An electron-phonon contribution to the stoner enhancement in GaMo4X8 compounds

Using the Stoner-Wohlfarth model of itinerant ferromagnetism for GaMo₄X₈ compounds, we determine the electron-phonon enhancement from their electronic specific heats. These compounds have unstable structures due to strong electron-phonon coupling. The room-temperature structure can be stabilized by doping with nonmagnetic isoelectronic impurities. The Curie temperature is decreased by a factor of three as a consequence of itinerant-electron parallel-spin scattering on nonmagnetic impurities. This property demonstrates that a long-range ferromagnetic interaction contributes to the Stoner enhancement. We speculate that the amplitude of this extra contribution is governed by the electron-phonon coupling, which depends on the mass. We are able to predict the experimental values of and the amplitude of the possible isotope effect.     

Nonlinear optical diagnostic of semimagnetic semiconductors Pb1−xYbxX (X = S, Se, Te)

Nonlinear optical measurements were performed to elucidate the influence of magnetic ions on the behavior of charge carriers in magnetic semiconductors—Pb_1−xYb_xX (X = S, Se, Te at x = 1-3%). It was shown that nonlinear optical methods could be used as sensitive tools for investigations of electron-phonon anharmonicity near low-temperature semiconductor-insulator phase transitions. There exists a difference between surface and bulk-like contributions to the nonlinear optical effects. It was shown that only low-temperature Two Photon Absorption (TPA) oscillator may be related to the number of the electron-phonon anharmonic modes responsible for the observed phase transformation. The explanation of the anomalous temperature dependences is given in accordance with dipole momentum's behaviors determined by low-temperature spin-spin interactions and by electron-phonon anharmonic interactions. We have discovered that low-temperature dependence of specific heat of Pb_1−xR_xTe (R = Yb, Pr with x = 3% and 1.6%, respectively) exhibits a non-magnetic order caused by large electron-phonon contributions and structural disorder effects.     

Anomalous Behavior of Low Temperature Specific Heat in HgCr2Se4

Low temperature specific heat (C _p ) characterization on high-quality HgCr₂Se₄ single crystal was performed between 0.5 K and 10 K. The result provides conceivable evidence for the existence of competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions in narrow temperature and temperature windows. The result suggests that magnetic interactions competition, as well as the Schottky anomaly, neglected previously in HgCr₂Se₄ due to the fragile AFM interaction and spin-lattice coupling, also play key roles in giving rise to the exotic magnetic behaviors.     

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5Se8 to Metallic Fe3Se4 Nanosheets

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂S₃, Cr₅Te₈). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe₂ to non-layered Fe-intercalated Fe₅Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.     

Low-temperature specific heats of some uranium ternary compounds UAsY (Y = S,Se, Te)

The specific heats of the UAsY (Y = S, Se, Te) ternary compounds have been studied in the temperature range 1–300K. -type anomalies are reported at 125.8, 108.8, and 62.8 K, respectively, corresponding to ferromagnetic ordering. The experimental results are analyzed in terms of the Debye model and electronic contributions. Debye temperatures, electronic specific heat coefficients, and magnetic entropies are derived. A comparison is made with the isostructural binary compounds UX₂ (X = P, As, Sb, Bi).     

Electronic characterization of alkali ruthenium hollandites: KRu4O8, RbRu4O8 and Cs0.8Li0.2Ru4O8

Transport, specific heat, and magnetic measurements have been performed on three alkali hollandites: KRu₄O₈, RbRu₄O₈, and a newly synthesized Cs analog, Cs_0.8Li_0.2Ru₄O₈, which was determined to have space group I4/m (#87) and lattice parameters, a = 10.0850(4) and c = 3.12180(20). In contrast to the ruthenium perovskites, which display a wide range of electrical and magnetic behavior, the alkali hollandites are simple paramagnetic metals.     

Low-temperature magnetic properties of EuMo6S8 and EuMo6Se8

The low-temperature magnetic properties of EuMo₆S₈ and EuMo₆Se₈ have been investigated by means of the¹⁵¹Eu Mössbauer effect between 0.06 and 300 K and by measuring dc and ac magnetic susceptibilities between 0.017 and 3 K. A sharp deviation of the dc susceptibility from Curie-Weiss behavior indicates that EuMo₆S₈ and EuMo₆Se₈ order magnetically at 0.40 and 0.85 K, respectively. A drastic broadening of the Mössbauer linewidth of both compounds occurs at 0.9 K. The broadening above the magnetic ordering temperature for EuMo₆S₈ is explained in terms of slow relaxation effects arising from the depopulation of higher crystal field levels of Eu²⁺ ions. The combination of Mössbauer and susceptibility results indicates that the magnetically ordered states of EuMo₆S₈ and EuMo₆Se₈ are strongly affected by the crystal field.     

Low-temperature magnetic properties of HoRh4B4

The low-temperature magnetic properties of HoRh₄B₄ have been studied by means of measurements of the magnetic susceptibility, magnetization, specific heat, thermal expansion, and magnetostriction. The ferromagnetic phase transition at T _M= 6.7 K shows almost ideal S = 1/2 mean field behavior in the specific heat. Crystal field effects due to the splitting of the J = 8 Hund's rule ground state of the Ho³⁺ ions result in Schottky anomalies in the specific heat and the thermal expansion and are also revealed in the low-field magnetic susceptibility and the magnetostriction. Information on the ground state doublet of the 4f electrons has been obtained from the nuclear contribution to the specific heat below 1 K and the high-field magnetization below T_M.This research was supported by the Schweizerische Nationalfonds zur Forderung der wissenschaftlichen Forschung (HRO), by the Department of Energy under Contract No. EY-76-S-03-0034-PA227-3 (LDW, MBM), and by the National Science Foundation under Grant No. NSF/DMR77-08469 (DCJ)     

Crystal structure of CuIn3Se5 and CuIn5Se8 ternary compounds

The structures of single crystals of CuIn₃Se₅ and CuIn₅Se₈ ternary compounds have been studied by X-ray diffraction for the first time. The unit cell parameters of CuIn₃Se₅ and CuIn₅Se₈ crystals are determined. It is established that CuIn₃Se₅ has a thiogallate lattice with c = 2a, which leads to the domain structure formation. CuIn₅Se₈ has a hexagonal lattice and exhibits cleavage along the (001) plane. Relationships between the atomic composition and crystal structure of these new ternary semiconductor compounds are discussed.     

Specific heat, electrical resistivity and thermoelectric power of YbNi4Si

The studies of the specific heat, electrical resistivity and thermoelectric power of YbNi₄Si are reported. These studies are supported by magnetic susceptibility and X-ray photoemission spectroscopy (XPS) measurements. YbNi₄Si does not order magnetically down to 4 K. Nearly in the whole temperature range studied the magnetic susceptibility follows a Curie law with μ_eff = 4.15 μ_B/f.u. This effective magnetic moment is close to the value expected for the 4f¹³ configuration (4.54 μ_B). The Yb²⁺ and Yb³⁺ peaks observed by XPS in the valence band region confirm the domination of the Yb³⁺ valence state. Based on the specific heat measurements, the electronic specific heat coefficient γ = 25 mJ/mol/K² and the Debye temperature θ_D = 320 K were derived. A quadratic dependence of electrical resistivity at low temperatures has been observed. The Kadowaki-Woods ratio has been discussed. The thermoelectric power has been analyzed in the framework of the two band model.     

Superconductivity and magnetism in the pseudoternary system Dy(Rux Rh 1−x)4B4

The pseudoternary system Dy(Ru_xRh_1?x)₄B₄ has been studied by means of ac magnetic susceptibility, electrical resistance, heat capacity, static magnetic susceptibility, and magnetization measurements. A low-temperature phase diagram was determined for the range of concentrations 0.1<x≤1. Superconductivity occurs forx?0.35, with a sharp depression of the superconducting critical temperature nearx～0.3. Long-range magnetic order exists for all concentrationsx; the rhodium-rich compounds exhibit antiferromagnetism, while the ruthenium-rich compounds are ferromagnetic. Pseudoternary compounds near the critical limit possibly exhibit reentrant antiferromagnetic superconductivity, a hitherto unobserved phenomenon. The competition between antiferromagnetism and ferromagnetism apparently leads to a mixed phase in a comparatively wide range of concentrations.     

Structural,Electrical and Magnetic Behaviour of FeTe0.5Se0.5 Superconductor

We report the synthesis as well as structural and physical properties of the bulk polycrystalline FeTe and FeTe_0.5Se_0.5 compounds. These samples are synthesised by the solid state-reaction method via vacuum encapsulation. Both studied compounds are crystallized in a tetragonal phase with space group P4/nmm. The parent FeTe compound shows an anomaly in resistivity measurement at around 78 K, which is due to the structural change along with a magnetic phase transition. The superconductivity in the FeTe_0.5Se_0.5 sample at 13 K is confirmed by the resistivity measurements. DC magnetisation along with an isothermal (M–H) loop shows that FeTe_0.5Se_0.5 possesses bulk superconductivity. The upper critical field is estimated through resistivity ρ (T,H) measurements using Gingzburg–Landau (GL) theory and is above 50 T with 50 % resistivity drop criterion. The origin of the resistive transition broadening under magnetic field is investigated by thermally activated flux flow. The magnetic field dependence of the activation energy of the flux motion is discussed.     

Superconducting and normal state properties of ErRh4B4 and LuRh4B4

Detailed heat capacity measurements of the ternary compounds ErRh₄B₄ and LuRh₄B₄ have been performed between 0.5 and 36 K and in magnetic fields up to 4 kG, yielding new information on crystal field effects in these materials and on the influence of externally applied magnetic fields on the coupled superconducting-ferromagnetic reentrant transition in ErRh₄B₄. Static magnetic susceptibility data on LuRh₄B₄ are presented which allow qualitative conclusions to be drawn regarding the magnitude of exchange enhancement and orbital paramagnetic effects in the RERh₄B₄ compounds. The electrical resistivity of ErRh₄B₄ has also been determined between 4 K and room temperature.This research was supported by the Department of Energy under Contract Number Ey-76-S-03-0034-PA227-3 (LDW, HBM, RWM, MBM) and by the National Science Foundation under Grant Number NSF/DMR77-08469 (DCJ). One of us (RWM) thanks the National Science Foundation for a postdoctoral fellowship.     

On the relationship between the electronic density of states and the superconducting critical temperature of disordered V3Si

The available data on the effect of neutron and α-particle irradiation on the superconducting properties and the low-temperature specific heat capacity of single- and polycrystalline V₃Si were analyzed in a unified manner. It was found that the electron-phonon renormalization parameter λ does not vary simply proportionally to the bare density of electron states at the Fermi level as had been earlier conjectured by Dynes and Varma, nor does it vary in exactly the same manner as found by Fradin and Williamson to hold for a variety of vanadium-based A15 structure compounds and pseudobinary alloys. When the behavior of V₃Si is compared with that of Nb₃Sn, Nb₃Al, and Nb₃Ge, it is seen that the response to disorder of parameters of importance in the superconductivity of these compounds does not follow any simple, universal behavior.     

Analysis of specific heat anomalies of high-T c superconductors based on a two-carrier high-T c superconductor model

We investigate the specific heat properties of a high-T _c superconductor model, the “two-carrier model” which includes an electron-phonon interaction driven by lattice defects such as oxygen deficiencies in the otherwise conventional BCS superconductor. The normal state is identified as a two-carrier model system which is composed of two groups of charge carriers: the normal and the renormalized Bloch carriers. An enhanced normal-state electronic specific heat is obtained, and there is good agreement between theoretical predictions about the specific heat anomalies at and aboveT _c and experimental data.     

Extremely large heat capacities between 4 and 10 K

This paper deals with the problem: which metallic materials have large heat capacities in the temperature range just above that of liquid helium temperatures?Experimental data are reported on the heat capacity of metallic compounds that are rich in gadolinium and in which the indirect magnetic interaction has been made small. The pseudobinary compounds Gd_x Er_1?x Rh are demonstrated to have high volumetric heat capacities as compared to eg Pb in the temperature range of interest (4 to 10 K).     

Ferromagnetism in the RERh4B4 compounds

The magnetic ordering temperatures of the primitive tetragonal rare earth (RE) rhodium boride compounds exhibit a peak at DyRh₄B₄, rather than GdRh₄B₄ as predicted from ordering via the indirect RKKY exchange interaction. This deviation is analyzed in terms of the depression of the superconducting transition temperature of LuRh₄B₄ by dilute amounts of RE impurity ions and the magnetic heat capacities of the RERh₄B₄ compounds for RE = Gd, Tb, Dy, and Ho. The strength of anisotropic crystalline electric field forces, as inferred from magnetic entropy considerations, may be the origin of the anomalous magnetic ordering temperatures. In addition, the depression of the superconducting transition temperatures of ErRh₄B₄ and TmRh₄B₄ from that of LuRh₄B₄ is shown to arise primarily from elastic exchange scattering of the conduction electrons by the magnetic RE ions.This research was supported by the U.S. Department of Energy under Contract EY-76-S-03-0034-PA227-3 (HBM, LDW, MBM) and by the National Science Foundation under Grant NSF/DMR77-08469 (DCJ).     

Physical properties of the quarternary chalcogenides Cu2IBIICIVX4 (BII = Zn,Mn, Fe,Co; CIV = Si,Ge, Sn; X = S,Se)

X-ray, electrical and magnetic measurements have been made on several quarternary chalcogenides of the type Cu₂^IB^IIC^IVX₄ with B^II = Zn, Mn, Fe, Co; C^IV = Si, Ge, Sn; X = S, Se. The structures of these compounds result from a superstructure of zincblende or wurtzite. The resistivities have been found to be extrinsic. The magnetic measurements show that the susceptibilities of the compounds containing zinc are independent of temperature, whereas those of the transition metal compounds obey the Curie Weiss law above 77 K. The sulfur compounds have a negative Weiss constant whereas the selenide compounds show a positive Weiss constant.     

Magnetic Properties of Ru-rich Ru2?x Fe x CrZ (Z=Si, Ge)

The properties of Heusler compounds Ru_2?x Fe _x CrGe are investigated and compared with those of Ru_1.9Fe_0.1CrSi. Ru₂CrGe is confirmed to exhibit an antiferromagnetic transition with Ne??l temperature 13?K by magnetic susceptibility and specific heat measurements. When Fe is substituted for Ru, a peak in the magnetic susceptibility is observed, and in the lower temperature region irreversibility between temperature dependences of magnetization for zero-field-cooling and field-cooling conditions is found. Nevertheless, in specific heat of Ru_1.9Fe_0.1CrGe, there is no anomaly to indicate phase transition. The specific heat is almost identical to that for Ru_1.9Fe_0.1CrSi. The above results demonstrate that the magnetic states in the low temperature region of Fe-substituted samples are spin-glass-like states. Slight substitution of Fe for Ru destroys the long-range-order and lead to peculiar spin-glass-like states.     

Transport (Electrical Resistivity/Magnetoresistance) and Specific Heat Measurements of Cu<Subscript>2 </Subscript><Emphasis Type="Italic">Ln</Emphasis>NbO<Subscript>8</Subscript>Sr<Subscript>2</Subscript> with <Emphasis Type="Italic">Ln</Emphasis> = Er,Dy

The magnetic, transport and specific heat measurements of 1212-type compounds crystallized in tetragonal symmetry with starting composition, Cu_2.1 LnNb_0.9O_7.9Sr₂; Ln = Er, Dy have been investigated. The Cu_2.1 LnNb_0.9O_7.9Sr₂; Ln = Er, Dy compounds showed non-superconducting properties down to 1.8 K in the magnetic susceptibility measurements. The magnetic parameter (μ _eff) obtained from the linear region of inverse molar magnetic susceptibility curves agrees with the calculated values for the respective free trivalent Er and Dy ions in Cu_2.1 LnNb_0.9O_7.9Sr₂; Ln = Er, Dy compounds. Though there is no strong evidence of magnetic ordering seen from magnetic susceptibilities, the hysteresis, M(H) curve obtained at 1.8 K looks like a canted antiferromagnet. The S-shaped (slightly curved) magnetization curve suggests weak ferromagnetic interactions that exist in the Ln = Er compound. A small hysteresis is observed with the coercive field (H _c) of 49 Oe and the remnant magnetization (M _r) of about 0.06 μ _B/f.u. for Ln = Er. Interestingly, such a hysteresis loop opening is not seen for the Ln = Dy compound. While specific heat anomaly (λ-like transition) was observed for Ln = Dy compound at low temperature (around 6 K), for Ln = Er compound such an anomaly was not seen in the specific heat measurements. The electrical resistivity versus temperature, ρ(T), data showed that the compounds Cu_2.1 LnNb_0.9O_7.9Sr₂; Ln = Er, Dy are not in metallic state and they are really semiconducting-in-nature. Interestingly, rather large magnetoresistance, MR (∼18%) was observed at low temperature ∼2 K for Er-based niobio-cuprate.