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M. A. Anisimov

Publications and source records attributed to M. A. Anisimov.

18 recordsLinked to original sources
arXiv2026

The analysis of heat capacity of MnGe metallic helimagnet

Zero-field heat capacity of metallic helimagnet MnGe was analyzed based on the results of resistivity decomposition published previously by our group for the same crystal. Current procedure allowed identifying along with ($i$) electronic ($\tildeγ$ $\approx$ 7 mJ/mol$\cdot$K$^2$) and ($ii$) phononic ($Θ_D$ $\approx$ 350 K) components ($iii$) the additional term, caused by the presence of spin fluctuations (SFs). The last contribution was found to exist in a wide range of temperatures in both paramagnetic (PM) and magnetically ordered states. However, its amplitude appears to be significantly lower in comparison with phononic component. The obtained value of spin fluctuation temperature $θ_{sf}$(MnGe) $\approx$ 330 K correlates well with previous estimations, as well as with results of various experiments, which predict the existence of SFs in MnGe at least up to 250 $-$ 300 K.

cond-mat.str-el↗
arXiv2018

High-Resolution Adiabatic Calorimetry of Supercooled Water

Liquid water exhibits anomalous behavior in the supercooled region. A popular hypothesis to explain supercooled water's anomalies is the existence of a metastable liquid-liquid transition terminating at a critical point. The hypothesized phase transition is not directly accessible in a bulk experiment because it is expected to occur in "no-man's" region below the kinetic stability limit of the liquid phase at about 235 K, the temperature of homogeneous ice formation. Therefore, verifications of this hypothesis are usually based on extrapolations from the experimentally accessible region. In this work, we present the results of high-resolution adiabatic calorimetry measurements of cold and supercooled liquid water in the range from 294 to 244 K, the lowest temperature of water's supercooling achieved so far in a bulk adiabatic-calorimetry experiment. The resolution of the measurements is also record-high, with the average statistical (random) error of about 0.1 %. The data are consistent with adiabatic-calorimetry measurements of supercooled water earlier reported by Tombari et al. [Chem. Phys Lett., Vol. 300, P. 749 (1999)] but significantly deviate from differential-scanning calorimetry measurements in emulsified water reported by Angell et al. [J. Phys. Chem., Vol. 86, P. 998 (1982)] and by Archer and Carter [J. Phys. Chem B., Vol. 104, P. 8563 (2000)]. Consequences of the new heat-capacity data in interpretation of the nature of water's anomalies are discussed.

physics.chem-ph↗
arXiv2018

Collective infrared excitation in rare-earth Gd$_x$La$_{1-x}$B$_6$ hexaborides

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides Gd$_x$La$_{1-x}$B$_6$, $x$(Gd)$=0$, 0.01, 0.1, 0.78, 1 are determined in the frequency range 30$-$35000$~cm^{-1}$. In all compounds, in addition to the Drude free-carrier spectral component, a broad excitation is discovered with the unusually large dielectric contribution $Δ$$\varepsilon$=5000 -- 15000 and non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with the dynamic cooperative Jahn-Teller effect of B$_6$ clusters. Analysis of the spectra together with the results of DC and Hall resistivity measurements shows that only 30$-$50$\%$ of the conduction band electrons are contributing to the free carrier conductivity with the rest being involved in the formation of an overdamped excitation, thus providing possible explanation of remarkably low work function of thermoemission of Gd$_x$La$_{1-x}$B$_6$ and non-Fermi-liquid behavior in GdB$_6$ crystals.

cond-mat.mtrl-sci↗
arXiv2017

Entropy-driven liquid-liquid separation in supercooled water

Twenty years ago Poole et al. (Nature 360, 324, 1992) suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of liquid-liquid separation of lower-density and higher-density water. Here we present an explicit thermodynamic model based on this hypothesis, which describes all available experimental data for supercooled water with better quality and with fewer adjustable parameters than any other model suggested so far. Liquid water at low temperatures is viewed as an 'athermal solution' of two molecular structures with different entropies and densities. Alternatively to popular models for water, in which the liquid-liquid separation is driven by energy, the phase separation in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the 'reaction' equilibrium constant. In particular, the model predicts the location of density maxima at the locus of a near-constant fraction (about 0.12) of the lower-density structure.

physics.chem-ph↗
arXiv2015

Suppression of Superconductivity in Lu$_x$Zr$_{1-x}$B$_{12}$ : Evidence of Static Magnetic Moments Induced by Non-Magnetic Impurities

Based on low temperature resistivity, heat capacity and magnetization investigations we show that the unusually strong suppression of superconductivity in Lu$_x$Zr$_{1-x}$B$_{12}$ BSC-type superconductors in the range $x$$<$0.08 is caused by the emergence of static spin polarization in the vicinity of non-magnetic lutetium impurities. The analysis of received results points to a formation of static magnetic moments with $μ_{eff}$$\approx$$3μ_B$ per Lu-ion. The size of these spin polarized nanodomains was estimated to be about 5 $Å$.

cond-mat.str-el↗
arXiv2014

Charge transport in Ho$_x$Lu$_{1-x}$B$_{12}$: Separating Positive and Negative Magnetoresistance in Metals with Magnetic Ions

The magnetoresistance (MR) $Δρ/ρ$ of cage-glass compound Ho$_x$Lu$_{1-x}$B$_{12}$ with various concentration of magnetic holmium ions ($x$$\leq$0.5) has been studied in detail concurrently with magnetization M(T) and Hall effect investigations on high quality single crystals at temperatures 1.9-120 K and in magnetic field up to 80 kOe. The undertaken analysis of $Δρ/ρ$ allows us to conclude that the large negative magnetoresistance (nMR) observed in vicinity of Neel temperature is caused by scattering of charge carriers on magnetic clusters of Ho$^{3+}$ ions, and that these nanosize regions with AF exchange inside may be considered as short range order AF domains. It was shown that the Yosida relation $-Δρ/ρ$$\sim$$M^2$ provides an adequate description of the nMR effect for the case of Langevin type behavior of magnetization. Moreover, a reduction of Ho-ion effective magnetic moments in the range 3-9$μ_B$ was found to develop both with temperature lowering and under the increase of holmium content. A phenomenological description of the large positive quadratic contribution $Δρ/ρ$$\sim$$μ_D^2 H^2$ which dominates in Ho$_x$Lu$_{1-x}$B$_{12}$ in the intermediate temperature range 20-120 K allows to estimate the drift mobility exponential changes $μ_D$$\sim$$T^{-a}$ with $a$=1.3-1.6 depending on Ho concentration. An even more comprehensive behavior of magnetoresistance has been found in the AF state of Ho$_x$Lu$_{1-x}$B$_{12}$ where an additional linear positive component was observed and attributed to charge carriers scattering on the spin density wave (SDW). High precision measurements of $Δρ/ρ=f(H,T)$ have allowed us also to reconstruct the magnetic H-T phase diagram of Ho$_{0.5}$Lu$_{0.5}$B$_{12}$ and to resolve its magnetic structure as a superposition of 4f (based on localized moments) and 5d (based on SDW) components.

cond-mat.str-el↗
arXiv2014

Minimum in the thermal conductivity of supercooled water: a computer simulation study

We report the results of a computer simulation study of the thermodynamic properties and the thermal conductivity of supercooled water as a function of pressure and temperature using the TIP4P-2005 water model. The thermodynamic properties can be represented by a two-structure equation of state consistent with the presence of a liquid-liquid critical point in the supercooled region. Our simulations confirm the presence of a minimum in the thermal conductivity, not only at atmospheric pressure, as previously found for the TIP5P water model, but also at elevated pressures. This anomalous behavior of the thermal conductivity of supercooled water appears to be related to the maximum of the isothermal compressibility or the minimum of the speed of sound. However, the magnitudes of the simulated thermal conductivities are sensitive to the water model adopted and appear to be significantly larger than the experimental thermal conductivities of real water at low temperatures.

cond-mat.stat-mech↗
arXiv2013

Universality versus nonuniversality in asymmetric fluid criticality

Critical phenomena in real fluids demonstrate a combination of universal features caused by the divergence of long-range fluctuations of density and nonuniversal (system-dependent) features associated with specific intermolecular interactions. Asymptotically, all fluids belong to the Ising-model class of universality. The asymptotic power laws for the thermodynamic properties are described by two independent universal critical exponents and by two independent nonuniversal critical amplitudes; other critical amplitudes can be obtained by universal relations. The nonuniversal critical parameters (critical temperature, pressure, and density) can be absorbed in the property units. Nonasymptotic critical behavior of fluids can be divided into two parts, symmetric ("Ising-like") and asymmetric ("fluid-like"). The symmetric nonasymptotic behavior contains a new universal exponent (Wegner exponent) and the system-dependent crossover scale (Ginzburg number) associated with the range of intermolecular interactions, while the asymmetric features are generally described by an additional universal exponent and by three nonasymptotic amplitudes associated with mixing of the physical fields into the scaling fields.

cond-mat.stat-mech↗
arXiv2011

Thermodynamics of supercooled water

We review the available experimental information on the thermodynamic properties of supercooled ordinary and heavy water and demonstrate the possibility of modeling these thermodynamic properties on a theoretical basis. We show that by assuming the existence of a liquid-liquid critical point in supercooled water, the theory of critical phenomena can give an accurate account of the experimental thermodynamic-property data up to a pressure of 150 MPa. In addition, we show that a phenomenological extension of the theoretical model can account for all currently available experimental data in the supercooled region, up to 400 MPa. The stability limit of the liquid state and possible coupling between crystallization and liquid-liquid separation are discussed. It is concluded that critical-point thermodynamics describes the available thermodynamic data for supercooled water within experimental accuracy, thus establishing a benchmark for further developments in this area.

physics.chem-ph↗
arXiv2010

Hall effect in PrB$_6$ and NdB$_6$

Hall effect was studied on the single crystals of antiferromagnets PrB$_6$ and NdB$_6$ at temperatures 2K$<$T$<300$K in magnetic fields up to 8T using the sample rotation technique. At low magnetic fields $μ_0$H$\leq1$T Hall coefficient R$_\mathrm{H}$, which is practically temperature independent in paramagnetic state at 8K$\leq$T$\leq70$K, is characterized by the values of R$_H$(PrB$_6$)$\sim-(4.2\pm0.1)\cdot10^{-4}$ cm$^3$/C and R$_H$(NdB$_6$)$\sim-(4.1\pm0.1)\cdot10^{-4}$ cm$^3$/C. Rather different behaviour of R$_H$ is observed in antiferromagnetic (AF) phases of these hexaborides. For PrB$_6$ the decrease of temperature below T$_N\approx6.7$K is accompanied by a noticeable ($ΔR_H/R_H\sim10%$) elevation of R$_H$($μ_0$H$=1$T) to the values of $-(3.8\pm0.1)\cdot10^{-4}$ cm$^3$/C. On the contrary, the low field Hall coefficient in NdB$_6$ diminishes by about 15% reaching the value R$_H\approx-(4.7\pm0.1)\cdot10^{-4}$ cm$^3$/C in AF state at 2.5K. The increase of magnetic field inducing magnetic transition in the commensurate magnetic phase of PrB$_6$ results in essential R$_H$ changes (up to 10%) at liquid helium temperatures. The anomalous behaviour of the charge transport parameters for RB$_6$ (R=Pr, Nd) found in vicinity of Neel temperature suggests the possible effect of $5d$-states spin density polarization of both in AF and paramagnetic states of the compounds under investigation.

cond-mat.str-el↗
arXiv2006

A droplet near the critical point: the divergence of Tolman's length

Application of "complete scaling" [Kim et al., Phys. Rev. E 67, 061506 (2003); Anisimov and Wang, Phys. Rev. Lett. 97, 25703 (2006)] to the interfacial behavior of fluids shows that Tolman's length, a curvature correction to the surface tension, diverges at the critical point of fluids much more strongly than is commonly believed. The amplitude of the divergence depends on the degree of asymmetry in fluid phase coexistence. A new universal amplitude ratio, which involves this asymmetry, is introduced. In highly asymmetric fluids and fluid mixtures the Tolman length may become large enough near criticality to be detected in precise experiments with microcapillaries and in simulations.

cond-mat.stat-mech↗
arXiv2006

Equation of State for Supercooled Water Near the Liquid-Liquid Critical Point

We have developed a scaled parametric equation of state to describe and predict thermodynamic properties of supercooled water. The equation of state, built on the growing evidence that the critical point of supercooled liquid-liquid water separation exists, is universal in terms of theoretical scaling fields and is shown to belong to the Ising-model class of universality. The theoretical scaling fields are postulated to be analytical combinations of the physical fields, pressure and temperature. The equation of state enables us to accurately locate the "Widom line" (the locus of stability minima) and determine that the critical pressure is considerably lower than predicted by computer simulations.

cond-mat.soft↗
arXiv2004

Probing structural relaxation in complex fluids by critical fluctuations

Complex fluids, such as polymer solutions and blends, colloids and gels, are of growing interest in fundamental and applied soft-condensed-matter science. A common feature of all such systems is the presence of a mesoscopic structural length scale intermediate between atomic and macroscopic scales. This mesoscopic structure of complex fluids is often fragile and sensitive to external perturbations. Complex fluids are frequently viscoelastic (showing a combination of viscous and elastic behaviour) with their dynamic response depending on the time and length scales. Recently, non-invasive methods to infer the rheological response of complex fluids have gained popularity through the technique of microrheology, where the diffusion of probe spheres in a viscoelastic fluid is monitored with the aid of light scattering or microscopy. Here we propose an alternative to traditional microrheology that does not require doping of probe particles in the fluid (which can sometimes drastically alter the molecular environment). Instead, our proposed method makes use of the phenomenon of "avoided crossing" between modes associated with the structural relaxation and critical fluctuations that are spontaneously generated in the system.

cond-mat.soft↗
arXiv2002

Scaling of demixing curves and crossover from critical to tricritical behavior in polymer solutions

In this paper we show that the virial expansion up to third order for the osmotic pressure of a dilute polymer solution, including first-order perturbative corrections to the virial coefficients, allows for a scaling description of phase-separation data for polymer solutions in reduced variables. This scaling description provides a method to estimate the Theta-temperature, where demixing occurs in the limit of vanishing polymer volume fraction $ϕ$ and infinite chain-length $N$, without explicit assumptions concerning the chain-length dependence of the critical parameters $T_c$ and $ϕ_c$. The scaling incorporates three limiting regimes: the Ising limit asymptotically close to the critical point of phase separation, the pure solvent limit and, the tricritical limit for the polymer-rich phase asymptotically close to the Theta point. We incorporate the effects of critical and tricritical fluctuations on the coexistence curve scaling by using renormalization-group methods. We present a detailed comparison with experimental and simulation data for coexistence curves and compare our estimates for the Theta-temperatures of several systems with those obtained from different extrapolation schemes.

cond-mat.soft↗
arXiv2002

Dynamic Crossover to Tricriticality and Anomalous Slowdown of Critical Fluctuations by Entanglements in Polymer Solutions

We have performed accurate dynamic light-scattering measurements near critical demixing points of solutions of polystyrene in cyclohexane with polymer molecular weight ranging from 200,000 to 11.4 million. Two dynamic modes have been observed, ''slow'' and ''fast'', that result from a coupling between diffusive relaxation of critical fluctuations of the concentration and visco-elastic relaxation associated with the entanglement network of the polymer chains. The coupling with the visco-elastic mode causes an additional slowdown of the critical mode on top of the uncoupled diffusion mode. By implementing crossover from critical to theta-point tricritical behavior for both static and dynamic properties, we are able to present a quantitative description of the phenomenon and to obtain a scaling of the visco-elastic parameters as a function of the molecular weight.

cond-mat.soft↗
arXiv2002

Competition of Mesoscales and Crossover to Tricriticality in Polymer Solutions

We show that the approach to asymptotic fluctuation-induced critical behavior in polymer solutions is governed by a competition between a correlation length diverging at the critical point and an additional mesoscopic length-scale, the radius of gyration. Accurate light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 200,000 up to 11.4 million clearly demonstrate a crossover between two universal regimes: a regime with Ising asymptotic critical behavior, where the correlation length prevails, and a regime with tricritical theta-point behavior determined by a mesoscopic polymer-chain length.

cond-mat.soft↗
arXiv2000

Scaling and Crossover to Tricriticality in Polymer Solutions

We propose a scaling description of phase separation of polymer solutions. The scaling incorporates three universal limiting regimes: the Ising limit asymptotically close to the critical point of phase separation, the "ideal-gas" limit for the pure-solvent phase, and the tricritical limit for the polymer-rich phase asymptotically close to the theta point. We have also developed a phenomenological crossover theory based on the near-tricritical-point Landau expansion renormalized by fluctuations. This theory validates the proposed scaled representation of experimental data and crossover to tricriticality.

cond-mat.soft↗