Low-temperature Entropy Research Articles

Enhanced Cryogenic Magnetocaloric Performance in Te Doped Dy2O3

The work aims to investigate the magnetocaloric effect (an eco-friendly and energy-efficient cooling technique) of Te doped nanosized dysprosia, which could be used as the best alternative for conventional chlorofluorocarbons based refrigeration systems. In this present work, Te doped nano-sized dysprosia (TNSD) is synthesized using the sol-gel technique. The particle characteristics and magnetocaloric properties of TNSD were investigated. The change in lattice parameters of NSD concerning doping of TNSD is analyzed by using Rietveld refinement. The synthesized nanoparticles were observed to be spherical and monophasic with a Ia-3 structure. At low temperature, the sample exhibited a non-saturated magnetic behavior due to the co-existence of ferromagnetic and antiferromagnetic phases, while at high temperature it exhibited a paramagnetic nature. The maximum entropy change of TNSD at a magnetic field of 50 kOe was found to be 30.6 JKg−1K−1. The significant magnetic transitions at low temperature and large magnetic entropy change make TNSD suitable material as a refrigerant for cryo-cooling systems.

The NuSTAR, XMM-Newton, and Suzaku View of A3395 at the Intercluster Filament Interface

Galaxy clusters are the largest virialized objects in the universe. Their merger dynamics and their interactions with the cosmic filaments that connect them are important for our understanding of the formation of large-scale structure. In addition, cosmic filaments are thought to possess the missing baryons in the universe. Studying the interaction of galaxy clusters and filaments therefore has the potential to unveil the origin of the baryons and the physical processes that occur during merger stages of galaxy clusters. In this paper, we study the connection between A3395 and the intercluster filament with NuSTAR, XMM-Newton, and Suzaku data. Since the NuSTAR observation is moderately contaminated by scattered light, we present a novel technique developed for disentangling this background from the emission from the intracluster medium. We find that the interface of the cluster and the intercluster filament connecting A3395 and A3391 does not show any signs of heated plasma, as was previously thought. This interface has low temperature, high density, and low entropy, thus we suggest that the gas is cooling, being enhanced by the turbulent or tidal “weather” driven during the early stage of the merger. Furthermore, our temperature results from the NuSTAR data are in agreement with those from XMM-Newton and from joint NuSTAR and XMM-Newton analysis for a region with ∼25% scattered light contamination within 1σ. We show that the temperature constraint of the intracluster medium is valid even when the data are contaminated up to ∼25% for ∼5 keV cluster emission.

Strange Metals from Melting Correlated Insulators in Twisted Bilayer Graphene.

Even as the understanding of the mechanism behind correlated insulating states in magic-angle twisted bilayer graphene converges toward various kinds of spontaneous symmetry breaking, the metallic "normal state" above the insulating transition temperature remains mysterious, with its excessively high entropy and linear-in-temperature resistivity. In this Letter, we focus on the effects of fluctuations of the order parameters describing correlated insulating states at integer fillings of the low-energy flat bands on charge transport. Motivated by the observation of heterogeneity in the order-parameter landscape at zero magnetic field in certain samples, we conjecture the existence of frustrating extended-range interactions in an effective Ising model of the order parameters on a triangular lattice. The competition between short-distance ferromagnetic interactions and frustrating extended-range antiferromagnetic interactions leads to an emergent length scale that forms stripy mesoscale domains above the ordering transition. The gapless fluctuations of these heterogeneous configurations are found to be responsible for the linear-in-temperature resistivity as well as the enhanced low-temperature entropy. Our insights link experimentally observed linear-in-temperature resistivity and enhanced entropy to the strength of frustration or, equivalently, to the emergence of mesoscopic length scales characterizing order-parameter domains.

Kondo physics in the T-shaped structure with two detuned quantum dots

The Kondo effect in the T-shaped double-quantum-dot structure is theoretically investigated, by considering different adjustments of the quantum dot levels. It is shown that when the quantum dot levels are shifted to the electron–hole symmetry point, the two-stage Kondo effect can be induced at the limit of weak interdot coupling. However, with the increase of interdot coupling, the interdot antiferromagnetic correlation modifies the Kondo effect efficiently, leading to the destruction of the conductance plateau. If the dot levels are detuned, the low-temperature entropy transition will exhibit new results and complicated transport behaviors occur. The reason also arises from the competition between the Kondo effect and the interdot antiferromagnetic correlation. We believe that these results are helpful for the further understanding of the Kondo physics in the T-shaped double-dot structure. • Kondo effects in the T-shaped structure are studied by considering different quantum dot levels. • When dot levels are shifted simultaneously to electron–hole symmetry point, the two-stage Kondo effect occurs. • If dot levels are detuned, entropy transition exhibits new results, leading to the discounted Kondo effect. • If one dot level is zero, Kondo effect induces various conductance results, with the shift of other dot level. • This work are helpful for further understanding the Kondo physics in T-shaped double-dot structures.

Low-temperature entropy in JT gravity

For ensembles of Hamiltonians that fall under the Dyson classification of random matrices with β ∈ {1, 2, 4}, the low-temperature mean entropy can be shown to vanish as 〈S(T)〉 ∼ κTβ + 1. A similar relation holds for Altland-Zirnbauer ensembles. JT gravity has been shown to be dual to the double-scaling limit of a β = 2 ensemble, with a classical eigenvalue density propto {e}^{S_0}sqrt{E} when 0 < E ≪ 1. We use universal results about the distribution of the smallest eigenvalues in such ensembles to calculate κ up to corrections that we argue are doubly exponentially small in S0.

Canted antiferromagnetic order and spin dynamics in the honeycomb-lattice compound Tb2Ir3Ga9

Single crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice $\rm Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy crystal-field doublet, the Tb$^{3+}$ ions form a canted antiferromagnetic structure below 12.5 K. Due to the Dzyalloshinskii-Moriya interactions, the Tb moments in the $ab$ plane are slightly canted towards $b$ by $6^\circ$ with a canted moment of 1.22 $\mu_{\rm B} $ per formula unit. A minimal $xxz$ spin Hamiltonian is used to simultaneously fit the spin-wave frequencies along the high symmetry directions and the field dependence of the magnetization along the three crystallographic axes. Long-range magnetic interactions for both in-plane and out-of-plane couplings up to the second nearest neighbors are needed to account for the observed static and dynamic properties. The $z$ component of the exchange interactions between Tb moments are larger than the $x$ and $y$ components. This compound also exhibits bond-dependent exchange with negligible nearest exchange coupling between moments parallel and perpendicular to the 4$f$ orbitals. Despite the $J_{{\rm eff}}=1/2$ moments, the spin Hamiltonian is denominated by a large in-plane anisotropy $K_z \sim -1$ meV. DFT calculations confirm the antiferromagnetic ground state and the substantial inter-plane coupling at larger Tb-Tb distances.

A connection between the ice-type model of Linus Pauling and the three-color problem

The ice-type model proposed by Linus Pauling to explain its entropy at low temperatures is here approached in a didactic way. We first present a theoretically estimated low-temperature entropy and compare it with numerical results. Then, we consider the mapping between this model and the three-colour problem, i.e. colouring a regular graph with coordination equal to 4 (a two-dimensional lattice) with three colours, for which we apply the transfer-matrix method to calculate all allowed configurations for two-dimensional square lattices of N oxygen atoms ranging from 4 to 225. Finally, from a linear regression of the transfer matrix results, we obtain an estimate for the case N → ∞ which is compared with the exact solution by Lieb.

Further insights into the thermodynamics of the Kitaev honeycomb model

Here we revisit the thermodynamics of the Kitaev quantum spin liquid realized on the honeycomb lattice. We address two main questions: First, we investigate whether there are observable thermodynamic signatures of the topological Majorana boundary modes of the Kitaev honeycomb model. We argue that for the time-reversal invariant case the residual low-temperature entropy is the primary thermodynamic signature of these Majorana edge modes, and verify using large-scale Monte Carlo simulations that this residual entropy is present in the full Kitaev model. When time-reversal symmetry is broken, the Majorana edge modes are potentially observable in more direct thermodynamic measurements such as the specific heat, though only at temperatures well below the bulk gap. % Second, we study the energetics, and the corresponding thermodynamic signatures, of the flux excitations in the Kitaev model. Specifically, we study the flux interactions on both cylinder and torus geometries numerically, and quantify their impact on the thermodynamics of the Kitaev spin liquid by using a polynomial fit for the average flux energy as a function of flux density and extrapolating it to the thermodynamic limit. By comparing this model to Monte Carlo simulations, we find that flux interactions have a significant quantitative impact on the shape and the position of the low-temperature peak in the specific heat.

Freezing and low temperature entropy: The case of mean-field Gaussian model

The freezing phenomenon is studied in connection with the low temperature behavior of the entropy of the Gibbs measure. In particular, in the case of the mean-field Gaussian spin-glass model, from the study of a functional relation between the free energies at two different temperatures, we show that the maximum value of the inverse temperature beyond which the specific entropy of the Gibbs measure cannot be strictly positive is given by β* = 4 log 2. The low temperature entropy of the random energy model is also discussed.

Optimizing the Phase-Separated Domain Size of the Active Layer via Sequential Crystallization in All-Polymer Solar Cells.

The proper domain size of the active layer plays a key role in determining the exciton dissociation and charge transport in all-polymer solar cells (all-PSCs). However, fine-tuning the domain size remains challenging due to low glass transition temperature and negligible mixing entropy in polymer blends. Herein, we systematically studied the influence of "crystallization kinetics" on the domain size and proposed that if the donor and acceptor crystallize simultaneously, they are prone to form a large domain, while if sequential crystallization of the donor and acceptor occurs, a fine phase separation structure with the proper domain size can be obtained. Taking PBDB-T/PNDI blends for instance, the domain size was decreased by using sequential crystallization; meanwhile, the crystallinity and molecular orientation were also optimized, boosting the power conversion efficiency from 6.55% to 7.78%. This work provides a novel way to finely tune the heterojunction phase separation structures.

Thermodynamic properties of TiC nanowire from first principles

We have investigated the thermodynamic properties of titanium carbide (TiC) nanowire within the framework of density functional theory and quasi-harmonic approximation via calculating the temperature dependence of a number of thermodynamic quantities including entropy, number of microstates, total and free energies, and specific heat. The level of disorder of the nanowire has been found to be larger than that of the bulk mainly due to expansion in only one direction, which accordingly results in acquiring more spatial degrees of freedom. A linear function of temperature has been also found for the low-temperature specific heat of the nanowire being in a remarkable agreement with the general $$T^{\text{n}}$$ -law for Debye systems. Results firmly establish a direct correlation between the spatial expansion of a TiC compound and its low-temperature specific heat and entropy.

Direct entropy measurement in a mesoscopic quantum system

The entropy of an electronic system offers important insights into the nature of its quantum mechanical ground state. This is particularly valuable in cases where the state is difficult to identify by conventional experimental probes, such as conductance. Traditionally, entropy measurements are based on bulk properties, such as heat capacity, that are easily observed in macroscopic samples but are unmeasurably small in systems that consist of only a few particles. In this work, we develop a mesoscopic circuit to directly measure the entropy of just a few electrons, and demonstrate its efficacy using the well understood spin statistics of the first, second, and third electron ground states in a GaAs quantum dot. The precision of this technique, quantifying the entropy of a single spin-$\frac{1}{2}$ to within 5\% of the expected value of $k_B \ln{2}$, shows its potential for probing more exotic systems. For example, entangled states or those with non-Abelian statistics could be clearly distinguished by their low-temperature entropy.

Quantum Critical Behavior in a Concentrated Ternary Solid Solution.

The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, Tc, goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess –TlnT (or power law) contribution to the low temperature heat capacity, and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x ≈ 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in this simple fcc material thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.

Phase transitions in a two-dimensional antiferromagnetic Potts model on a triangular lattice with next-nearest neighbor interactions

Phase transitions (PTs) and frustrations in two-dimensional structures described by a three-vertex antiferromagnetic Potts model on a triangular lattice are investigated by the Monte Carlo method with regard to nearest and next-nearest neighbors with interaction constants J1 and J2, respectively. PTs in these models are analyzed for the ratio r = J2/J1 of next-nearest to nearest exchange interaction constants in the interval |r| = 0–1.0. On the basis of the analysis of the low-temperature entropy, the density of states function of the system, and the fourth-order Binder cumulants, it is shown that a Potts model with interaction constants J1 0 and J2 < 0, frustrations arise in the range 0.5 < |r| < 1.0, while, in the interval 0 ⩽ |r| ⩽ 1/3, the model exhibits a second-order PT.

Can Thermal Input from a Prior Universe Account for Relic Graviton Production and Imply Usage of the Bogomolnyi Inequality, as a Bridge between Brane World Models and Loop Quantum Gravity in Early Universe Conditions?

The author uses a low temperature and low entropy pre inflation state to create a bridge between String theory and loop quantum gravity. We use this analysis in lieu of the CMB barrier as of z = 1000 since it is our way to come up with a working model of quintessence scalar fields, which permits relic generation of dark matter and dark energy. Not only referencing this bridge, we do it in such a way as to utilize the low entropy condition which the Brane world model of Randal and Sundrum creates, and to show how it is in common with what Caroll and Chen wrote up in 2005., i.e. when the universe was about 1000 times smaller and 100,000 times younger than today.

Can Thermal Input from a Prior Universe Account for Relic Graviton Production? Implications for the Cosmological Landscape

The author presents how to make a link between the low temperature and low entropy of pre big bang state of cosmology as given by Carroll and Chen in 2005, to the quantum cosmology conditions predicted by Weinberg when the temperature reaches 1032 degrees Kelvin. We do this bridge building in our model construction as a way to get about the fact that cosmological CMB is limited by a red shift about z = 1100, so in order to get our suppositions consistent with observations, we also examine what happens in our model when we introduce quantization via a shift in values of the Hartle-Hawking wave function from a lower value of nearly zero to one which is set via an upper bound of the Planck’s constant of the order of 360 times the square of the Planck’s mass.

Suppression of Pauling's residual entropy in the dilute spin ice(Dy1−xYx)2Ti2O7

Around 0.5 K, the entropy of the spin-ice Dy$_2$Ti$_2$O$_7$ has a plateau-like feature close to Pauling's residual entropy derived originally for water ice, but an unambiguous quantification towards lower temperature is prevented by ultra-slow thermal equilibration. Based on specific heat data of (Dy$_{1-x}$Y$_x$)$_2$Ti$_2$O$_7$ we analyze the influence of non-magnetic dilution on the low-temperature entropy. With increasing x, the ultra-slow thermal equilibration rapidly vanishes, the low-temperature entropy systematically decreases and its temperature dependence strongly increases. These data suggest that a non-degenerate ground state is realized in (Dy$_{1-x}$Y$_x$)$_2$Ti$_2$O$_7$ for intermediate dilution. This contradicts the expected zero-temperature residual entropy obtained from a generalization of Pauling's theory for dilute spin ice, but is supported by Monte Carlo simulations.

High temperature enthalpy increments and thermodynamic functions of ZrCo: An experimental and theoretical study

The thermodynamic properties of ZrCo intermetallic were investigated both experimentally and theoretically to determine its applicability for the storage of hydrogen isotopes. The enthalpy increments of ZrCo were measured using a high temperature inverse drop calorimeter in the temperature range 642–1497K. The enthalpy increments and heat capacity obtained for Zr0.5Co0.5(s) in the temperature range 642–1497K were fitted into following polynomial expressions:Hm0(T)−Hm0(298.15 K)(J·mol−1)=25.682·(TK)+29.807·10−4·(TK)2+2.1864·105·(KT)−8655.5Cp.m0(J·K−1·mol−1)=25.682+5.961·10−3·(TK)−2.1864·105·(KT)2The enthalpy of formation of ZrCo(s) at 298.15K was calculated to be −55.96kJmol−1 using the ab-initio method. The temperature dependent thermodynamic functions were calculated from the Debye–Grüneisen quasi-harmonic approximation. A set of thermodynamic functions for ZrCo alloy were also calculated using experimental heat capacity data obtained in this study. The theoretically calculated low temperature heat capacity (Cp,m0) and entropy (Sm0) values of ZrCo smoothly joined with that of experimental high temperature data.

Quantum discord in a three-spin system in an external magnetic field

We study quantum correlations in a bipartite heteronuclear 2×1 system with three spins S = 1/2 in an external magnetic field at high and low temperatures. We analytically determine the minimum of the system conditional entropy at high temperatures after measurements on the one-spin subsystem for any relations between the spin frequencies of both subsystems. As a result, we obtain analytic expressions for the measure of quantum correlations, i.e., for the quantum discord in three-qubit systems, for the first time in the high-temperature case. We obtain the minimum of the low-temperature conditional entropy numerically. We investigate the quantum discord evolution and the relation between the classical and quantum correlations.

Conductivity tensor in a holographic quantum Hall ferromagnet

The Hall and longitudinal conductivities of a recently studied holographic model of a quantum Hall ferromagnet are computed using the Karch–O'Bannon technique. In addition, the low temperature entropy of the model is determined. The holographic model has a phase transition as the Landau level filling fraction is increased from zero to one. We argue that this phase transition allows the longitudinal conductivity to have features qualitatively similar to those of two dimensional electron gases in the integer quantum Hall regime. The argument also applies to the low temperature limit of the entropy. The Hall conductivity is found to have an interesting structure. Even though it does not exhibit Hall plateaux, it has a flattened dependence on the filling fraction with a jump, analogous to the interpolation between Hall plateaux, at the phase transition.