Yu.B. Paderno

Magnetic and transport properties of TmB12, ErB12, HoB12 and DyB12

Abstract We present measurements of the magnetization and electrical resistivity of TmB 12 , ErB 12 and HoB 12 singlecrystalline samples as well as of a DyB 12 polycrystalline sample. Data were taken in the paramagnetic state as well as in the magnetically ordered regime. To our knowledge, this is the first systematic investigation of TmB 12 , ErB 12 , HoB 12 and DyB 12 on the magnetically ordered phase by measurements of magnetization and electrical resistivity. All samples investigated undergo antiferromagnetic ordering in the lower Kelvin temperature range. From the magnetization data, the Neel temperatures, the paramagnetic Curie temperatures, and the effective magnetic moments of the corresponding rare earth ions are determined. It is shown that the indirect exchange interaction of RKKY type is the dominating mechanism leading to the observed antiferromagnetic ordering. The measured temperature dependences of the electrical resistivity show a pronounced effect of superzone boundaries near Neel temperatures. The spin wave scattering of conduction electrons below T N is discussed, too.

Low-temperature thermal properties of yttrium and lutetium dodecaborides

The heat capacity (Cp) and dilatation (?) of YB12 and LuB12 are studied. Cp of the zone-melted YB12 tricrystal is measured in the range 2.5?70?K, of the zone-melted LuB12 single crystal in the range 0.6?70?K, and of the LuB12 powder sample in the range 4.3?300?K; ? of the zone-melted YB12 tricrystal and LuB12 single crystals is measured in the range 5?200?K. At low temperatures a negative thermal expansion (NTE) is revealed for both compounds: for YB12 at 50?70?K, for LuB12 at 10?20?K and 60?130?K. Their high-temperature NTE is a consequence of nearly non-interacting freely oscillating metal ions (Einstein oscillators) in cavities of a simple cubic rigid Debye lattice formed by B12 cage units. The Einstein temperatures are ~254 and ~164?K, and the Debye temperatures are ~1040?K and ~1190?K for YB12 and LuB12 respectively. The LuB12 low-temperature NTE is connected with an induced low-energy defect mode. The YB12 superconducting transition has not been detected up to 2.5?K.

Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy

We have investigated the intermediate valence narrow-gap semiconductor SmB6 at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of SmB6, did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero - bias conductance are in the diffusive Maxwell regime. They had spectra with only small non-linearities. Contacts with small zero - bias conductance are in the tunnelling regime. They had larger anomalies, but still indicating a finite 45 % residual quasiparticle density of states at the Fermi level at low temperatures of T = 0.1 K. The density of states derived from the tunelling spectra can be decomposed into two energy-dependent parts with Eg = 21 meV and Ed = 4.5 meV wide gaps, respectively.

Tunneling and magnetic characteristics of superconducting ZrB12 single crystals

Bulk and surface properties of high-quality single crystals of zirconium dodecaboride have been studied in the temperature range from 4.5 K up to the superconducting transition temperature which is found to be nearly 6.06 K. Scanning tunnelling spectroscopy data, together with dc and ac magnetization measurements, are consistent with the conventional s-wave pairing scenario, whereas they disagree in estimates of the electron-phonon coupling strength. We explain the divergence, supposing a great difference between the surface and bulk superconducting characteristics of the compound. This assertion is supported by our findings of a non-linear magnetic response to an amplitude-modulated alternating magnetic field, testifying to the presence of surface superconductivity in the ZrB

Analysis of a Zirconium Diboride Single Crystal, ZrB2 (0001), by XPS

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Transport and magnetic properties of mixed valent SmB6

samples at dc fields exceeding the thermodynamic critical field.

The micromechanical properties of melted borides of rare earth metals

X-ray photoelectron spectroscopy (XPS) was used to study the clean surface of ZrB2 (0001). The clean surface exhibits Zr 3d5/2 and Zr 3d3/2 peaks at 179.2 and 181.6 eV, respectively. However, angle resolved XPS indicated ZrO2 peaks at 183.5 and 185.7 eV at higher emission angles, indicating that further cleaning was necessary. After additional cleaning cycles, these oxide peaks were no longer observed at high emission angles. This result demonstrates the necessity to probe the near surface region with higher emission angles on very reactive surfaces, in order to completely establish the cleanliness of the surface.

Anomalous electron-phonon coupling probed on the surface of superconductor ZrB12

Abstract We present the results of transport and magnetic measurements performed on two intermediate valent SmB6 single crystals. The received results above 15 K are in agreement with the results obtained by other groups. At the lowest temperatures, however, the resistivity, magnetization, and susceptibility seem to be strongly influenced by the presence of impurities in the investigated samples.

Ground state formation in intermediate valent SmB6

Abstract The compounds of boron with metals are characterized by a high strength of the interatomic bonds and a low relaxation ability of the internal stress. As a result they are very fragile materials. The value of the microfragility of these compounds may be used for a comparative estimation of their strength and of their possible practical application. The structure of the surface disturbance and the relative microfragility (microelasticity) of a number of alkaline and rare earth metal hexaborides obtained by melting (sometimes accompanied by zone purification) were determined by electron microscopy and measurements of the microhardness and microfragility. The modulus of elasticity and the effective surface energy of a number of materials were determined by the method of continuous indentation. It was established that, if we place these compounds in the sequence in which their microelasticity decreases, we have the following order: SmB 6 →BaB 6 →EuB 6 →CaB 6 → YbB 6 →LaB 6 →PrB 6 →NdB 6 →GdB 6 . In this order there is a noticeable increase in the length of the metal-metal and metal-B bonds which have a metallic character and are weakened when passing from samarium to gadolinium. The ability of the metal atoms to transfer some of their electrons to the boron atoms increases in the same direction. This also increases the fragility of the compounds because of the strong acceptor ability of the boron atoms and their tendency to form a stable sp 3 electron state.

Antiferro-quadrupole resonance in CeB6

Magnetization measurements under hydrostatic pressure up to 10.5 kbar in zirconium dodeca- boride ZrB12 superconductor (Tc ≃ 6.0 K at p = 0) were carried out. A negative pressure effect on Tc with dTc/dp = −0.0225(3) K/kbar was observed. The electron-phonon coupling constant �el ph decreases with increasing pressure with dlnel ph/dp ≃ −0.20%/kbar. The magnetic field penetration depthwas studied in the Meissner state and, therefore, probes only the surface of the sample. The absolute values ofand the superconducting energy gap at ambient pressure and zero temperature were found to be �(0) =140(30) nm and �0 =1.251(9) meV, respectively. �0 scales linearly with Tc as 2�0/kBTc = 4.79(1). The studies of the pressure effect onreveal that � 2 increases with pressure with dln� 2 (0)/dp = 0.60(23) %/kbar. This effect can not be explained within the framework of conventional adiabatic electron-phonon pairing, suggesting that close to the surface, an unconventional non-adiabatic character of the electron-phonon coupling takes place.