Porous manganese-based magnetocaloric material for magnetic refrigeration at room temperature

The powder metallurgy technique has been exploited as a means to prepare porous magnetocaloric materials. The alloy Mn_1.1Fe_0.9P_0.46As_0.54 was previously synthesized by mechanical alloying followed by a solid-state reaction for crystallization and homogenization. Subsequently, the alloy was comminuted and sintered at 1298 K. The obtained sintered product is aimed to be tested in a magnetic regenerator of a prototype machine.     

Specific Heat of Heavy Fermion Compound CeRu2Si2 in Magnetic Field

The high field anomalies of the linear term in the specific heat of CeRu₂Si₂ are shown to be caused by a reconstruction of the quasiparticle band due to the magnetostriction and by a destruction of short range magnetic order.     

Magnetic moments contribution to the specific heat and susceptibility of heavy fermion system

In non-magnetic heavy fermion systems, valence fluctuations prevent formation of long-range magnetic correlations. The RKKY exchange fields fluctuate due to f-electron transfers to the conduction band. To simulate these processes, we use the Ising-type model with fluctuating fields. The linear temperature dependence of the specific heat and temperature independence of susceptibility suggest that the f-electron contribution to these quantities can be considered as complementary to the one originating from Kondo resonance.     

On planar fermions with quartic interaction at finite temperature and density

We study the breaking of parity symmetry in the 2+1 Gross-Neveu model at finite temperature with chemical potential μ, in the presence of an external magnetic field. We find that the requirement of gauge invariance, which is considered mandatory in the presence of gauge fields, breaks parity at any finite temperature and provides for dynamical mass generation, preventing symmetry restoration for any non-vanishing μ. The dynamical mass becomes negligibly small as temperature is raised. Received 4 April 2002 / Received in final form 12 July 2002 Published online 15 October 2002 RID="a" ID="a"e-mail: cabra@venus.fisica.unlp.edu.ar     

复叠式低温保存箱系统设计关键问题分析

以R404A/R23为制冷剂,利用二元单级复叠制冷循环,开发出-86℃大容量低温保存箱。文中对蒸发器优化布置、冷凝蒸发器中间温度确定、毛细管设计以及如何保证设备安全运行等关键问题进行了分析。     

Reduced magnetic disorder at low temperature in Ca3Co2O6 via zinc substitution

ABSTRACT

The compound Ca₃Co₂O₆ undergoes a transition into a spin-density wave (SDW) state near 24?K. Below ～10?K, this unstable SDW state coexists with a nearly- degenerate commensurate antiferromagnetic state as well as short-range magnetic order. Clear signatures of this strong magnetic disorder have been observed in the response of entropy to changing magnetic field and temperature. We performed a calorimetry study of Ca₃Co₂O₆ and Ca₃Co_1.9Zn_0.1O₆ in order to compare their entropic responses at low temperature. Our results for Ca₃Co₂O₆ reveal that ΔS(T, H)?≡?S(T, H)?S(T, H?=?0) increases as either temperature or magnetic field increase. In contrast, ΔS data for Ca₃Co_1.9Zn_0.1O₆ were relatively unresponsive to changes in temperature or field, suggesting that Zn substitution may reduce the low-temperature magnetic disorder observed in Ca₃Co₂O₆. These results are discussed within the context of two cases (Ca₃Co₂O₆ under applied pressure and Ca_2.75R_0.25Co₂O₆ (R?=?Dy, Lu)) in which a single magnetic ground state is stabilised.     

An automated thermal relaxation calorimeter for operation at low temperature (0.5 K<T<10 K)

We describe an automated calorimeter for measurement of specific heat in the temperature range 10 K>T>0.5 K. It uses sample of moderate size (100–1000 mg), has a moderate precision and accuracy (2%–5%), is easy to operate and the measurements can be done quickly with He⁴ economy. The accuracy of this calorimeter was checked by measurement of specific heat of copper and that of aluminium near its superconducting transition temperature.     

Zn含量对重电子金属CeCu6-xZnx低温性质的影响

报道了掺Zn的重电子金属CeCu6-xZnx(x=0.05,0.1,0.15,0.2,0.3)样品的非常规制备方法以及低温电阻和低温比热容的测量结果,研究Zn含量对CeCu6-xZnx低温性质的影响及其物理原因.发现低温下Kondo散射项系数c随Zn含量的加大而减小,电阻极大值温度Tmax随Zn含量的加大而降低,CeCu6-xZnx的德拜温度ΘD随Zn含量的加大而略有上升 关键词： 重费密子系统 低温比热容 低温电阻 相干散射     

Channeling study of boron implanted silicon: Liquid nitrogen temperature implantation

Silicon samples have been boron implanted at 150 keV at liquid nitrogen temperature to a dose of 3.6 × 10¹⁵/cm². This dose rendered the implanted layer amorphous as viewed by helium ion backscattering. Four kinds of room temperature measurements were made on the same set of samples as a function of the isochronal annealing temperature. The measurements made were the determination of the substitutional boron content by the channeling technique using the B¹¹(p, α) nuclear reaction, observation of the disorder by helium ion backscattering, determination of the carrier concentration by van der Pauw Hall measurements, and the sheet resistivity by four point probe measurements. These measurements are compared with results from samples implanted at room temperature. The carrier concentration correlates well with the substitutional boron content for both room temperature and liquid nitrogen temperature implantations. Following annealing temperatures in the 600 to 800°C range, a much larger percentage of the boron lies on substitutional lattice sites, and therefore the carrier concentration is larger, if the implantation is done at liquid nitrogen temperature rather than at room temperature. Following liquid nitrogen temperature implantation, reverse annealing is observed from 600 to 800°C in the substitutional boron content, carrier concentration and sheet resistivity. The boron is more than 90 per cent substitutional after annealing to 1100°C for both the room temperature and liquid nitrogen temperature implantations. The low temperature implantation produced a buried amorphous layer, and this layer was observed to regrow from both the surface and substrate sides at approximately equal rates.     

The DNA electronic specific heat at low temperature: The role of aperiodicity

The electronic specific heat spectra at constant volume (_CV$C_V$) of a long-range correlated extended ladder model, mimicking a DNA molecule, is theoretically analyzed for a stacked array of a double-stranded structure made up from the nucleotides guanine G, adenine A, cytosine C and thymine T  . The role of the aperiodicity on _CV$C_V$ is discussed, considering two different nucleotide arrangements with increasing disorder, namely the Fibonacci and the Rudin–Shapiro quasiperiodic structures. Comparisons are made for different values of the band fillings, considering also a finite segment of natural DNA, as part of the human chromosome Ch22.     

极端高温环境下空气源热泵冷热水机组制冷实验研究

针对现有空气源热泵冷热水机组高温环境运行效果差、效率低、排气温度过高导致停机等问题,设计一套基于准双级压缩循环理论,以R410A为制冷剂的中压补气型空气源热泵冷热水机组。在50℃极端环境温度下,采用中压补气技术,对系统的制冷性能进行实验研究。结果表明:(1)系统出水温度由10℃增至15℃时,制冷量增加77.28%,EER提高59.02%,系统的制冷量、功率和EER均随出水温度的升高而增加;(2)相较不补气模式,系统排气温度由111.9℃降至106.23℃,制冷量由14.14 kW增至16.05 kW,可有效降低排气温度,提升制冷量,能更好提高系统超高温制冷时的稳定性。     

重电子金属CeCu(6-x)Nix低温电阻与比热的研究

测量了重电子金属CeCu6-xNix(x=0,005,01,015,02)01K—250K的低温电阻和5K—70K低温比热,发现样品电阻的极大值温度随着掺Ni含量的增大而急剧下降,这一现象反映少数与Ni邻近的Ce离子在极低温下磁矩的加强和整个Ce离子点阵对导电电子相干散射的减弱.与此相反,低温电子比热系数γ在较低温度下近于常数,而在8K附近因有效质量变大而明显上升,但γ明显上升的温度,对Ni的含量却不敏感,表明绝大部分Ce离子的状况并未受到影响 关键词： 重费米子系统 低温比热 低温电阻     

Magnetic transitions and thermomagnetic properties of GdCu6

We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu₆. These results clearly reveal the evidences of multiple magnetic transitions in GdCu₆. In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic-antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu₆ is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu₆ as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature.     

Random-phase approximation for the grand-canonical potential of composite fermions in the half-filled lowest Landau level

We reconsider the theory of the half-filled lowest Landau level using the Chern-Simons formulation and study the grand-canonical potential in the random-phase approximation (RPA). Calculating the unperturbed response functions for current- and charge-density exactly, without any expansion with respect to frequency or wave vector, we find that the integral for the ground-state energy converges rapidly (algebraically) at large wave vectors k, but exhibits a logarithmic divergence at small k. This divergence originates in the k^-2 singularity of the Chern-Simons interaction and it is already present in lowest-order perturbation theory. A similar divergence appears in the chemical potential. Beyond the RPA, we identify diagrams for the grand-canonical potential (ladder-type, maximally crossed, or a combination of both) which diverge with powers of the logarithm. We expand our result for the RPA ground-state energy in the strength of the Coulomb interaction. The linear term is finite and its value compares well with numerical simulations of interacting electrons in the lowest Landau level. Received: 19 February 1998 / Revised: 25 March 1998 / Accepted: 17 April 1998     

Improving the gas barrier properties of a-SiOxCyNz film at low temperature using high energy and suitable nitrogen flow rate

Amorphous silicon oxycarbonitride thin films were synthesized on polyethylene terephthalate (PET) substrates at low temperatures (～80 °C) by plasma-enhanced chemical vapor deposition (PECVD). A high ion flux and suitable nitrogen flow rate improved the gas barrier properties and deposition rate of the resulting a-SiO_xC_yN_z film. The a-SiO_xC_yN_z films were deposited at a high deposition rate and low water WVTR properties as a result of the high ion flux and nitrogen chemistry. The high ion flux modified the chemical structure and nitrogen atomic composition of the resulting a-SiO_xC_yN_z film coatings. The substrate temperature was characterized using a thermometer. In addition, the coating properties were characterized by Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) and the water vapor transmission rate (WVTR).     

Thermodynamic properties of two-dimensional charged spin-1/2 Fermi gases

Based on the mean-field theory, we investigate the thermodynamic properties of the two-dimensional (2D) charged spin-1/2 Fermi gas. Landé factor g is introduced to measure the strength of the paramagnetic effect. There is a competition between diamagnetism and paramagnetism in the system. The larger the Landé factor, the smaller the entropy and specific heat. Diamagnetism tends to increase the entropy, while paramagnetism leads to the decrease of the entropy. We find that there exists a critical value of Landé factor for the transition point due to the competition. The entropy of the system increases with the magnetic field when g < 0.58. With the growth of paramagnetism, when g > 0.58, the entropy first decreases with the magnetic field, then reaches a minimum value, and finally increases again. Both the entropy and specific heat increase with the temperature, and no phase transition occurs. The specific heat tends to a constant value at the hightemperature limit, and it approaches to zero at very low temperatures, which have been proved by the analytical calculation.     

重电子金属CeCu5.8M0.2（M=Ni,Zn, Cd）低温物性的比较

研究了不同元素等量掺杂下重电子金属CeCu_5.8M_0.2(M=Ni，Zn，Cd)低温电阻、比热容和磁化率随温度的变化关系.分析讨论了掺杂元素M(M=Ni，Zn，Cd)的磁性、价态及原子尺寸对近藤温度T_K、相干温度（即电阻极大值的温度Ｔ^R_max）、每个Ce离子在较高温度时的平均磁矩μ_Ｂ以及在温度降低时被传导电子自旋屏蔽过程的影响. 关键词： 重电子系统 低温电阻 低温比热容 磁化率     

Lattice specific heat and local density of states of Ni-based dilute alloys at low temperature

A detailed theoretical study of the low-temperature lattice specific heat of Ni-based dilute alloys has been carried out. Lattice Green’s function method has been used to calculate the local density of states of substitutional impurities and lattice specific heat in different alloys. The resonance condition has been investigated for possible occurrence of resonance modes. Except in NiCr and NiMn, low-frequency resonance modes have been obtained in all the alloys. However, no localized mode was obtained. The impurity-induced increase in lattice specific heat is explained on the basis of the obtained resonance modes. The calculation shows an excellent agreement with the measured lattice specific heat in these alloys     

Electron cooling in three‐dimensional Dirac fermion systems at low temperature: Effect of screening

Hot electron cooling rate P, due to acoustic phonons, is investigated in three‐dimensional Dirac fermion systems at low temperature taking account of the screening of electron–acoustic phonon interaction. P is studied as a function of electron temperature T_e and electron concentration n_e. Screening is found to suppress P very significantly for about T_e < 0.5 K and its effect reduces considerably for about T_e > 1 K in Cd₃As₂. In Bloch–Grüneisen (BG) regime, for screened (unscreened) case the T_e dependence is P ～ T_e⁹(T_e⁵) and the n_e dependence gives P ～ n_e^–5/3 (n_e^–1/3). The T_e dependence is characteristic of 3D phonons and n_e dependence is characteristics of 3D Dirac fermions. The plot of P /T_e⁴ vs. T_e shows a maximum at temperature T_em which shifts to higher values for larger n_e. Interestingly, the maximum is nearly same for different n_e and T_em/n_e^1/3 being nearly constant. More importantly, we propose, the n_e dependent measurements of P would provide a clearer signature to identify 3D Dirac semimetal phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)