Low-temperature Contribution Research Articles

Study of low-dimensional magnetism in zeolitic imidazolate frameworks

Applying the procedure of a rapid synthesis in the aqueous system, nanocrystalline form of ZIF-8 and Cu-ZIF was obtained. The composition of the latter was inferred from the comparison of vibration spectra and lattice specific heats of both compounds which become nearly identical. The nanocrystalline character of both compounds prevented direct comparison of powder XRD patterns and manifested by the appearance of a boson peak in the diamagnetic ZIF-8 specific heat. The low-temperature magnetic contribution in Cu-ZIF specific heat and a Curie-Weiss tail in low-temperature susceptibility may originate from the effect of surface spins while the round maximum observed in the susceptibility at 135 K indicates the presence of strong low-dimensional correlations with the exchange coupling ∼200 K. Corresponding magnetic contribution cannot be observed in the specific heat due to the huge overlap by the high-temperature lattice contribution. Negligible magnetization values at low temperatures correspond to the aforementioned strong antiferromagnetic coupling. Future experimental scenarios are discussed which should provide better understanding of the bulk and surface magnetism in this unique quantum system.

Casimir Contribution to the Interfacial Hamiltonian for 3D Wetting.

Previous treatments of three-dimensional (3D) short-ranged wetting transitions have missed an entropic or low-temperature Casimir contribution to the binding potential describing the interaction between the unbinding interface and wall. This we determine by exactly deriving the interfacial model for 3D wetting from a more microscopic Landau-Ginzburg-Wilson Hamiltonian. The Casimir term changes the interpretation of fluctuation effects occurring at wetting transitions so that, for example, mean-field predictions are no longer obtained when interfacial fluctuations are ignored. While the Casimir contribution does not alter the surface phase diagram, it significantly increases the adsorption near a first-order wetting transition and changes completely the predicted critical singularities of tricritical wetting, including the nonuniversality occurring in 3D arising from interfacial fluctuations. Using the numerical renormalization group, we show that, for critical wetting, the asymptotic regime is extremely narrow with the growth of the parallel correlation length characterized by an effective exponent in quantitative agreement with Ising model simulations, resolving a long-standing controversy.

Specific Heat of the Kagome Antiferromagnet Herbertsmithite in High Magnetic Fields

Measuring the specific heat of herbertsmithite single crystals in high magnetic fields (up to $34$ T) allows us to isolate the low-temperature kagome contribution while shifting away extrinsic Schottky-like contributions. The kagome contribution follows an original power law $C_{p}(T\rightarrow0)\propto T^{\alpha}$ with $\alpha\sim1.5$ and is found field-independent between $28$ and $34$ T for temperatures $1\leq T\leq4$ K. These are serious constrains when it comes to replication using low-temperature extrapolations of high-temperature series expansions. We manage to reproduce the experimental observations if about $10$ % of the kagome sites do not contribute. Between $0$ and $34$ T, the computed specific heat has a minute field dependence then supporting an algebraic temperature dependence in zero field, typical of a critical spin liquid ground state. The need for an effective dilution of the kagome planes is discussed and is likely linked to the presence of copper ions on the interplane zinc sites. At very low temperatures and moderate fields, we also report some small field-induced anomalies in the total specific heat and start to elaborate a phase diagram.

Systems layouts with additional heat storages for low-temperature PVT collectors assisting Heat Pumps for DHW production

Systems based on the coupling of heat pumps (HP) with solar hybrid photovoltaic and thermal technology (PVT) for domestic hot water (DHW) production are a valid alternative to conventional electric production systems. In previous research, simulation models were developed in order to study the dynamic behaviour of a plant set-up based on the integration of an air-to-water heat pump aimed at DHW production with a rated power of 700 W and a storage tank of 500 lt. It was coupled with a plant of 6 PVT flat uncovered hybrid solar collectors with a peak power of 300 W for each module. Starting from the results obtained in the pre-sizing stage, in this study, a plant configuration was analysed to maximize the exploitation of the low-temperature contribution provided by the adopted PVT collectors. With this aim, the efficacy of the introduction of a solar water preheating storage in addition to the one normally integrated into the HP was investigated. Different operational scenarios have been studied to evaluate the best energy management strategies to be implemented. They consider the influence that the thermal capacity of the solar storage could have on the system performance according to its volume and the setpoint temperatures. The obtained results show that the proposed plant solution generally allows an increase in solar energy exploitation for DHW production and a reduction in HP electricity consumption.

High-temperature 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 multiferroic: Microstructure, ferroelectric properties, and Mössbauer effect

Research findings of the microstructure, dielectric, ferroelectric characteristics, and Mössbauer effect of solid solution ceramics with 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 composition in a wide temperature range are presented. The examined ceramic chip surface allows one to draw conclusions about the internal homogeneity of grains and the absence of pores inside them. It was shown that Fe3+ iron cations in the material are valence and they are found only in seven locally different states, which is associated with disorder in the solid solution structure. The Néel temperature is TN ~ 445 K. The anomalous behavior at T < 30 K becomes clear when analyzing the dielectric spectra of 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 ceramics in the range of 10 … 1000 K. It is explained by the appearance of a spin-glass state in the object. The presence of contributions to the dielectric response in ceramics at T > 300 K is revealed. It is claimed that the ferroelectric–relaxor → paraelectric phase transition caused the low-temperature contribution, and the second one is a manifestation of the Maxwell–Wagner polarization and the corresponding non-Debye type dielectric relaxation. The causes of the revealed regularities and the prospects for using the material in the thin films form are discussed.

Dressed excitations, thermodynamics and relaxation in the XXZ Heisenberg model

In this note, we discuss the low and high temperature contribution of thermodynamic Bethe ansatz (TBA) dressed excitations in the thermodynamics and energy-magnetization relaxation within the generalized hydrodynamics approach in the linear response regime. In particular, we show how the temperature dependent dispersions of the excitations reproduce well known behavior of the specific heat, magnetic susceptibility, spin and energy Drude weights. In this context, we derive a further formulation of the Drude weights from the finite wavevector relaxation. Furthermore, we contrast the TBA description of thermodynamics and dynamics in terms of a multitude of string excitations to that in terms of a single quasi-particle in low energy effective theories.

Magnetoreflection and Kerr Effect in La2/3Ba1/3MnO3 Films with a Variant Structure

A correlation between the effect of magnetoreflection of natural light and tunnel magnetoresistance has been found in La2/3Ba1/3MnO3 films with a variant structure grown on ZrO2(Y2O3) substrates. It is shown that the magnetoreflection effect and giant magnetoresistance in these films are maximum in the region of magnetic ordering near room temperature (TC ≈ 295 K). Magnetoreflection spectra of a La2/3Ba1/3MnO3 film with a variant structure are formed via the same mechanisms as for films without a variant structure and can be described within the theory of the magnetorefractive effect. The field and temperature dependences of the magnetoreflection exhibit the presence of an additional low-temperature contribution to the reflection of a La2/3Ba1/3MnO3 film due to the tunneling of spin-polarized electrons through structural domain boundaries.

Магнитоотражение и эффект Керра в пленках La-=SUB=-2/3-=/SUB=-Ba-=SUB=-1/3-=/SUB=-MnO-=SUB=-3-=/SUB=- с вариантной структурой

A correlation was found between the magnetoreflection of natural light and tunneling magnetoresistance in La2 / 3Ba1 / 3MnO3 films with a variant structure grown on ZrO2 (Y2O3) substrates. It was shown that the magnetoreflection as well as the colossal magnetoresistance, is maximum in the region of magnetic ordering near room temperature (Tc ~ 295 K) of the films. The magnetoreflection spectra of a La2 / 3Ba1 / 3MnO3 film with the variant structure are formed by the same mechanisms as in the case of films without the variant structure and can be described in terms of the theory of the magnetorefractive effect. The field and temperature dependences of magnetoreflection demonstrate the presence of an additional low-temperature contribution to the reflection of the La2 / 3Ba1 / 3MnO3 film due to tunneling of spin-polarized electrons through the boundaries of structural domains.

Peculiarities of magneto-infrared reflectivity of nanostructured manganite films

The electrical resistance and reflectance of unpolarized infrared radiation have been studied in the La2/3Ba1/3MnO3 films with and without a variant structure. Applying of an external magnetic field leads to appearance of magnetoreflection and colossal magnetoresistance effects with maxima near the Curie temperature of the films. The high-angle boundaries markedly enhance the electric and magnetooptical properties of nanostructured epitaxial films with the variant structure. For example, an additional low-temperature contribution to the magnetoreflection has been revealed in La2/3Ba1/3MnO3/ZrO2(Y2O3) films. This peculiarity is associated with the change of the high-frequency conductivity of the films with the variant structure due to the processes of tunnelling of spin-polarized electrons across the boundaries of structural domains far below the Curie temperature of the sample. The detection of “tunnel magnetoreflection” in the doped lanthanum manganite films could promote extension of the scope of their application in the modern optoelectronics.

Multiwavelength spectroscopy of the black hole candidate MAXI J1813-095 during its discovery outburst

Abstract MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ∼70 d and peaked at LX(0.5–10 keV) ∼ 7.6 × 1036 erg s−1, assuming a distance of 8 kpc. Swift/XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4–1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ∼30 per cent fractional root-mean-square amplitude of the fast variability (0.1–50 Hz) inferred from the only XMM–Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ∼ 0.2 keV), which barely contributes to the total flux (≲8 per cent). The Comptonization y-parameter (∼1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the Very Large Telescope and Gran Telescopio Canarias telescopes during outburst and quiescence, respectively. In both cases, the spectrum lacks emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca ii triplet and H α in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the H α equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G–K star. This favours a distance ≳3 kpc for the X-ray transient.

Debye–Einstein model and anomalies of heat capacity temperature dependences of solid solutions at low temperatures

An approach is proposed for analysing the deviations of the heat capacity Cp(T) of solid solutions from the Kopp–Neumann rule (KNR) ΔC(T) = Cp(T) − CKNR(T). Temperature dependences of the heat capacity Cp(T) of selected compositions of systems (InP)x (InAs)1−x and (GaAs)x (InAs)1−x at temperatures of 5–300 K are analysed in the Debye–Einstein approximation. It was established that in the case of substitution of atoms in the cation subsystem (Ga3+ ↔ In3+) with the same subsystem of anions (As3−), the positive values of ΔC(T) at T 100 K are the result of a decrease in the fraction of the Debye contribution without changing the upper limit of the oscillation frequency. In the case of substitution in the cation subsystem (P3− ↔ As3−) with the invariant cation subsystem (In3+) to the low-temperature positive contribution of the additional low-frequency Einstein mode, a positive part is added from the modified Debye mode having the characteristic temperature θD below the additive value θDKNR. The adequacy of this model is confirmed by Raman scattering data.

Monte Carlo investigation of how interfacial magnetic couplings affect blocking temperature distributions in exchange bias bilayers

Exchange bias in ferromagnetic (F)/antiferromagnetic (AF) bilayers is a function of both the bulk properties of the AF layer and the interfacial properties determining the effective interfacial couplings between the F and AF layers. The distinction between bulk and interface can be clearly revealed in blocking temperature distributions, where AF grain volume distribution results in a high-temperature peak while disordered interfacial magnetic phases produce a low-temperature contribution. However, the coupling conditions producing such bimodal blocking temperature distributions remain to be specified. In this article, we use a granular model which accounts for the disordered interfacial phases by considering small magnetic grains (SGs) with weaker anisotropy and coupling with the F grains at the F/AF interface. The SG are included in the AF material. The coupling conditions producing bimodal blocking temperature distributions were determined. Then, using Monte Carlo simulations, these conditions were validated and the effect of interfacial F-SG coupling on distributions was investigated. We next determined how the ratio between F-SG and F-AF couplings could be used to estimate the surface coverage of the disordered interfacial phases from experimental data.

Low-temperature contribution to the resonant tunneling conductance of a disordered N–I–N junction

A formula for the contribution ΔG res(T) to the resonant tunneling conductance of the N–I–N junction (where N is a normal metal and I is an insulator) with a weak (low impurity concentrations) structural disorder in the I layer from the low-temperature “smearing” electron Fermi surfaces in its N shores is obtained. It is shown that the temperature dependence ΔG res(T) in such a “dirty” junction qualitatively differs from the corresponding dependence ΔG 0(T) in a “pure” (without resonant impurities in the I layer) junction: ΔG res(T) < 0, d(ΔG res)/dT < 0; ΔG 0(T) > 0, d(ΔG 0)/dT > 0, which can serve as an experimental test of the presence of impurity tunneling resonances in the disordered I layer.

Thermal Conductivity through the Quantum Critical Point in YbRh_{2}Si_{2} at Very Low Temperature.

The thermal conductivity of YbRh_{2}Si_{2} has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively, below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point.

Bimodal distribution of blocking temperature for exchange-bias ferromagnetic/antiferromagnetic bilayers: a granular Monte Carlo study with less stable magnetic regions spread over the interface

In exchange bias based devices, ferromagnetic/antiferromagnetic disordered interfacial spins which exhibit low freezing temperatures contribute to the alteration of the exchange bias properties. In particular, the blocking temperature distributions earlier observed experimentally show two contributions: the common high-temperature peak due to the antiferromagnetic grain volume distribution and a less usual low-temperature contribution presumably attributed to disordered interfacial spins. Here, in order to test this assumption Monte Carlo simulations based on a granular level model are used to calculate blocking temperature distribution. Small magnetic grains with weaker anisotropy and interfacial coupling are introduced at the ferromagnetic/antiferromagnetic interface to account for the disordered interfacial spins. As a result, the bimodal character of blocking temperature distributions is reproduced and varying the amount of these smaller magnetic grains tunes the low-temperature contribution of the distribution. These simulations therefore validate the assumption that less stable magnetic regions spread over the ferromagnetic/antiferromagnetic interface may originate the bimodal character of blocking temperature distributions, as earlier brought forward by experimentalists to explain their results. In addition, because of the heterogeneities of the interface in the presence of less stable small grains, our simulations show that such bilayers cannot be described using a simple model of uniform rotation of the ferromagnetic layer interacting with an average interfacial coupling.

Exchange bias and blocking temperature distribution of Fe-film/CoO-nanoparticle hybrid bilayers

We fabricated and characterized Fe-film/CoO-nanoparticle hybrid bilayers and studied their temperature-dependent magnetic properties. The magnetic nanoparticles are uncoupled or weakly coupled in the particle films, which essentially resemble the antiferromagnetic grains in the deposited thin films. The size and size distribution of the nanoparticles can be well-controlled by chemical synthesis prior to assembling, providing additional route for engineering the antiferromagnetic layer in exchange-bias systems. Using cycled annealing-cooling measurements, we observed a unimodal blocking-temperature distribution in our samples that is a reflection of the intrinsic antiferromagnetic monodisperse “grain” size distribution. The absence of the low-temperature contribution is likely attributed to both the fabrication process and the nanoparticle morphology.

Low Temperature Delayed Recombination Decay in Complex Oxide Scintillating Crystals

We study the low temperature contribution to delayed recombination decay in several complex oxide scintillating crystals. We experimentally test the previously suggested hypothesis that the losses of fast scintillation light even at the lowest temperatures can be due to quantum effects. The results obtained for several material systems confirm that quantum tunneling between the luminescence center and a nearby defect is a good candidate for the origin of the observed phenomena.

Anharmonicity and Disorder Effect on the Free Energy of Crystalline Solids

Dealing with the anharmonic phonon-electron problem with the help of double time temperature dependent Green’s Function theory of quantum dynamics the general and explicit expressions for the lattice energy, partition function, free energy have been derived for an impurity induced anharmonic crystalline solid. The effects of mass and force constant change terms between the impurity and host lattice atoms are taken into account, and the cubic and quartic anharmonic terms are also retained in an almost complete Hamiltonian to develop the many body theory. The general trend of low and high temperature contributions to Helmholtz free energy is obtained via three dimensional graphics.

Quantum tunneling and low temperature delayed recombination in scintillating materials

We study the origin of the low temperature contribution to delayed recombination decay in scintillating materials. This contribution represents the loss of fast scintillation light even at the lowest temperatures. The possible role of quantum effects, as previously suggested, is tested both experimentally and theoretically. The experiments were performed on Lu2Si2O7:Pr3+ crystals. The results suggest that quantum tunneling between the luminescence center and a nearby defect is a good candidate for explaining the observed phenomena.

Specific heat of Ce x La1 − xB6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality CexLa1 − xB6 (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB6 samples with various boron isotope compositions (10B, 11B, natB) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature ΘE ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature ΘD ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5−2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.