Pressure-induced transition in a heavy fermion YbPd2Si2

We report the results of a room-temperature investigation of the thermoelectric and the dilatometric properties of a heavy fermion system YbPd₂Si₂ (itterbium-palladium-silicon, 1-2-2) at high pressure P up to 22 GPa; YbPd₂Si₂ is a less-studied representative of the RM₂X₂ family (R=Ce, Yb, U; M=transition metal; X=Si, Ge) with the tetragonal ThCr₂Si₂-type structure of the I4/mmm space group. Around P∼6±0.5 GPa, a phase transition in Yb-Pd-Si was registered by the drastic changes in the pressure dependencies of the electrical resistance R, the thermopower (Seebeck effect) S, a temperature difference along a sample ΔT, and a sample's thickness Δx (related to compressibility). Both a nature of the found phase transition and a presumable P-T phase diagram of YbPd₂Si₂ are discussed.     

The surface state of URu2Si2

The ‘hidden-order’ (HO) transition of URu₂Si₂ remains a puzzle after 25 years of research. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we found that a hole-like band around Γ having its band maximum at E = −35 meV, and previously thought to be a bulk band of the system, is indeed a surface state not related to the HO phase transition. Here we present our detailed investigations to assign that state to a surface feature, and discuss on the possible origins of this surface band.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Magnetic structures of PrCO2Si2 and TbCo2Si2 compounds

Neutron diffraction studies of polycrystalline PrCo₂Si₂ and TbCo₂Si₂ compounds were carried out at 4.2 and 293 K. Both samples have collinear antiferromagnetic order below T_N(31(1) and 46(1) K for Pr and Tb compound respectively), with their magnetic moments parallel to the c axis. The ordered magnetic moment values of Pr and Tb at 4.2 K (3.19 and 9.12 μ_B respectively), are close to the saturation value of the free ions. The corresponding magnetic space group P_l4/mnc (Sh⁴¹⁰₁₂₈) is body-anticentered ($\text{k =}\text{111}\text{222}$ refering to P_l cell).     

Magnetoresistance of the compound CeRu2Ge2

In this work we present a magnetoresistance study on the CeRu₂Ge₂ compound. We analyze the ρ(T) curves for several applied magnetic fields using the electron–magnon scattering model for a ferromagnetic spin arrangement. From this analysis, the field dependence of the energy gap of the magnon spectrum is obtained. The magnetoresistance ρ(H) at various temperatures arises from a normal metal contribution with an additional scattering mechanism due to electron–magnon interaction.     

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu2Si2

The heavy fermion compound CeCu₂Si₂ is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ～ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T_c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T_c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E_F.     

Exotic superconducting state embedded in the hidden order state of URu2Si2

The heavy-fermion compound URu₂Si₂ has mystified researchers since the superconducting state (T_c = 1.45 K) is embedded within the enigmatic ‘‘hidden order” phase (T_h = 17.5 K). Here, we report charge and thermal transport measurements on ultraclean single crystals of URu₂Si₂ with very large residual-resistivity-ratio down to 30 m K (∼T_c/50), which reveal a number of unprecedented superconducting properties. The results provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology. We propose a gap function with chiral d-wave form Δ(k) = Δ₀k_z(k_x + ik_y). We also demonstrate that a distinct flux line lattice melting transition with outstanding characters occurs well below the upper critical fields even at sub-Kelvin temperature. The intriguing superconducting state of URu₂Si₂ adds a unique and exciting example to the list of unconventional superconductors.     

Destruction of pressure-induced superconductivity by long-range antiferromagnetic order in Tm2Fe3Si5

Reentrant superconductivity has been detected in a well characterized sample of Tm₂Fe₃Si₅ by ac susceptibility measurements under applied pressure. The superconducting transition temperature displays a pronounced, non-linear behavior under pressure, starting at low pressures with an increase which is among the largest known for any compound (dT_c1/dp = 0.47 K/kbar). The magnetic transition temperature is relatively pressure independent.     

Neutron diffraction studies of the magnetic ordering in the rare earth compounds TbNi2Si2, HoCo2Si2 and TbCo2Si2

Neutron diffraction studies and magnetic measurements on the compounds TbNi₂Si₂ (1), HoCo₂Si₂ (2) and TbCo₂Si₂ (3) revealed a collinear antiferromagnetic order below T_N = 10 ± 1 K (1), T_N = 13 ± 1 K (2) and T_N = 30 ± 2 K (3) with the rare earths moments oriented along the c-axis [m₀ = 8.8 ± 0.2 μ_B (1), m₀ = 8.1 ± 0.2 μ_B (2), m₀ = 8.8 ± 0.2 μ_B (3)] and the corresponding wavevector are $\text{k}\text{= [}\text{1}\text{2}\text{1}\text{2}\text{0] (1)}\text{and}\text{k}\text{= [ 0 0 1] (2) (3)}$. The magnetic structure of the compounds HoCo₂Si₂ and TbCo₂Si₂ consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically (+?+?) while for TbNi₂Si₂ the ordering within (0 0 1) plane is antiferromagnetic and the planes (0 0 1) are indeed decoupled.     

Superconductivity in CeCu2Si2

Resistivity measurements of CeCu₂Si₂ are carried out under pressures p up to 12 kbars. Unlike polycrystalline samples, no traces of superconductivity have been observed in CeCu₂Si₂ at ambient pressure. When pressure is applied, CeCu₂Si₂ monocrystals become superconducting with anomalously large ratio H_c2(0)/T_c (0) = 34 K0e/K and with the derivative dH_c2/dT(T=T_c) = 140 K0e/K     

Hidden order, collective excitations, and entropy loss in the heavy fermion superconductor URu2Si2

We have clarified the ‘hidden order’ of URu₂Si₂, and the spin density wave, and calculated the large entropy loss on Kondo channel transition to the one channel Kondo fixed point of the crystal. The three collective excitations, particularly the one discovered recently, have been either derived or explained. These and other findings, in excellent agreement with outstanding data, are of general applicability to the analogues of URu₂Si₂ including the nonstoichiometric cuprates. The results show the presence of multichannel Kondo effect in these crystals.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

Neutron diffraction determination of the magnetic structures of NpCo2Si2 and NpCu2Si2

A small polycrystalline ingot sample of NpCo₂Si₂ (weight ≈ 1.5 g) has been studied by neutron diffration between 2 and 160 K on the multi-detector D1B of ILL, Grenoble. At 100 K, the crystal structure is body-centered tetragonal (space group 14/mmm) with a = 3.886 Å and c =9.649 Å. Below T_N = (44 ± 2) K, seven superlattice lines are observed which correspond to a simple tetragonal lattice with lattice constants as above. They are consistent with a type I antiferromagnetic structure of the Np (2a) sublattice, with (001) ferromagnetic sheets coupled antiferromagnetically according to the sequence +-+-. At 6 K, the neptunium moment obtained from the diffracted intensities is: (1.48 ± 0.20)_μuB, and makes an angle 52° ± 15° with the c axis. The cobalt moment is certainly smallet than 0.3_μuB. The Np moment correlates well with the ²³⁷Np hyperfine field deduced from Mos?sbauer spectroscopy; the sublattice magnetization-temoperature curve follows very well the $\text{J=}\text{1}\text{2}$ brillouin curve. The magnetism is therefore probably of lovalized character in this compound. An isomorphous sample of NpCu₂Si₂ (a = 3.990 Å c = 9.920 Å) was shown to be ferromagnetic below (41 ± 2) K, with the Np moment [1.5 ± 0.2)_μuB] aligned along the c axis.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Magnetocaloric effect in high-energy ball-milled Gd5Si2Ge2 and Gd5Si2Ge2/Fe nanopowders

Evolution of structure and magnetocaloric properties in ball-milled Gd₅Si₂Ge₂ and Gd₅Si₂Ge₂/0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (ΔS_M) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd₅Si₂Ge₂ alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.     

Search for a spin glass state in PrAu2Si2 by μSR

μSR spectroscopy down to 40 mK was carried out on PrAu₂Si₂. This compound and heavy fermion URh₂Ge₂ are the only stoichiometric spin glasses reported among the tetragonal FT₂X₂ (F=rare earth or actinide, T=transition metal, X=metalloid) materials. Although bulk measurements on PrAu₂Si₂ exhibit all the typical features of a canonical spin glass with a freezing temperature of ∼3 K, no evidence for the formation of a frozen spin-glass state was found with μSR. Instead, the data clearly demonstrate that the magnetic moments in PrAu₂Si₂ remain dynamic down to the lowest temperatures. The discrepancy between these observed dynamics and the spin-glass-like response in bulk measurements is not understood. Analogous measurements on PrAu₂Ge₂ and PrAu₂(Ge_0.8Si_0.2)₂ showed the expected antiferromagnetic signals, demonstrating that in this type of alloy muons detect magnetic properties in the usual manner.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Magnetic phase transitions in PrCo2Si2

Magnetic phases in PrCo₂Si₂ have been studied by measurements of magnetization, neutron diffraction and electrical resistivity. For <9 K, the magnetic structure with a propagation vector k = (0,0,1) [2π/c] is stable. Incommensurate structures k = (0,0,0.926) and (0,0,0.777) appear for 9 K < T <17 K and 17 K < T <30 K, respectively.     

X-ray absorption spectroscopic study of mixed valence systems EuCu2Si2, YbCu2Si2 and Sm4Bi3

X-ray absorption spectroscopy technique is employed to determine the valence of the rare earth ions in EuCu₂Si₂, YbCu₂Si₂ and Sm₄Bi₃. In each case, two absorption peaks corresponding to two different valence states of respective rare earth ions have been observed. Low temperature (77 K) study of EuCu₂Si₂ indicates distinct change in the relative intensities of the absorption peaks compared to those registered at room temperature (300 K). It is inferred from the change in the relative intensities that the population of Eu²⁺ in EuCu₂Si₂ decreases at liquid nitrogen temperature compared to Eu³⁺. Conclusions drawn from these results agree well with those reported by others using different experimental techniques. In Sm₄Bi₃, Sm²⁺ and Sm³⁺ are found to occur in the ratio of 3:1.     

The magnetic structure of HoCo2Si2

We have carried out neutron diffraction on a HoCo₂Si₂ powder sample at 4.2 K. The magnetic structure of this compound is collinear antiferromagnetic with the holmium magnetic moments parallel to the c-axis of the crystal. The magnetic moment value of holmium is 9.85 μ_B. The magnetic space group is I4/mm'm' (Sh⁴¹⁰₁₂₈) k = 000 The ordering temperature is t_n = 12(1) K.