ThCr2Si2-type Crystal Structure Research Articles

Observation of parabolic electron bands on superconductor LaRu2As2

Abstract Ru-based superconductor LaRu2As2 has been discovered exhibiting the highest critical temperature of ~7.8 K among iron-free transition metal pnictides with the ThCr2Si2-type crystal structure. However, microscopic research on this novel superconducting material is still lacking. Here, we utilize scanning tunneling microscopy/spectroscopy to uncover the superconductivity and surface structure of LaRu2As2. Two distinct terminating surfaces are identified on the cleaved crystals, namely the As surface and the La surface. Atomic missing line defects are observed on the La surface. Both surfaces exhibit a superconducting gap of ~1.0 meV. By employing quasiparticle interference techniques, we observe standing wave patterns near the line defects on the La atomic plane. These patterns were attributed to quasiparticle scattering from two electron type parabolic bands.

Optical evidence of local and itinerant states in Ce- and Yb-heavy-fermion compounds

The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the 4f electrons and the Kondo coupling between 4f and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity (σ(ω)) spectra. Remarkably, the temperature variation in the σ(ω) spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the σ(ω) spectra of various Ce and Yb compounds with a tetragonal ThCr2Si2-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the σ(ω) spectra is mainly due to the electron–phonon interaction, while the temperature dependence below is due to the Kondo effect.

Magnetism and magnetocaloric effect of rare earth <italic>RET</italic><sub>2</sub><italic>X</italic><sub>2</sub> compounds

Magnetic refrigeration (MR) technology is considered to be an energy-saving alternative to the commonly used gas compression/expressing refrigeration technology. Currently, searching or designing magnetic solids with an excellent magnetocaloric effect (MCE) that works at different temperature regions is one of the most important targets. Therefore, the investigation of magnetism and MCE of rare-earth-based compounds is a prominent topic in the field of refrigeration. Among them, the ternary rare-earth compounds of RET 2 X 2 ( T = 3d, 4d, or 5d transition elements; X = B, P, Si, and Ge) with a ThCr2Si2-type crystal structure have attracted considerable attention owing to their unique physical and chemical properties. In this study, the crystal structure, magnetism, and MCE of RE Mn2(Si, Ge)2, RE Ru2(Si, Ge)2, RE Co2B2 and related compounds are briefly reviewed. Some of these compounds show excellent magnetocaloric properties with potential application in the field of active cryogenic MR.

Electronic, transport, and magnetic properties of (Ca, Ba)0.9La0.1Fe1.9Pt0.1As2 compounds

Superconductivity and magnetic properties were studied for La and Pt substituted (Ca, Ba)[Formula: see text]La[Formula: see text]Fe[Formula: see text]Pt[Formula: see text]As2 samples using structural, resistivity and magnetic measurement techniques. All bulk samples were synthesized by solid-state reaction method and annealed under a specific annealing technique with a time-dependent annealing process in vacuumed quartz tubes. ThCr2Si2-type crystal structure was concluded for both samples varying with in- and out-of-plane lattice parameters. The superconducting critical temperatures were determined by resistivity and under H = 20 Oe magnetization measurements, which were performed between the temperature ranges of 0–200 K. The upper and lower critical fields were determined and possible Meissner effects were roughly figured out to understand the level of shielding from M–H measurements. The maximum critical temperature was obtained from Ca[Formula: see text]La[Formula: see text]Fe[Formula: see text]Pt[Formula: see text]As2.

Magnetic Phase Transitions in Compounds with a Layered Crystal Structure

Compounds with a layered ThCr2Si2-type crystal structure exhibit a great variety of magnetic structures and magnetic phase transitions. This has aroused the interest of investigators in their physical properties and allows us to consider these compounds as advanced materials for magnetocaloric, magnetostrictive, and magnetoresistive applications. In the present work, the new results related to the magnetic and thermal properties of the compounds are reviewed.

Magnetic characteristics of RPd2Si2 (R = Rare earth)

Magnetic characteristics of HoPd2Si2 and ErPd2Si2 with the ThCr2Si2-type crystal structure have been studied from measurements of magnetic susceptibility χ(T) and magnetization M(B) on the single crystals. The compound HoPd2Si2 orders antiferromagnetically below TN = 5.1 K with the magnetic easy direction of [110]. The [110] magnetization process is a three-step metamagnetic one; metamagnetic transitions at B1 = 0.9 T, B2 = 1.5 T and B3 = 3.4 T. The [100] process is a two-step one. Between [110] and [100], strong magnetic anisotropy is seen for high magnetic fields. ErPd2Si2 shows an antiferromagnetic behavior with the magnetic easy direction of [001] below TN=3.9 K. The χ(T) shows four anomalies, which may come from magnetic transitions, at Tmax = 5.1 K, TN = T1 = 3.9 K, T2 = 3.1 K and T3 = 2.2 K. The [001] magnetization process at 2 K is a three-step metamagnetic process; metamagnetic transitions at B1 = 0.5 T, B2 = 1.0 T and B3 = 1.3 T. In the basal plane magnetization processes, a very small metamagnetic transition is at 1.9 T. Crystalline electric field (CEF) analysis was performed, and anisotropic magnetic behaviors are explained. The magnetic data of the RPd2Si2 compound series are summarized.

Structural and Superconductivity Properties of BaFe2−x Pt x As2

Extraordinary magnetic behaviors, resistivity properties, and lattice parameters of the main sample BaFe2As2 and BaFe2−xPtxAs2 in the variation of x from 0 to 0.4 with the step of 0.1 were investigated. The bulk materials have been prepared by the solid-state reaction method and sealed into a quartz tube. The crystal structure of all samples exhibited the ThCr2Si2-type crystal structure which is in harmony with earlier studies in the literature. The superconducting states with magnetization measurements have been detailed in the wide temperature range 5–170 K, up to a field of 20 Oe. Increasing Pt and decreasing Fe elements in the BaFe2−xPtxAs2 compound deteriorated superconductivity. Using magnetization measurement data, we present the variation of superconducting critical temperature (Tc) correlating with a Pt dopant rate from x = 0 to x = 0.4. The dopant rate of x = 0.3 exhibited the limit rate for maximum Tc; deterioration of superconductivity was revealed with a dopant rate of more than x = 0.3. This should be explained by varying Tc related to a lattice shrinking and pressure effect (geometric factor).

Electronic structure and X-ray spectroscopic properties of YbNi2P2

X-ray absorption spectrum at the Yb L3 edge and X-ray emission spectra of Ni and P at the K and L2,3 edges have been studied experimentally and theoretically in the mixed valent compound YbNi2P2 with ThCr2Si2 type crystal structure. The electronic structure of YbNi2P2 is investigated using the fully relativistic Dirac linear muffin-tin orbital (LMTO) band-structure method. The effect of the spin–orbit (SO) interaction and Coulomb repulsion U on the electronic structure of YbNi2P2 is examined in the frame of the LSDA+SO+U method. The core-hole effect in the final states as well as the effect of the electric quadrupole E2 transitions have been investigated. A good agreement between the theory and the experiment was found. Both the trivalent and the divalent Yb ions in YbNi2P2 are reflected in the experimentally measured Yb L3 X-ray absorption spectrum simultaneously. We found that the best agreement between the experimental spectrum and sum of the theoretically calculated Yb2+ and Yb3+ spectra is achieved with 73% ytterbium ions in 2+ state and 27% ions in 3+ state.

複合極限環境下（低温・磁場・圧力下）における超音波測定

In this article, we review recent advances in ultrasonic measurements under multi-extreme conditions such as high pressures, high magnetic fields and low temperatures. We have developed a clamped piston-cylinder high-pressure cell for the application of hydrostatic pressure and uniaxial one as well, and succeeded to perform the ultrasonic measurement under multi-extreme conditions, even in a pulsed magnetic field for the first time. We show the technical details, the current problems and some of recent results obtained in compounds with a ThCr2Si2-type crystal structure such as EuRh2Si2 and BaFe2As2.

Thermodynamic and electrical transport properties of PrAg2Si2

Physical properties of PrAg2Si2, crystallizing in the tetragonal ThCr2Si2-type crystal structure, was investigated by means of magnetic, calorimetric and electrical transport measurements. The compound was found to order magnetically below 14K, and exhibit two subsequent order–order magnetic phase transitions at 11.5K and 8K. The low-temperature magnetic structure encloses a substantial ferromagnetic component but the exact nature of the different spin arrangements in PrAg2Si2 remains unclear. The compound is a good metallic conductor.

High pressure effects on the superconductivity in rare-earth-doped CaFe2As2

High pressure superconductivity in a rare-earth-doped Ca0.86Pr0.14Fe2As2 single-crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using the designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low-temperature X-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with Tc as high as ∼51 K at 1.9 GPa, presenting the highest Tc reported in the intermetallic class of 122 iron-based superconductors. The resistive transition observed suggests a possible existence of two superconducting phases at low pressures of 0.5 GPa: one phase starting at Tc1 ∼ 48 K and the other starts at Tc2 ∼ 16 K. The two superconducting transitions show distinct variations with increasing pressure. High pressure and low-temperature structural studies indicate that the superconducting phase is a collapsed tetragonal ThCr2Si2-type (122) crystal structure.

Electronic band calculation of BaPd2Sb2: Which polymorph is stable?

Iron-pnictide superconductor (Ba,K)Fe2As2 and nickelate superconductor BaNi2As2 both have the ThCr2Si2-type crystal structure. On the other hand, platinum superconductor SrPt2As2 has the CaBe2Ge2-type crystal structure. We focused on BaPd2Sb2, which contains Pd located between Ni and Pt on the periodic table. We have calculated the electronic structure of BaPd2Sb2 from first-principles, and found that the CaBe2Ge2-type structure is more stable. The Fermi surfaces of BaPd2Sb2 are two-dimensional for the CaBe2Ge2-type structure, and are three-dimensional for the ThCr2Si2-type structure. The calculated D(EF) is 32.1 States/Ry, which is comparable with that in SrPt2As2. These results strongly suggest that superconductivity also occurs in BaPd2Sb2.

Magnetothermal study of La1−xNdxMn2Si2 (x=0.35 and 0.40) compounds

Magnetic properties of La1−xNdxMn2Si2 (x=0.35 and 0.40), crystallizing with the tetragonal ThCr2Si2-type crystal structure, are investigated by means of magnetic and calorimetric measurements. La1−xNdxMn2Si2 (x=0.35 and 0.40) compounds have complex magnetic states. Anomalies are observed in the heat capacity data which refer to different magnetic states. In addition, the electronic, magnetic and lattice contributions to the heat capacity of the compound are determined. Moreover, the characteristic Debye (ΘD) temperatures of both compounds are estimated. We present the comparative results of magnetothermal properties of several compounds in the R1−xR′xMn2Si2 family which show reentrant ferromagnetism at low temperatures due to rare-earth elements.

Peculiar magnetic phase diagrams of GdPd2Si2 single crystal

High field magnetization measurement were performed up to 45 T using a long pulse magnet on a GdPd2Si2 single crystal compound having the tetragonal ThCr2Si2-type crystal structure and an antiferromagnetic ordering of TN=16.7 K. The magnetization processes are almost typical antiferromagnetic ones with a weak magnetic anisotropy though small anomalies exist in the processes. The magnetization saturates 7 μB/f.u. above Bs=32 T. The B001-T and B100-T magnetic phase diagrams were constructed. There are three phases, the phase I, II and III, under non-magnetic field in the ordered state for both diagrams. The field-induced phase, phase IV, appears below the paramagnetic phase by the [100] magnetic field, B100, which is accompany with a re-entrant behaviour. The B100 makes the phase II be stable and the phase I and III be instable. These strange behaviours indicate the direction of the magnetic moment changes in each phase.

Electronic structure of LaPt2Si2

Iron–pnictide superconductor (Ba,K)Fe2As2 has the ThCr2Si2-type crystal structure. On the other hand, recently found arsenide superconductor SrPt2As2 has the CaBe2Ge2-type structure, which is a different polymorphic form of the ThCr2Si2-type structure. LaPt2Si2 shows superconductivity with Tc=2.2K and the above-shown two different crystal structures have been reported previously. In this paper we have calculated the electronic structure of LaPt2Si2 from first-principles. From the total energy calculation, we show that the CaBe2Ge2-type structure is more stable in this compound. The density of states at the Fermi level (D(EF)) mainly consists of Pt1-d orbitals, while the Pt2-d orbitals contribute to D(EF) only half of Pt1-d orbitals. In contrast to iron–pnictide system, the total density of states at the Fermi level does not depend so much on the distortion of the PtX4 tetrahedra and/or the height of the X atom from the two-dimensional Pt plane. The calculated electronic specific heat coefficient is 7.0mJ/molK2 for LaPt2Si2, which is comparable with that of SrPt2As2, but larger than the experimental value 4mJ/molK2.

Specific heat studies of RCu 2Si 2 (R=Tb, Ho) and RCu 2Ge 2 (R=Gd, Tb) compounds

Magnetic properties of RCu 2Si 2 (R=Tb, Ho) and RCu 2Ge 2 (R=Gd, Tb), crystallizing with the tetragonal ThCr 2Si 2-type crystal structure, were investigated by means of magnetic and calorimetric measurements. All compounds exhibit antiferromagnetic ordering below T N equal to 11.2 K for TbCu 2Si 2, 5.3 K for HoCu 2Si 2, 12.0 K for GdCu 2Ge 2 and 12.5 K for TbCu 2Ge 2. At temperatures of T t = 8.5 K for TbCu 2Si 2 and 9.8 K for TbCu 2Ge 2 additional phase transitions were observed. Apart from the investigation on magnetic phase transitions, magnetic contributions to the specific heat were analyzed in terms of Schottky effect. The corresponding crystal electric field (CEF) level splitting schemes were calculated. Moreover, the phononic characteristic temperatures were estimated.

Magnetic structure of TmCu2Ge2

TmCu2Ge2 compound crystallizes in the tetragonal ThCr2Si2-type crystal structure. The neutron diffraction reveals the presence of an incommensurate antiferromagnetic order below TN=2.5K. The Tm magnetic moment of 5.0(1)μB at 0.47K is parallel to the c-axis. The order is described by the propagation vector k=[kx, kx, 0], where kx=0.117(3). The increase of the values of the components kx near the Néel temperature is observed.

Novel magnetic behaviour of GdPd2Si2 single crystal

Magnetic and specific heat measurements have been performed on a GdPd2Si2 single crystal compound having the tetragonal ThCr2Si2-type crystal structure. Specific heart shows two λ-type anomalies at TN=16.7 K, T1=12.3 K and a small hump with hysteresis at T2=6.8 K; there are three phases below TN. The magnetic entropy of Rln6 is released below TN. The temperature dependence of magnetic susceptibility shows a very peculiar behaviour. With decreasing temperature, the magnetic susceptibility along the [001] direction increases through a small kink at TN down to T1 where a clear cusp is seen, decreases rapidly below TN, then shows an upturn and a clear kink at T2 followed by a gradual increase. This behaviour examines at various magnetic field along [001] and [100]. The B001-T and B100-T magnetic phase diagrams have been constructed. The field-induced phase appears near by the 100] magnetic field, B100.

Nature of magnetic phase transitions in TbCu 2X 2 (X = Si, Ge) and HoCu 2Si 2 compounds

Physical properties of TbCu 2X 2 (X = Si, Ge) and HoCu 2Si 2, crystallizing with the tetragonal ThCr 2Si 2-type crystal structure, were investigated by means of neutron diffraction as well as by magnetic and calorimetric measurements. All compounds exhibit antiferromagnetic ordering below T N equal to 12 K for TbCu 2Si 2, 13 K for TbCu 2Ge 2 and 6.4 K for HoCu 2Si 2. Neutron diffraction data indicate collinear antiferromagnetic structure described by the propagation vector k → = [ ( 1 / 2 ) , 0 , ( 1 / 2 ) ] at low temperatures. While increasing temperature the direction of the Tb magnetic moment changes from [1 1 0] to [0 1 0] at T t = 8.5 K for TbCu 2Si 2 and 9.7 K for TbCu 2Ge 2. In HoCu 2Si 2 a change of the magnetic structure from the collinear to a modulated one described by the propagation vector k → = [ k x , 0 , k z ] was found at T t = 4.7 K.

Magnetic and thermodynamic properties of Nd T2Ge 2 ( T=Pd, Ag) compounds

Physical properties of NdPd 2Ge 2 and NdAg 2Ge 2, crystallizing with the tetragonal ThCr 2Si 2-type crystal structure, were investigated by means of magnetic, calorimetric, electrical transport as well as by neutron diffraction measurements. The specific heat studies and neutron diffraction measurements were performed down to 0.30 K and 0.47 K, respectively. Both compounds exhibit antiferromagnetic ordering below T N equal to 1.5 K for NdPd 2Ge 2 and 1.8 K for NdAg 2Ge 2. Neutron diffraction data for the latter germanide indicate antiferromagnetic collinear structure described by the propagation vector k =(0.5, 0, 0.5). The Nd magnetic moments equal to 2.24(5) μ B at 0.47 K are aligned along the a-axis and have the +− sequence within the crystal unit cell. For NdPd 2Ge 2 only very small Bragg peaks of magnetic origin were observed in the neutron diffraction patterns measured below T N, thus hampering determination of the magnetic structure. Both compounds exhibit metallic-like electrical conduction. From the specific heat data the crystal electric field (CEF) levels schemes were determined. Difference between the overall CEF splitting in the two compounds is correlated with their structural parameters.