Landscapes of Glasses

Evidence of a correlation between the density of two-level systems and the fragility of glasses are presented for a variety of glasses. More fragile glasses tend to have higher excess low temperature specific heat (T < 1 K) and thus higher density of tunneling states. This correlation can be explained in terms of the different topography of potential energy landscapes between strong and fragile glasses in the configuration space. 97 Despite the success of the tunneling model of two-level systems (TLS) in ex­ plaining most of the low temperature properties of glasses, 1 ) the microscopic origins of tunneling states are still not known for almost all the glasses studied to date. One of the fundamental question is that what are the other key aspects of the materials that are closely related to the properties governed by TLS's. It was found that glass forming process or sample preparation procedure has direct effects to the low tem­ perature properties of glasses. 2 ), 3 ) Thus the relevant properties might be those that are related to glass transitions at which the structural disorder of the supercooled liquid is frozen in the materials. It was first recognized that an empirical correlation exists between glass transition temperature, T 9 , and the phonon mean free path derived from the low temperature thermal conductivity. 3 ), 4 ) Since the phonon mean free path is inversely proportional to the spectral density of TLS, 1 ) the relation im­ plies a correlation between the density of TLS's and T 9 . The correlation was tested in several glass systems undergoing systematically modification. 6 )- 8 ) Partial sup­ port was found in fluoride glasses, electrolyte glasses, water-doped nitrate glasses, and neutron-irradiated and doped silica glasses. 6 ), 7 ) But an extensive analysis of the density of TLS's and T 9 for a large number of different glasses yields no evidence to support the correlation, 8 ) and contradictory cases were found in alkali doped borate glasses. 9 ) It appears that although the correlation exists in some glasses undergoing systematic modification, there is no general dependence of the density of states of TLS on T 9 . 8 ), 9 )

Low temperature properties of glasses: a preliminary study of double well potentials microscopic structure

In this paper we report preliminary results concerning the microscopic structure of double well potentials observed in simulations of a two- dimensional model of glass. In spite of their diversity, it appears that the observed double well potentials share some general features.

Defect interactions in metallic glasses: Acoustic probes.

If the low-temperature properties of glasses are determined by strongly interacting defects, then in a superconducting metallic glass the apparent density of two-level-system (TLS) glass should be greater in the superconducting than in the normal state. This difference affects the acoustic properties of the material. Published measurements of sound attenuation and velocity in ${\mathrm{Pd}}_{30}$${\mathrm{Zr}}_{70}$ exhibit anomalies consistent with an increased density of TLS's in the superconducting state, but further experiments are needed to distinguish the effects caused by changes in the interactions between TLS's as opposed to changes in the coupling between each TLS and the conduction electrons.

Frustrated interactions and tunneling: Two-level systems in glasses.

It is argued that the standard model of weakly interacting two-level systems (TLS) is compatible with the proposal that low-temperature properties of glasses are determined by strongly interacting defects. Typical strongly interacting defects have their tunneling suppressed, and observable tunneling occurs only for TLS which appear to be weakly coupled because their interactions are frustrated. Localized TLS are generated rather than collective modes because tunneling lowers the energy of the system. This explains both the success and the universality of the TLS model.

On the relation between two approaches in the theory of universal low-temperature properties of glasses

Abstract The relation between the soft-configuration model and the recently proposed alternative approach in the theory of the universal low-temperature properties of glasses is briefly discussed. Some features of both approaches are considered and compared to properties of glassy systems.

Theory of low-temperature thermal expansion of glasses.

We have developed a theory of low-temperature thermal expansion of glasses explaining a number of existing experimental data. We assume that thermal expansion, like many other low-temperature properties of glasses, is determined by associated two-level systems (TLS's); this concept has been introduced to explain these properties by Anderson, Halperin, and Varma and by Phillips. Our theory is based on the Karpov-Klinger-Ignat'ev model of two-level systems in glasses. The deformation potential of the TLS's is calculated. We have shown that it consists of two parts: The larger part (of the order of 0.3 eV) is responsible for the observed transport properties of glasses; however, it does not contribute to the thermal expansion of glasses. The latter is caused by a relatively small second part of the deformation potential which is, within logarithmic accuracy, proportional to the TLS's interlevel spacing E.This is why at low temperatures the coefficient of thermal expansion of glasses is approximately a linear function of the temperature. Its sign is determined by a microscopic structure of the TLS. We have calculated the Gr\"uneisen parameter \ensuremath{\Gamma}. It appears to be of the order of (${\mathrm{scrE}}_{a}$/\ensuremath{\Elzxh}${\ensuremath{\omega}}_{D}$${)}^{2/3}$\ensuremath{\simeq}100, where ${\mathrm{scrE}}_{a}$ is an energy of the order of 30 eV and ${\ensuremath{\omega}}_{D}$ is the Debye frequency. Such large values of \ensuremath{\Gamma} are connected with the softness of local anharmonic potentials that produce the TLS's in glasses. Our principal result is the dependence of the coefficient of thermal expansion \ensuremath{\alpha} on the time of experiment, ${\ensuremath{\tau}}_{\mathrm{expt}}$. It is shown that if \ensuremath{\alpha}&lt;0, then after heating glass it is at first contracted and afterwards, after the time about ${10}^{\mathrm{\ensuremath{-}}8}$ sec (at T=0.3 K), a slow expansion begins. At ${\ensuremath{\tau}}_{\mathrm{expt}\mathrm{\ensuremath{\simeq}}1}$ sec the parameter \ensuremath{\Gamma} can have the absolute value of about (1/3) of that at ${\ensuremath{\tau}}_{\mathrm{expt}\mathrm{\ensuremath{\simeq}}{10}^{\mathrm{\ensuremath{-}}8}}$ sec. Such behavior of the thermal expansion coefficient is due to the fact that the contribution of the TLS's with large relative tunnel splitting (${\ensuremath{\Delta}}_{0}$/E\ensuremath{\simeq}1) is negative while that of the TLS's with small relative tunnel splitting (${\ensuremath{\Delta}}_{0}$/E\ensuremath{\ll}1) is positive. The latter, however, have large times of relaxation which can be comparable with the time of experiment. Finally, we discuss the relative role of the TLS's and free-electron contributions to the thermal expansion of metallic glasses.

A microscopic model for the two-level systems in glasses

A microscopic model is introduced, in which the two-level systems are due to the non-trivial topological structure of any glass. Glasses are punctured by Z 2 line defects (characterized by their oddness). Their free energy is of the Yang-Mills type, with both matter and gauge fields belonging to the rotation group SO(3). The gauge symmetry is associated with the local invariance or overall homogeneity of glasses. The solutions of the classical field equations belong to either of two distinct classes. The two classical solutions are related to each other by a gauge transformation: they can therefore tunnel into each other and yield the familia two-level systems responsible for the anomalous elementary excitations and low temperature properties of glasses.

Kapitza conductance of localized surface excitations

Abstract The existing concepts for the universal low temperature properties of glasses are used to calculate the contribution of supposed localized centers in the surface of solids to the heat transfer into adjacient helium. With a similar coupling constant and density of states as in bulk glass, the mechanism can account for the strength and the experimental features of the “anomalous Kapitza conductance” which is universal for all but the most perfect surfaces.