Ashok Razdan
Anomalous Values of Debye temperature have been obtained from Lamb Mossbauer factor measurements of Ge-Se-Ag glasses. We show that this anomaly may arise due to the presence of anharmonic potential at the high spin ferrous site. In this paper, we use q Lamb Mossbauer factor and anharmonic Lamb Mossbauer factor to study this anharmoncity.
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Below 1 K, the specific heat Cp of glasses depends approximately linearly on temperature T, in contrast with the cubic dependence observed in crystals, and which is well understood in terms of the Debye theory. That linear contribution has been ascribed to the existence of two-level systems as postulated by the Tunnelling Model. Therefore, a least-squares linear fit Cp = C_1 T + C_3 T^3 has been traditionally used to determine the specific-heat coefficients, though systematically providing calorimetric cubic coefficients exceeding the elastic coefficients obtained from sound-velocity measureme…
Specific heat of dipolar glasses does not obey Debye law. It is of interest to know if the non-Debye specific heat can be accounted for in terms of Schottky-type specific heat arising from rotational tunneling states of the dipoles. This paper deals with rotational tunneling spectra of NH$_{4}^{+}$ ions and the non-Debye specific heat of mixed salts (e.g. (NH$_{4})_{x}$Rb$_{1-x}$Br) of ammonium and alkali halides which are known to exhibit dipolar glass phase. We have measured specific heat of above mixed salts at low temperatures (1.5 K $< T <$ 15 K). It is seen that while the specific heat o…
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 aging for mill…
Over the past 40 years measurements of the nucleation rates in a large number of silicate glasses have indicated a breakdown in the widely used Classical Nucleation Theory (CNT) for temperatures below that of the peak nucleation rate. The data show that instead of steadily decreasing with decreasing temperature, the work of critical cluster formation enters a plateau, and even starts to increase. While many explanations have been offered to explain this anomaly, none have provided a satisfactory answer. We present the first experimental results that demonstrate that the anomaly is not real, bu…
The atomic structure, electronic structure and physical properties of $(TiZrNbCu)_{(1-x)}Ni_x$ (x $\leq$ 0.5) metallic glasses (MG) were studied in both the high-entropy (0<x<0.35) and the higher Ni concentration range (x $\geq$ 0.35). Atomic structure studies performed with X-ray diffraction and synchrotron powder diffraction provided average atomic volumes, structure factors, radial distribution functions, coordination numbers and packing densities. Electronic structure studies performed using photoemission spectroscopy and low-temperature specific heat provided information about the electro…
A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys. Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static quantities near 250 K. To check whether corresponding observations can be made for other monohydroxy alcohols as well, these experimental methods were applied to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular backbone. All studied liquids exhibit a change of behavior near 250 K which is tentatively ascribed to effects of hydrogen bond cooper…
The glass transition temperature (Tg) is the temperature, after which the supercooled liquid undergoes a dynamical arrest. Usually, the glass network modifiers (e.g., Na2O) affect the behavior of Tg. However, in aluminosilicate glasses, the effect of different modifiers on Tg is still unclear and show an anomalous behavior. Here, based on molecular dynamics simulations, we show that the glass transition temperature decreases with increasing charge balancing cations field strength (FS) in the aluminosilicate glasses, which is an anomalous behavior as compared to other oxide glasses. The results…
Boson peak in the dynamical structure factor of network- and packing-type glasses
研究通过分子动力学模拟和有效介质理论，揭示了玻璃中玻色峰的微观起源，将其与振动激发的波数解析结构联系起来。
We numerically study the evolution of the vibrational density of states $D(ω)$ of zero-temperature glasses when their kinetic stability is varied over an extremely broad range, ranging from poorly annealed glasses obtained by instantaneous quenches from above the onset temperature, to ultrastable glasses obtained by quenching systems thermalised below the experimental glass temperature. The low-frequency part of the density of states splits between extended and quasi-localized modes. Extended modes exhibit a boson peak crossing over to Debye behaviour ($D(ω) \sim ω^2$) at low-frequency, with a…
The low-temperature properties of glasses present important differences with respect to crystalline matter. In particular, models such as the Debye model of solids, which assume the existence of an underlying regular lattice, predict that the specific heat of solids varies with the cube of temperature at low temperatures. Since the 1970s' at least, it is a well established experimental fact that the specific heat of glasses is instead just linear in $T$ at $T \sim 1K$, and presents a pronounced peak when normalized by $T^{3}$, known as the boson peak. Here we present a new approach which sugge…
Analyzing the atomic structure of glassy materials is a tremendous challenge both experimentally and computationally, and the lack of direct, detailed insights into glass structure hinders our ability to navigate structure-property relationships. For instance, the structural origin of the density anomaly in silica glasses - the negative thermal expansion coefficient - is still poorly understood. Simulations based on molecular dynamics (MD) produce atomically resolved structures, but quantifying the role of disorder in the density anomaly is challenging. Here, we propose to use a a graph-theore…
Recent studies (J. Alloys Compd. 695 (2017) 2661) of the electronic structure and properties of $(TiZrNbCu)_{1-x}$$Ni_{x}$ (x$\leq$0.25) amorphous high entropy alloys (a-HEA) have been extended to x=0.5 in order to compare behaviours of a-HEA and conventional Ni-base metallic glasses (MG). The amorphous state of all samples was verified by thermal analysis and X-ray diffraction (XRD). XRD indicated a probable change in local atomic arrangements, i.e. short-range-order (SRO) for x$\geq$0.35. Simultaneously, thermal parameters, such as the first crystallization temperature $T_{x}$ and the liquid…
Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we studied the halomethane family C-Br_n-Cl_4-n (n = 0,1,2) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For n = 1,2 dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br …
Temperature-invariant heat conductivity from compensating crystalline and glassy transport: from the Steinbach meteorite to furnace bricks
研究结合第一性原理与热反射实验，发现晶体传播与玻璃态隧穿在鳞石英中可相互补偿，形成温度不变的 PTI 热导率，并揭示其在陨石与工业耐火砖中的意义。
We investigated the dielectric properties of Pb(Fe1/2Nb1/2)1-xTixO3 single crystals below room temperature. Two dielectric anomalies were detected in sample A while only one was detected in sample B in the temperature range 90~300 K. A Debye-like relaxation with strong frequency dispersion was detected in both samples. The pre-edge XAFS suggests that this dielectric anomaly is induced by the hopping conductivity between Fe2+ and Fe3+. The EXAFS results give us a clear picture of the local structure of iron ions. The weak frequency dependent dielectric anomaly only observed in sample A is suppo…
The low-temperature normal-state specific heat and resistivity curves of various nonmagnetic intermetallic compounds manifest an anomalous thermal evolution. Such an anomaly is exhibited as a break in the slope of the linearized C/T versus T^2 curve and as a drop in the R versus T curve, both at the same T_βγ. It is related, not to a thermodynamic phase transition, but to an anomaly in the density of states curves of the phonon or electron subsystems. On representing these two anomalies as additional Dirac-type delta functions, situated respectively at kB.θ_L (for lattice) and kB.θ_E (for elec…
Physics-Informed Symbolic Regression for Predicting the Glass Transition Temperature of Alkali Borate Glasses
针对碱金属硼酸盐玻璃$T_g$难预测的问题，本研究采用物理启发符号回归推导了具物理可解释性的$T_g$闭式表达式，验证了参数物理一致性，明确了模型不确定性与硼异常的关联，为玻璃性能预测提供了新的透明建模方案。
A fresh look at the vibrational and thermodynamic properties of liquids within the soft potential model
本文在soft potential model框架内重新分析了液体的振动和热力学性质，指出该模型能定性解释液体振动态密度不服从Debye定律及热容随温度单调递减等现象，且计算结果与实验和模拟数据一致。
Anomalies in the temperature dependences of the recoil-free factor, f, and the average center shift, <CS>, measured by 57-Fe Mossbauer Spectroscopy, were observed for the first time in the archetype of the σ-phase alloys system, Fe-Cr. In both cases the anomaly started at the temperature close to the magnetic ordering temperature, and in both cases it was indicative of lattice vibrations hardening. As no magnetostrictive effects were found, the anomalies seem to be entirely due to a spin-phonon coupling. The observed changes in f and in <CS> were expressed in terms of the underlying changes in…
Binding of 57Fe atoms in a metallic chromium was investigated in a Cr-Fe alloy, containing less than 0.1 at% Fe enriched to ~95 % in 57Fe isotope, using 57Fe-site Mossbauer spectroscopy. The binding force was derived from the Debye temperature, T_D, that, in turn, was calculated from the temperature dependence of the central shift of the Mossbauer spectra recorded in the range of 80 to 330 K. Following a temperature dependence of the line width that shows a minimum at ~155 K, two temperature intervals were considered: a low temperature one (LT) ranging from 80 to 155 K, and the T_D - value of …
Stringlet Excitation Model of the Boson Peak
本文提出玻色峰的弦元激发模型，通过指数分布的弦元尺寸给出玻色峰频率的无自由参数解析预测，并与模拟结果定量吻合，同时解释了玻色峰随温度软化和与剪切模量的标度关系。
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