Low-temperature Expansion Research Articles

High-performance carbon-nanotube-based cold cathode electron beam with low-thermal-expansion gate electrode

The emission of a high-performance electron beam via a carbon nanotube cold cathode requires a higher electron transmission through the gate electrode. The transmittance of electrons through the gate mesh electrode strongly depends on the gate electrode structure and material properties. Therefore, thermal expansion of the gate electrode induced by the thermal load owing to the gate leakage current is a significant hurdle to be overcome. Using a high-thermal-expansion gate electrode comprised of SUS304 grid mesh, electron emission was brought to saturation when the mesh was bent upward, which was the result of a reduction of the effective electric field under the grid mesh. To mitigate the effect of this bending, a Mo grid mesh material possessing low thermal expansion introduced. The Mo grid material properties of low linear temperature expansion coefficient, high tensile strength, and low resistivity are necessary. With this grid mesh improvement, the electron emission current increased to ten times that of the SUS304 mesh grid.

Negative 2D thermal expansion in the halogen bonded acetone bromine complex

The complex formed between acetone and bromine shows both negative 2D thermal expansion at low temperature and colossal thermal expansion.

Features of low-temperature thermal expansion of n-type Bi2Se3 single crystals in magnetic field

Tensometric study of n-type Bi2Se3 single crystals in dc magnetic fields to 6 T in a temperature range of 7–23 K detected a weak negative thermal expansion (NTE) in the basal plane. The NTE increases with the field strength and depends on its orientation with respect to the trigonal c axis. In a magnetic field of 6 T, parallel to the c axis, the linear NTE coefficient reaches −7 · 10−7 K−1, and a minimum sample length is reached at a temperature of 13 K, where a Hall carrier concentration maximum is also detected. The found magnetoelastic anomaly can be associated with the topological insulator state.

Effects of interactions on dynamic correlations of hard-core bosons at finite temperatures

We investigate how dynamic correlations of hard-core bosonic excitation at finite temperature are affected by additional interactions besides the hard-core repulsion which prevents them from occupying the same site. We focus especially on dimerized spin systems, where these additional interactions between the elementary excitations, triplons, lead to the formation of bound states, relevant for the correct description of scattering processes. In order to include these effects quantitatively we extend the previously developed Br\"uckner approach to include also nearest-neighbor (NN) and next-nearest neighbor (NNN) interactions correctly in a low-temperature expansion. This leads to the extension of the scalar Bethe-Salpeter equation to a matrix-valued equation. Exemplarily, we consider the Heisenberg spin ladder to illustrate the significance of the additional interactions on the spectral functions at finite temperature which are proportional to inelastic neutron scattering rates.

High refractive index coating of phosphor-in-glass for enhanced light extraction efficiency of white LEDs

The coating of phosphor-in-glass with high refractive index was successfully fabricated for enhanced light extraction efficiency of white LEDs through a multilayer screen-printing and low-temperature sintering procedure. The effects of sintering temperature on the B2O3–SiO2–ZnO glass coating and YAG:Ce phosphor concentration on the phosphor-in-glass coating were investigated. The B2O3–SiO2–ZnO glass matrix possesses low glass transition temperature and appropriate thermal expansion coefficient. With the addition of La2O3 and WO3, high refractive index of 1.82 at 460 nm is acquired. The amorphous glass coating, sintered at 600 °C, yields an optimized transmittance of 50%. Furthermore, negligible thermal degradation is observed in the phosphor-in-glass coating, and the emission peak under the 460-nm excitation is at ~540 nm. When increasing the concentration of phosphor, luminous efficacy and correlated color temperature are both improved, but color rendering index is deteriorated. As the refractive index increases, the light extraction efficiency of white LEDs is enhanced due to the reason that the match of refractive index between the glass matrix and the phosphor decreases the total internal reflection at the interface.

Low-temperature abnormal thermal expansion property of Mn doped cubic NaZn13-type La(Fe, Al)13 compounds

Low-temperature abnormal thermal expansion (ATE) materials have been recently developed because of their significant applications for cryogenic engineering. However, the challenge still remains for the control of ATE effect at cryogenic temperature and adjustable ATE is of fundamental interest. In this paper, we report the isotropic ATE in La(Fe, Al)13 compounds over a wide adjusting temperature range by partially substituting Fe by Mn. It is found that all samples crystallize in the cubic NaZn13-type structure with the space group. The introduction of nonmagnetic Mn atoms reduces the Fe-Fe exchange interaction, therefore, the itinerant electron system needs less energy to break the magnetic order in ferromagnetic (FM) state at low temperature. The negative thermal expansion (NTE) operation-temperature window moves towards lower temperatures accompanied with the decrease of Curie temperature (TC) by increasing Mn elements. Moreover, the composite combining Mn 0 and Mn 57 broadens the zero thermal expansion (ZTE) behavior occurring in the whole tested temperature range. The present studies could be useful to control the thermal expansion, and indicate the potential applications of ATE materials in cryogenic engineering.

Low temperature stacking fault nucleation and expansion from stress concentrators in 4H-SiC

Despite of intensive studies during the last two decades of nucleation and expansion of Shockley-type single or adjacent stacking faults (double stacking faults) in 4H-SiC, the involved mechanisms are far from being deeply understood. Thus, in this paper, the temperature and carrier injection dependence of nucleation and expansion of these defects from local stress concentrators have been investigated by cathodoluminescence and low energy electron beam irradiation. Their corresponding behavior depends on their multiplicity. Upon annealing, the single stacking faults are found to be nucleated at lower temperature than the double stacking faults and their mobility is lower. Under electron injection at room temperature, only single stacking faults are nucleated. The involved partial dislocations dragging the stacking faults are assumed to have different nature cores.

Local thermal energy as a structural indicator in glasses

Identifying heterogeneous structures in glasses-such as localized soft spots-and understanding structure-dynamics relations in these systems remain major scientific challenges. Here, we derive an exact expression for the local thermal energy of interacting particles (the mean local potential energy change caused by thermal fluctuations) in glassy systems by a systematic low-temperature expansion. We show that the local thermal energy can attain anomalously large values, inversely related to the degree of softness of localized structures in a glass, determined by a coupling between internal stresses-an intrinsic signature of glassy frustration-anharmonicity and low-frequency vibrational modes. These anomalously large values follow a fat-tailed distribution, with a universal exponent related to the recently observed universal [Formula: see text] density of states of quasilocalized low-frequency vibrational modes. When the spatial thermal energy field-a "softness field"-is considered, this power law tail manifests itself by highly localized spots, which are significantly softer than their surroundings. These soft spots are shown to be susceptible to plastic rearrangements under external driving forces, having predictive powers that surpass those of the normal modes-based approach. These results offer a general, system/model-independent, physical/observable-based approach to identify structural properties of quiescent glasses and relate them to glassy dynamics.

Partition function of free conformal fields in 3-plet representation

Simplest examples of AdS/CFT duality correspond to free CFTs in d dimensions with fields in vector or adjoint representation of an internal symmetry group dual in the large N limit to a theory of massless or massless plus massive higher spins in AdSd+1. One may also study generalizations when conformal fields belong to higher dimensional representations, i.e. carry more than two internal symmetry indices. Here we consider the case of the 3-fundamental (“3-plet”) representation. One motivation is a conjectured connection to multiple M5-brane theory: heuristic arguments suggest that it may be related to an (interacting) CFT of 6d (2,0) tensor multiplets in 3-plet representation of large N symmetry group that has an AdS7 dual. We compute the singlet partition function Z on S1 × Sd−1 for a free field in 3-plet representation of U(N) and analyse its novel large N behaviour. The large N limit of the low temperature expansion of Z which is convergent in the vector and adjoint cases here is only asymptotic, reflecting the much faster growth of the number of singlet operators with dimension, indicating a phase transition at very low temperature. Indeed, while the critical temperatures in the vector (Tc ∼ Nγ, γ > 0) and adjoint (Tc ∼ 1) cases are finite, we find that in the 3-plet case Tc ∼ (log N)−1, i.e. it approaches zero at large N. We discuss some details of large N solution for the eigenvalue distribution. Similar conclusions apply to higher p-plet representations of U(N) or O(N) and also to the free p-tensor theories invariant under [U(N)]p or [O(N)]p with p ≥ 3.

The nature of three dimensions: Non-local behavior in the three-dimensional (3D) Ising model

In this work, we first reveal the nature of three dimensions: the existence of non-local behavior. Taking the three-dimensional (3D) Ising model as an example, we analyze the transfer matrices as well as the partition function to discuss in details the origin of non-local behavior in three dimensions. We then point out that fail to deal with non-local behavior is one of the main disadvantages of several theoretical approaches (such as conventional low-temperature expansions, conventional high-temperature expansions, perturbative renormalization group and Monte Carlo methods, etc), all of which take local environments into account only. This work provides a deep understanding on the natural difference between 3D bulk materials and 2D materials, and shows that how to treat the non-local behavior and the interaction between spins along the third dimension is a key point for solving exactly the problems of the 3D systems with the nearest neighboring interactions. Finally, we will briefly review recent advance in the 3D Ising model and related topics.

Vacancy-induced dissolution of precipitates in out-of-equilibrium systems: A test case ofFeX(X=C,N,O) alloys

Various out-of-equilibrium processes produce a supersaturation of vacancies in the material. When these defects show attractive binding energy with solutes, they will form stable point defect-solute clusters which will stabilize solutes in the solid solution with respect to the equilibrium case. Hence the out-of-equilibrium dynamic solubility limit increases, which can lead to the dissociation of solute precipitates. This vacancy-induced precipitate (VID) dissolution mechanism is an alternative to the well-known ballistic mixing effect (BAL) under irradiation, and it is also relevant for quenching, ball milling, and severe plastic deformation. Under irradiation, a BAL is efficient at low temperature only, whereas a VID is expected to be effective at intermediate temperatures. A quantitative and consistent prediction of the interstitial solute solubility limit increase generated by both BAL and VID mechanisms is presented starting from ab initio binding energies and migration energies of solutes and point defects, and using a low-temperature expansion of the free energy of the solid solution. These results are discussed for three alloys: FeC, FeN, and FeO, the latter being relevant to discuss the stability of oxide dispersed strengthened alloys microstructure under irradiation. We also suggest an experiment that would be able to determine steady-state vacancy supersaturations from the measure of solute partial pressures in out-of-equilibrium systems.

Cubic ideal ferromagnets at low temperature and weak magnetic field

The low-temperature series for the free energy density, pressure, magnetization and susceptibility of cubic ideal ferromagnets in weak external magnetic fields are discussed within the effective Lagrangian framework up to three loops. The structure of the simple, body-centered, and face-centered cubic lattice is taken into account explicitly. The expansion involves integer and half-integer powers of the temperature. The corresponding coefficients depend on the magnetic field and on low-energy effective constants that can be expressed in terms of microscopic quantities. Our formulas may also serve as efficiency or consistency check for other techniques like Green's function methods, where spurious terms in the low-temperature expansion have appeared. We explore the sign and magnitude of the spin-wave interaction in the pressure, magnetization and susceptibility, and emphasize that our effective field theory approach is fully systematic and rigorous.

Interaction Corrections to Spin-Wave Theory in the Large-S Limit of the Quantum Heisenberg Ferromagnet

The Quantum Heisenberg Ferromagnet can be naturally reformulated in terms of interacting bosons (called spin waves or magnons) as an expansion in the inverse spin size. We calculate the first order interaction correction to the free energy, as an upper bound in the limit where the spin size $S \to \infty$ and $\beta S$ is fixed ($\beta$ being the inverse temperature). Our result is valid in two and three spatial dimensions. We extrapolate our result to compare with Dyson's low-temperature expansion. While our first-order correction has the expected temperature dependance, in higher orders of the perturbation theory cancellations are necessary.

Quantum chromodynamics, antiferromagnets and XY models from a unified point of view

Antiferromagnets and quantum XY magnets in three space dimensions are described by an effective Lagrangian that exhibits the same structure as the effective Lagrangian of quantum chromodynamics with two light flavors. These systems all share a spontaneously broken internal symmetry O(N)→O(N−1). Although the respective scales differ by many orders of magnitude, the general structure of the low-temperature expansion of the partition function is the same. In the nonabelian case (N≥3), logarithmic terms of the form T8ln⁡T emerge at three-loop order, while for N=2 the series only involves powers of T2. The manifestation of the Goldstone boson interaction in the pressure, order parameter, and susceptibility is explored in presence of an external field.

Unprecedented Integral-Free Debye Temperature Formulas: Sample Applications to Heat Capacities of ZnSe and ZnTe

Detailed analytical and numerical analyses are performed for combinations of several complementary sets of measured heat capacities, for ZnSe and ZnTe, from the liquid-helium region up to 600 K. The isochoric (harmonic) parts of heat capacities, CVh(T), are described within the frame of a properly devised four-oscillator hybrid model. Additional anharmonicity-related terms are included for comprehensive numerical fittings of the isobaric heat capacities, Cp(T). The contributions of Debye and non-Debye type due to the low-energy acoustical phonon sections are represented here for the first time by unprecedented, integral-free formulas. Indications for weak electronic contributions to the cryogenic heat capacities are found for both materials. A novel analytical framework has been constructed for high-accuracy evaluations of Debye function integrals via a couple of integral-free formulas, consisting of Debye’s conventional low-temperature series expansion in combination with an unprecedented high-temperature series representation for reciprocal values of the Debye function. The zero-temperature limits of Debye temperatures have been detected from published low-temperature Cp(T) data sets to be significantly lower than previously estimated, namely, 270 (±3) K for ZnSe and 220 (±2) K for ZnTe. The high-temperature limits of the “true” (harmonic lattice) Debye temperatures are found to be 317 K for ZnSe and 262 K for ZnTe.

Strong correlations, strong coupling, ands-wave superconductivity in hole-dopedBaFe2As2single crystals

We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient is observed with both decreasing band filling and isovalent substitution (K, Rb, Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our Density-Functional Theory + Slave-Spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d-wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.

Field-Tuned Order by Disorder in Frustrated Ising Magnets with Antiferromagnetic Interactions.

We demonstrate the appearance of thermal order by disorder in Ising pyrochlores with staggered antiferromagnetic order frustrated by an applied magnetic field. We use a mean-field cluster variational method, a low-temperature expansion, and MonteCarlo simulations to characterize the order-by-disorder transition. By direct evaluation of the density of states, we quantitatively show how a symmetry-broken state is selected by thermal excitations. We discuss the relevance of our results to experiments in 2D and 3D samples and evaluate how anomalous finite-size effects could be exploited to detect this phenomenon experimentally in two-dimensional artificial systems, or in antiferromagnetic all-in-all-out pyrochlores like Nd_{2}Hf_{2}O_{7} or Nd_{2}Zr_{2}O_{7}, for the first time.

Exact low-temperature series expansion for the partition function of the zero-field Ising model on the infinite square lattice.

In this paper, we provide the exact expression for the coefficients in the low-temperature series expansion of the partition function of the two-dimensional Ising model on the infinite square lattice. This is equivalent to exact determination of the number of spin configurations at a given energy. With these coefficients, we show that the ferromagnetic–to–paramagnetic phase transition in the square lattice Ising model can be explained through equivalence between the model and the perfect gas of energy clusters model, in which the passage through the critical point is related to the complete change in the thermodynamic preferences on the size of clusters. The combinatorial approach reported in this article is very general and can be easily applied to other lattice models.

Low-temperature thermal expansion behavior of the geometrically frustrated pyrochlore Tb2Ti2O7

Abstract The low-temperature lattice behavior of the pyrochlore magnet Tb 2 Ti 2 O 7 has been investigated by thermal expansion measurements. The thermal expansion coefficient of Tb 2 Ti 2 O 7 shows no anomaly except for a shoulder around 8 K, which can be explained by a Schottky type excitation arising from the energy gap between the ground-state and first excited CEF doublets. The analysis of the thermal expansion behavior combined with the heat capacity data indicates the lack of strong spin-lattice coupling in the thermal expansion at the low temperature region, where the spin liquid state emerges.

Thermodynamics of the d = 3 + 1 quantum XY model

Within effective field theory we explore the properties of the d = 3 + 1 quantum XY model at low temperatures and in weak magnetic or staggered fields. For this parameter regime only few results appear to be known, and furthermore are restricted to one-loop order. In the present study we systematically analyze the thermodynamics of the d = 3 + 1 quantum XY model up to three-loop order. In the low-temperature expansion of the free energy density, the free Bose gas term of order T 4 receives corrections of order T 6 and T 8. The discussion also includes the pressure, (staggered) magnetization and susceptibility. In particular, we show how these quantities are influenced by the spin-wave interaction.