Quadrupolar susceptibility and magnetic phase diagram of PrNi2Cd20 with non-Kramers doublet ground state

ABSTRACT

In this study, ultrasonic measurements were performed on a single crystal of cubic PrNi₂Cd₂₀, down to a temperature of 0.02?K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant (C₁₁?C₁₂)/2, which is related to the Γ₃-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ～30?K, which indicates that the present system has a Γ₃ non-Kramers doublet ground state. A leveling-off of the elastic response appears below ～0.1?K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ₃-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28?T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ～26?T are also detected by high-magnetic-field measurements.     

Physical properties and crystal chemistry of Ce2Ga12Pt

Single crystals of the new ternary compound Ce(2)Ga(12)Pt were prepared by the self-flux technique. The crystal structure with the space group P4/nbm was established from single crystal x-ray diffraction data and presents a derivative of the LaGa(6)Ni(0.6) prototype. Magnetic susceptibility measurements show Curie-Weiss behaviour due to local Ce(3+) moments. At high temperatures, the magnetic anisotropy is dominated by the crystal-electric-field (CEF) effect with the easy axis along the crystallographic c direction. Ce(2)Ga(12)Pt undergoes two antiferromagnetic phase transitions at T(N,1) = 7.3 K and T(N,2) = 5.5 K and presents several metamagnetic transitions for the magnetic field along c. Specific-heat measurements prove the bulk nature of these magnetic transitions and reveal a doublet CEF ground state. The 4f contribution to the resistivity shows a broad maximum at T(max) ≈ 85 K due to Kondo scattering off the CEF ground state and excited levels.     

Antiferroquadrupolar ordering in a Pr-based superconductor PrIr(2)Zn(20)

An antiferroquadrupolar ordering at T(Q)=0.11 K has been found in a Pr-based superconductor PrIr(2)Zn(20). The measurements of specific heat and magnetization revealed the non-Kramers Γ(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T(Q)=0.11 K. The increment of T(Q) in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T(Q). The entropy release at T(Q) is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T(c)=0.05 K.     

Iridium-rare earth dilute ferromagnetism

Dilute solutions of Gd in Ir ( > 300 ppm) are ferromagnetic at 39 K and there is an enhanced moment. Other rare earths in Ir also order. For larger quantities of Gd, the elements Ru, Rh, Os and Ir show evidence of ordering in their eutectics and exhibit susceptibility cusps in low magnetic fields. Low concentrations of Gd in Rh and Pt also order tat 35 and 12 K, respectively.     

Magnetic and electrical properties of the equiatomic cerium germanides CeMGe (M = Rh, Ir, Pd, Pt)

From X-ray Guinier powder data the crystal structures of the ternary germanides Ce(Rh or Ir)Ge and Ce(Pd or Pt)Ge were confirmed to be of the TiNiSi-type and the CeCu₂-type, respectively. Magnetic, electrical and thermodynamic properties of these materials have been studied in the temperature range 1.5 K T 300 K. CeRhGe orders antiferromagnetically below 9.3 K, whereas in CeIrGe cerium is in an intermediate valence state and no magnetic transition is observed down to 1.5 K. Resistivity and specific heat measurements reveal a magnetic transition near 3.4 K for both CePdGe and CePtGe. According to the present investigations the compounds CeMGe, with M = Rh, Pd, Pt are classified as magnetically ordered Kondo systems.     

Observation of modulated quadrupolar structures in PrPb3

Neutron diffraction measurements have been performed on the cubic compound PrPb(3) in a [001] magnetic field to examine the quadrupolar ordering. Antiferromagnetic components with q = (1/2 +/- delta 1/2 0), (1/2 1/2 +/- delta 0) (delta approximately 1/8) are observed below the transition temperature T(Q) (0.4 K at H = 0) whose amplitudes vary linear with H and vanish at zero field, providing the first evidence for a modulated quadrupolar phase. For H < 1 T, a nonsquare modulated state persists even below 100 mK suggesting quadrupole moments associated with a Tau(3) doublet ground state to be partially quenched by hybridization with conduction electrons.     

Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru,Rh, Pd,Os, Ir,Pt): a DFT study

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.     

Adsorption of hydrogen on rhodium; Comparison with hydrogen adsorption on platinum and iridium

The adsorption of hydrogen on Rh has been studied (i) on a single crystal tip using field electron microscopy, and (ii) on a filament carrying this tip, using thermal desorption spectroscopy. The results are compared to those of other Group VIII metals. An isosteric heat of adsorption of 19 kcal/mole was found at low coverage, decreasing slightly with increasing coverage. This heat is substantially lower than that on Ru and Ir, determined by the same method. The work function increases by 0.4 eV, a value comparable to data reported for Ni and Ru, but significantly larger than those of Ir and Pt. An electropositive state of hydrogen as observed for Pt and Ir was not found for Rh. A small fraction of the adsorbed hydrogen is not desorbed at temperatures where other transition metal surfaces are completely denuded. This β₂-hydrogen which is desorbed only at 600–800 K, is tentatively assigned to a subsurface species.     

Structurally tuned superconductivity in heavy-fermion CeMIn5 (M=Co, Ir, Rh)

We discuss systematic trends in the high-temperature physical properties of the heavy Fermion superconductors (HFS) CeCoIn₅ (T_c=2.3 K, γ=300 mJ/molK²), CeIrIn₅ (T_c=0.4 K, γ=750 mJ/molK²), and CeRhIn₅ (P_c=16 kbar, T_c=2.1 K, γ=400 mJ/molK²) in terms of crystalline-electrical-field effects(CEF). We suggest the possibility that the interplay between the symmetry of the CEF ground-state (or low-T CEF scheme of levels) and the f–s hybridization could generate spin fluctuations relevant to the superconducting pairing mechanism in these materials. This hypothesis may provide insight into the fact that some crystal structures appear to favor superconductivity. Further, CeMIn₅ (M=Co, Ir, Rh) appear to be structural relatives of the cubic heavy Fermion superconductor CeIn₃, but with much higher T_c's. We argue that structural layering inherent in the tetragonal CeMIn₅ crystal structure determines the magnetic and electronic anisotropy responsible for the enhanced T_c's. We also describe similarities and differences between these compounds and the high-T_c cuprates.     

稀土化合物CeRhGe和CeRhSn晶场的理论计算

在晶场理论和分子场理论的基础上,通过模拟Taiki Ueda和M.S.Kim等人测量的磁化率实验结果,得到了稀土化合物CeRhGe和CeRhSn的晶场分裂,分析了晶场效应对化合物磁性质的影响.计算结果表明,Ce3 晶场分裂得到了双基态,两种化合物的第一激发能和总激发能分别是723 K、610 K和970 K、1130 K,与实验获得了较好的吻合.     

The magnetic properties of RuF5

The structural unit of the pentafluorides MF₅ (M = Ru, Rh, Os, Ir, Pt) consists of M₄F₂₀ tetramers with the transition elements occupying the corners of the rhombus. In the case of RuF₅ the magnetic susceptibility vs. temperature curve shows a broad maximum at T ? 40 K. Neutron diffraction experiments below and above this temperature show that this maximum does not correspond to a magnetic three-dimensional ordering. Theoretical calculations based on isolated tetra-nuclear clusters of identical $\text{S =}\text{3}\text{2}$ spins have been applied. The intracluster exchange interactions are antiferromagnetic. The fitting of the susceptibility data leads to an exchange constant of $\text{J}\text{k}\text{= ?8.3 K}$.     

Spin-liquid state in the S=1/2 hyperkagome antiferromagnet Na4Ir3O8

A spinel related oxide, Na(4)Ir(3)O(8), was found to have a three dimensional network of corner shared Ir(4+) (t(2g)(5)) triangles. This gives rise to an antiferromagnetically coupled S = 1/2 spin system formed on a geometrically frustrated hyperkagome lattice. Magnetization M and magnetic specific heat C(m) data showed the absence of long range magnetic ordering at least down to 2 K. The large C(m) at low temperatures is independent of applied magnetic field up to 12 T, in striking parallel to the behavior seen in triangular and kagome antiferromagnets reported to have a spin-liquid ground state. These results strongly suggest that the ground state of Na(4)Ir(3)O(8) is a three dimensional manifestation of a spin liquid.     

High-pressure sequence of Ba3NiSb2O9 structural phases: new S = 1 quantum spin liquids based on Ni2+

Two new gapless quantum spin-liquid candidates with S = 1 (Ni(2+)) moments: the 6H-B phase of Ba(3)NiSb(2)O(9) with a Ni(2+)-triangular lattice and the 3C phase with a Ni(2/3)Sb(1/3)-three-dimensional edge-shared tetrahedral lattice were obtained under high pressure. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures θ(CW) of -75.5 (6H-B) and -182.5 K (3C), respectively. Below ~25 K, the magnetic susceptibility of the 6H-B phase saturates to a constant value χ(0) = 0.013 emu/mol, which is followed below 7 K by a linear-temperature-dependent magnetic specific heat (C(M)) displaying a giant coefficient γ = 168 mJ/mol K(2). Both observations suggest the development of a Fermi-liquid-like ground state. For the 3C phase, the C(M) perpendicular T(2) behavior indicates a unique S = 1, 3D quantum spin-liquid ground state.     

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.     

Strong magnetic instability in correlated metallic Bi(2)Ir(2)O(7)

We report an experimental/theoretical study of single-crystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50?mK; (2) strongly field-dependent coefficients of the low-temperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W)?≈?53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80?K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7).     

The heat capacity of LaPd3 and PrPd3 at low temperature

Values of γ=0.33±0.01 mJ/K²·g atom and θ_D=176±1 K were found for LaPd₃. PrPd₃ shows magnetic ordering below 0.6 K and a Schottky anomaly whose maximum lies around 1.8 K. The crystalline field at the Pr³⁺ site creates a Γ₃ (doublet) excited state lying 4 K above the Γ₅ (triplet) ground state.     

Pressure induced superconductivity in CaFe2As2

CaFe2As2 has been found to be exceptionally sensitive to the application of hydrostatic pressure and can be tuned to reveal all the salient features associated with FeAs superconductivity without introducing any disorder. The ambient pressure, 170 K, structural/magnetic, first-order phase transition is suppressed to 128 K by 3.5 kbar. At 5.5 kbar a new transition is detected at 104 K, increasing to above 300 K by 19 kbar. A low temperature, superconducting dome (T(c) approximately 12 K) is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity too dramatically.     

Raman spectroscopy of CaSnO3 at high temperature: a highly quasi-harmonic perovskite

Calcium stannate perovskite (CaSnO(3)) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8 nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Grüneisen parameters and thermal line broadening following Γ=Acothθ/T, where the quantum temperature θ is determined by the phonon branch without further coupling with other degrees of freedom. The geometrical nature of phonon lines has been identified. High-temperature powder x-ray diffraction measurements provide thermal expansion coefficients of α(x)=13.9 × 10(-6) K(-1), α(y)=2.7 × 10(-6) K(-1), α(z)=14.3 × 10(-6) K(-1). The strongly quasi-harmonic behaviour observed and the lack of any indication of instability with respect to the post-perovskite structure points to the strongly first-order character of the reported perovskite to post-perovskite phase transition in this material, which appears to behave as a very good analogue to MgSiO(3) in the Earth's interior.     

Antiferromagnetic order and 4f-instability of CePb3

We have measured the static magnetic susceptibility of CePb₃ and LaPb₃ between 0.5 and 1,000 K and 1.5 and 900 K, respectively and the specific heat of CePb₃ between 1.5 and 40 K. The data show clearly an antiferromagnetic phase transition at 1.25 K. The order occurs in a doublet 4f groundstate whose entropy is removed only partially by long range order. There are strong indications for quadrupole-quadrupole interactions enhancing the ordering energy over that from spin spin interactions alone. The CEF splittings seem to depend on temperature. The 4f-instability in CePb₃ is quite weak.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.