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1.
K. Ishida Y. Kawasaki T. Mito C. Thessieu G.-q. Zheng Y. Kitaoka C. Geibel F. Steglich 《Hyperfine Interactions》2000,128(1-3):193-204
In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have
performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2 polycrystals: Ce0.99Cu2.02Si2 (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and
Ce0.975Cu2Si2 (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction
of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although Tc stays nearly constant in the measured pressure range, P≤14.4 kbar. This shows that the P-induced transition from the A to
the SC phases is of a first-order type, without passing through a quantum (T→0) phase transition. However, no superconductivity
has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample
quality.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
From 29Si NMR study, we present evidence for spatially inhomogeneous development of antiferromagnetic (AF) ordering below T(o) = 17.5 K in URu2Si2. In the pressure range between 3.0 and 8.3 kbar, we have observed the 29Si NMR lines arising from the AF region as well as the previously observed 29Si NMR line which correspond to the nonmagnetic region in the sample. The AF volume fraction is enhanced by applied pressure, whereas the magnitude of internal field at the Si site remains constant (910 Oe) up to 8.3 kbar. In the AF region, the ordered moment is about an order of magnitude larger than 0.03 mu(B)/U. 相似文献
3.
《Solid State Communications》1986,59(9):603-606
Nuclear magnetic resonance (NMR) and relaxation studies on 29Si have been carried out on the heavy Fermion system URu2Si2. Above the Kondo temperature of about 60 K, the nuclear relaxation time T1 is nearly temperature independent, which is consistent with the occurrence of fluctuations of localized U moments. Below about 60 K T1 is inversely proportional to temperature suggesting that the system behaves like a Fermi liquid. A sharp increase in T1 occurs below 17 K which is probably associated with the opening of an energy gap at the Fermi surface due to the formation of a spin density wave state. Below about 10 K, T1 reacquires the inverse temperature dependence observed in the 17 K ∼ 60 K temperature range. 相似文献
4.
The structural parameters with stability upon Si incorporation and elastic, electronic, thermodynamic and optical properties of Ti3Al1−xSixC2 (0≤x≤1) are investigated systematically by the plane wave pseudopotential method based on the density functional theory (DFT). The increase of some elastic parameters with increasing Si-content renders the alloys to possess higher compressive and tensile strength. The Vickers hardness value obtained with the help of Mulliken population analysis increases as x is increased from 0 to 1. The solid solutions considered are all metallic with valence and conduction bands, which have a mainly Ti 3d character, crossing the Fermi level. The temperature and pressure dependences of bulk modulus, normalized volume, specific heats, thermal expansion coefficient, and Debye temperature are all obtained through the quasi-harmonic Debye model with phononic effects for T=0−1000 K and P=0−50 GPa. The obtained results are compared with other results available. Further an analysis of optical functions for two polarization vectors reveals that the reflectivity is high in the visible–ultraviolet region up to ∼10.5 eV region showing promise as a good coating material. 相似文献
5.
We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe2As2, BaFe2As2, and (Ba0.55K0.45)Fe2As2, by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe2As2 and BaFe2As2 (lightly doped with Sn) display structural phase transitions near 170 and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba0.55K0.45)Fe2As2 is superconducting for T<30 K. The effect of pressure on BaFe2As2 is to shift the onset of the crystallographic transformation down in temperature at the rate of ~−1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba0.55K0.45)Fe2As2 is to shift the onset of superconductivity to lower temperatures at the rate of ~−0.21 K/kbar. The effect of pressure on CaFe2As2 is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P<5 kbar. However, higher pressures bring about another phase transformation characterized by reduced-resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (Hc2) data in (Ba0.55K0.45)Fe2As2 and CaFe2As2 are discussed. 相似文献
6.
Ae Ran Lim 《Physica B: Condensed Matter》2012,407(5):833-837
The structural properties and relaxation mechanisms of Li2KH(SO4)2 crystals were determined using the temperature dependences of NMR spectra and the spin-lattice relaxation times (T1) of their 1H, 7Li, and 39K nuclei. The results obtained were compared with the previously reported physical properties of LiKSO4 crystals. The substitution of the potassium ions with protons in the LiKSO4 crystals were variations in the phase transition temperatures, and the non-appearance of ferroelastic properties. The 7Li T1 for the Li2KH(SO4)2 crystals was much shorter than the 7Li T1 for the LiKSO4 crystals, and these findings indicate that the presence of the protons in Li2KH(SO4)2 causes the Li ions to move with greater freedom. 相似文献
7.
Ekaterina Leonova Jekabs Grins Mohammadreza Shariatgorji Leopold L. Ilag Mattias Edn 《Solid state nuclear magnetic resonance》2009,36(1):11-18
We compare 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra from the two modifications of silicon nitride, α-Si3N4 and β-Si3N4, with that of a fully (29Si, 15N)-enriched sample 29Si315N4, as well as 15N NMR spectra of Si315N4 (having 29Si at natural abundance) and 29Si315N4. We show that the 15N NMR peak-widths from the latter are dominated by J(29Si–15N) through-bond interactions, leading to significantly broader NMR signals compared to those of Si315N4. By fitting calculated 29Si NMR spectra to experimental ones, we obtained an estimated coupling constant J(29Si–15N) of 20 Hz. We provide 29Si spin-lattice (T1) relaxation data for the 29Si315N4 sample and chemical shift anisotropy results for the 29Si site of β-Si3N4. Various factors potentially contributing to the 29Si and 15N NMR peak-widths of the various silicon nitride specimens are discussed. We also provide powder X-ray diffraction (XRD) and mass spectrometry data of the samples. 相似文献
8.
Elastic constants and thermodynamic properties of Mg2SixSn1-x from first-principles calculations
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<正>This paper stuides the elastic constants and some thermodynamic properties of Mg2SixSnn-1(x=0,0.25,0.5, 0.75,1) compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory,within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg2SixSnn-1 were calculated.It shows that,at 273 K,the elastic constants of Mg2Si and Mg2Sn are well consistent with previous experimental data.The isotropy decreases with increasing Sn content.The dependences of the elastic constants,the bulk modulus,the shear modulus and the Debye temperatures of Mg2Si and Mg2Si0.5Sn0.5 on pressure were discussed.Through the quasi-harmonic Debye model,in which phononic effects were considered,the specific heat capacities of Mg2SixSn1-x at constant volume and constant pressure were calculated.The calculated specific heat capacities are well consistent with the previous experimental data. 相似文献
9.
The very recently discovered pseudo-binary hexagonal silicide SrNixSi2−x, which exhibits low-TC superconductivity, was examined theoretically to understand the effect of the unusual doping type (partial replacement of Si by Ni) on the electronic band structure of this material. Besides, the possible factors of the stabilization of the hexagonal AlB2-type structure of SrSi2 upon substitution of Ni for Si, and the solubility limit of Ni in SrNixSi2−x are discussed in terms of competing Si–Si, Si–Ni, and Ni–Ni bonds. 相似文献
10.
H. Chudo H. Sakai S. Kambe Y. Homma Y. Haga T.D. Matsuda H. Yasuoka 《Physica B: Condensed Matter》2009,404(19):3216-3219
We present 27Al NMR studies for a single crystal of the Np-based superconductor NpPd5Al2. We have observed a five-line 27Al NMR spectrum with a center line and four satellite lines separated by first-order nuclear quadrupole splittings. The Knight shift clearly drops below Tc. The temperature dependence of the 27Al nuclear spin-lattice relaxation rate shows no coherence peak below Tc, indicating that NpPd5Al2 is an unconventional superconductor with an anisotropic gap. The analysis of the present NMR data provides evidence for strong-coupling d-wave superconductivity in NpPd5Al2. 相似文献
11.
F.L. Boariu A. Nuber M. Klein P. Lejay 《Journal of Electron Spectroscopy and Related Phenomena》2010,181(1):82-87
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. 相似文献
12.
L.M. Holanda E.M. Bittar P.G. Pagliuso R.W. Hu S. Maquilon D.L. Huber 《Physica B: Condensed Matter》2009,404(19):2964-2968
Electron spin resonance (ESR) experiments in YbRh2Si2 Kondo lattice at different field/frequencies and H⊥c revealed: (i) a strong field dependent Yb3+ spin-lattice relaxation, (ii) a weak field and T-dependent effectiveg-value, (iii) a suppression of the ESR intensity beyond 15% of Lu-doping, and (iv) a strong sample and Lu-doping (≤15%) dependence of the ESR data. These results suggest that the ESR signal in YbRh2Si2 may be due to a coupled Yb3+-conduction electron resonant collective mode with a subtle field-dependent spins dynamic. 相似文献
13.
Daniel Martins de Almeida Carlos Angelo Nunes Gilberto Carvalho Coelho 《Journal of magnetism and magnetic materials》2009,321(17):2578-2581
In this work the Mn5Si3 and Mn5SiB2 phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn5Si3 and near single-phase Mn5SiB2 microstructures. The magnetic behavior of the Mn5Si3 phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn5SiB2 phase shows a ferromagnetic behavior presenting a saturation magnetization Ms of about 5.35×105 A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments. 相似文献
14.
B. Podmiljšak B. Markoli P.J. McGuiness S. Kobe 《Journal of magnetism and magnetic materials》2009,321(4):300-304
Gd5Si2Ge2-based alloys can exhibit a giant magnetocaloric effect (GMCE); this gives them the potential for use in cooling technologies. It has also been reported that a small addition of iron reduces the hysteresis losses in Gd5Si2Ge2-based alloys, thus increasing the net refrigerating capacity. In this investigation, we have been the first to look at the effect on the microstructure and magnetic properties of Gd5Si2Ge2 resulting from a wide range of substitutions of Si by Fe. The macrostructures of the arc-melted buttons revealed some very unusual surface morphologies, and the analytical results revealed a gradual substitution of the Gd5(Si,Ge,Fe)4-type phase by a Gd5(Si,Ge,Fe)3-type phase and the presence of three grain-boundary phases, two of which contain substantial amounts of iron. The magnetic measurements indicated that larger amounts of iron reduced the hysteresis losses, but at the same time reduced the Curie temperatures to below lower values that would make the material useful in practice. 相似文献
15.
Ab initio molecular orbital calculations (Hartree–Fock, HF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting 29Si and 17O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the 29Si and 17O chemical shifts (δiSi and δiO), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δiSi and δiO have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δiSi values agree well (within ±1 ppm) with experimental data. The calculated 17O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δiO for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting 29Si and 17O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. 相似文献
16.
Hiromitsu Terao Yoshihiro Furukawa Satomi Miki Fukue Tajima Masao Hashimoto 《Hyperfine Interactions》2004,159(1-4):211-216
The crystal structure of [C(NH2)3]3Sb2Br9 was determined at 143 K: monoclinic, space group C2/c, Z = 4, a = 15.695 (3), b = 9.039(2), c = 18.364(3) Å, β = 96.94(1)°. The structure consists of two crystallographically independent guanidinium ions and two-dimensional corrugated sheets of (Sb2Br9 3?) n , in which SbBr6 octahedra are connected through three bridging Br atoms each other. One of the cations situates in a cavity of the (Sb2Br9 3?) n layer with statistical disorder, while the other situates between the layers without disorder. Three 81Br NQR resonance lines were assignable to terminal Br atoms, while only one line was found for two inequivalent bridging Br atoms. All the 81Br NQR resonance lines were subjected to fade-out at low temperatures. The temperature dependence curve of 1H NMR T 1 showed well defined two minima, which were explained by postulating the C3 reorientations of two types of cations with very different activation energies. The DTA (DSC) measurement revealed a phase transition of a first-order type at 444 K. 相似文献
17.
The influences of boron addition on the phase formation, Curie temperature and magnetic entropy change of the NaZn13-type La(Fe0.9Si0.1)13 compound have been investigated. Eight boron containing La(Fe0.9Si0.1)13Bx samples were prepared with x=0, 0.03, 0.06, 0.1, 0.2, 0.3, 0.5 and 0.6, respectively. Experimental results show that a small amount of B addition in La(Fe0.9Si0.1)13 forms the solid solution NaZn13-type structure phase by substituting B for Si or doping B into interstitial position of the lattice, preserves its giant magnetocaloric effects due to their first-order structural/magnetic transition, as well as increase its Curie temperature Tc slightly. The maximum magnetic entropy changes in the magnetic field change of 0–1.6 T are around 20 J kg–1 K–1 for the samples with Boron addition less than 0.3, while improving the Curie temperatures by 2 K. 相似文献
18.
In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that β-C3N2 is the only phase under high pressure, and α-C3N2 does not exist. β-C3N2 is a covalent crystal composed of strong CC and CN covalent bonds. Band gap of β-C3N2 increases with pressure. The width of antibonding state, shown in partial density of states (PDOS), keeps about 5 eV with rising pressures, which brings stable CN or CC covalent bonds. At sufficiently low temperatures, heat capacity (Cv) is proportional to T3; and at intermediate temperatures, Cv is governed by the details of vibrations of the atoms; finally, Cv reaches to β-C3N2's Dulong–Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (α) increases with temperature, α is less than 1×10−5 K−1. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures. 相似文献
19.
On the basis of temperature dependences of the acoustic wave velocity and the acoustic attenuation in Sn2P2(Se0.28S0.72)6 crystals under normal conditions and hydrostatic pressures, we show that increasing pressure causes these crystals to move away from the tricritical point and enter into the region of first-order phase transitions (PTs) in (P,?T,?x)-space. Still higher hydrostatic pressures lead to an increase in the attenuation of the acoustic wave with the velocity v 22 and broadening of its anomaly in the PT region, thus hinting at splitting of the PT, with the appearance of an intermediate incommensurate phase approximately at P?=?3.3?kbar. We offer a (P,?T,?x)-phase diagram for the solid solutions Sn2P2(Se x S1? x )6, which includes two special lines of PTs, tricritical and triple ones, which can intersect at the Lifshitz point at negative hydrostatic pressures. The variations of the surface of acoustic wave velocities occurring with changing temperature are obtained for Sn2P2(Se0.28S0.72)6 crystals under the atmospheric pressure. 相似文献
20.
Y. Kohori Y. Yamato Y. Iwamoto T. Kohara 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):601-604
We have carried out 115In nuclear quadrupole resonance (NQR) measurements in CeRhIn5. At ambient pressure, CeRhIn5 undergoes an antiferromagnetic AF phase transition at K. The 115In NQR spectrum has shown the appearance of a small internal field in the direction perpendicular to the tetragonal c-axis. With application of a hydrostatic pressure, the AF state is suppressed and the superconductivity appears just above
the critical pressure (P
= 17 kbar). The nuclear spin lattice relaxation rate 1/T1 of 115In measured at P
= 27 kbar indicates the occurrence of the superconductivity in the nearly AF region. In the superconducting state, 1/T1 has no Hebel-Slichter coherence peak just below of 2 K and has a power law T-dependence (T3) down to 300 mK. This is consistent with anisotropic superconductivity, with line nodes in the superconducting energy gap:
non-s-wave superconductivity occurs in CeRhIn5.
Received 5 July 2000 相似文献