Low-temperature Specific Heat Research Articles

Experimental observation of magnetic dimers in diluted Yb:YAlO3

We present a comprehensive experimental investigation of Yb magnetic dimers in Yb$_{0.04}$Y$_{0.96}$AlO$_3$, an Yb-doped Yttrium Aluminum Perovskite (YAP) YAlO$_3$ by means of specific heat, magnetization and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and $\sim 7$\% of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the $c$-axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at $\Delta \approx 0.2$~meV in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intra-dimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, YbAlO$_3$ [arXiv:1904.11513, arXiv:1902.04112], and in contrast to Yb$_2$Pt$_2$Pb [arXiv:1606.01309, arXiv:1907.01067]. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.

Low temperature specific heat of 12442-type KCa2Fe4As4F2 single crystals

Low-temperature specific heat (SH) is measured for the 12442-type KCa$_2$Fe$_4$As$_4$F$_2$ single crystal under different magnetic fields. A clear SH jump with the height of $\Delta C/T|_{T_c}$ = 130 mJ/mol K$^2$ is observed at the superconducting transition temperature $T_c$. It is found that the electronic SH coefficient $\Delta\gamma (H)$ quickly increases when the field is in the low-field region below 3 T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicates the presence of the $T^2$ term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and placed this system in the middle between the BCS limit and the Bose-Einstein condensation.

Unconventional superconducting properties of noncentrosymmetric Re5.5Ta

Rhenium based noncentrosymmetric superconductors crystallizing in $\alpha$-Mn structure have become potential candidates to exhibit an unconventional superconducting ground-state. Here we report a detailed investigation on the superconducting and normal state properties of Re$_{5.5}$Ta, that also has the $\alpha$-Mn structure. Magnetization, specific heat, and transport measurements confirm the bulk superconducting transition \textit{T}$_{C}$ at 8.0 K. Upper critical field value (H$_{C2}$(0)) calculated from magnetization, specific heat and AC transport measurements exceed the Pauli paramagnetic limit (14.7 T), indicating that the superconducting properties of Re$_{5.5}$Ta are probably unconventional in nature. However, low-temperature specific heat and transverse-field muon spin rotation measurements suggest a surprising nodeless isotropic superconducting gap, although with strong electron-phonon coupling.

Calorimetric investigation of the low temperature specific heat of the amorphous RE-Cu-Y metallic glasses

The specific heat, Cp of the RE0.40Cu0·37Y0.23 (RE = Gd, Tb, Dy, Ho, Er, and Lu) metallic glasses have been measured from liquid helium to liquid nitrogen temperatures using the traditional pulse and continuous heating techniques. The average values of the electronic and lattice specific heat coefficients, γ and β were estimated to be of order of 2.35 mJ/mol,K2, and 0.62 mJ/mol,K4 respectively. The corresponding average Debye temperature, θD(0) was estimated to be 145 K. The magnetic contributions to the specific heat, Cm are quite apparent in all samples. Cm has been separated from the nonmagnetic terms with an acceptable accuracy method; by subtracting the nonmagnetic specific heat, of the amorphous Lu0.40Cu0·37Y0.23 sample, which has been prepared with Lu rather than RE. The magnetic phase transition in these magnetic amorphous has been confirmed by the ac susceptibility results, which exhibits quite sharp maximum at the freezing temperature Tf, the spin glass freezing temperature. The magnetic specific heats were characterized by the quite broad maximum at temperature Tm and by the low-temperature linear term in Cm. Values of Tm are higher than Tf values for most of the samples. The magnetic entropy changes, Sm in the temperature range T = 0 K to T = Tf have also been estimated. Values of Sm and the effective value of the magnetic spin number J were found to be less than the expected maximum values of Sm(max) = Rℓn(2J + 1) and the well-known value of the magnetic number J of the rare earth metal respectively. The apparent features of the present amorphous magnetic samples agree quite well with traditional behaviors of the condensed spin glass systems.

Vibrational dynamics of the type-I clathrates A8Sn44□2 (A = Cs, Rb, K) from lattice-dynamics calculations, inelastic neutron scattering, and specific heat measurements

We report on a joint theoretical and experimental study of the vibrational dynamics of the type-I clathrates A8Sn44□2 (A = K, Rb, Cs, and □ stands for a vacancy) by high-resolution inelastic neutron scattering experiments combined with low-temperature specific heat measurements (2–300 K). Ab initio lattice dynamics calculations were performed on hypothetical vacancy-free A8Sn46 clathrates in order to determine the phonon dispersions and vibrational density of states Z(ω). The temperature dependence of the generalized vibrational density of states (GVDOS) was traced from 420 K down to 50 K, paying particular attention to the low-energy region of the GVDOS spectra. In the Cs and Rb compounds, the inelastic signal at low energies is dominated by several peaks mainly associated with the dynamics of the alkali metal atoms A in the polyhedral cages of the clathrate structure. In contrast, the low-energy spectrum of the K compound features a more pronounced contribution of the weighed modes of the framework Sn atoms. Upon cooling, the A-weighted modes soften regardless of the nature of the alkali element. The shift observed is similar for the three compounds and of small amplitude, suggestive of a dominant quasi-harmonic behavior above 50 K. The distinct dynamics of the K atoms in comparison to Cs and Rb is further demonstrated by the analyses of the low-temperature specific heat data, indicating that the low-energy Sn-weighted modes cannot be described by a Debye model with Einstein-like contributions.

Unveiling the first post-graphene member of silicon nitrides: A novel 2D material

We have theoretically predicted a new two-dimensional allotrope of the silicon nitrides family, being found during our effort to construct a nanosheet from the unit cell of hexagonal β-Si3N4. Electronic structure calculations at the DFT-PBE level of theory have revealed that this new material is an insulator with a band gap of about 4 eV. Finite-temperature density functional molecular dynamics simulations applied at the unit-cell level have also verified that the new compound is thermodynamically more stable than the layered form of hexagonal β-Si3N4 due to having both stronger Si–N bonds and a more compact crystal structure. Furthermore, phonon dispersion calculations have tested and proved the dynamic stability of the new monolayer to a great accuracy with accordingly no soft modes being observed. Results obtained within the quasi-harmonic approximation also demonstrate that our predicted material is indeed a Debye solid with a low-temperature specific heat proportional to T2 as a consequence of its two-dimensional nature, possessing a Debye temperature of about 860 K considerably higher than those of α,β, and γ phases of Si3N4.

Superconductivity in single crystalline LuPd2Si2 probed by heat capacity measurements

Single-crystalline samples of LuPd2Si2 were studied by means of low-temperature specific heat measurements performed in a magnetic field oriented along two main crystallographic directions. The compound was found to be superconducting below 0.6 K, in agreement with previous reports. Detailed analysis of the phase-transition anomaly revealed that LuPd2Si2 is a BCS-like superconductor with anisotropic properties and a possible two-gap character.

Low-temperature properties of pyrochlore Lu2Ir2O7

Low-temperature properties of Lu2Ir2O7 were studied by means of magnetization, ac-susceptibility and specific heat measurements. Our low-temperature measurements reveal a bifurcation of ZFC and FC magnetization data below 140 K, which is further pronounced as a broad anomaly in the temperature development of specific heat and ac-susceptibility. The anomaly can be attributed to the same physical mechanism as discussed for other A2Ir2O7 with A = magnetic rare-earth element, i.e. to short-range correlations among Ir4+ magnetic moments, rather than to a long-range order of Ir sublattice reported earlier. The ac-susceptibility data revealed a strong frequency dependence of the signal, a paradigmatic sign of a spin-glass transition. The analysis of low-temperature specific heat data brought important information on physical properties of Lu2Ir2O7, in striking agreement with our density functional theoretical (DFT) calculations, concluding relatively weak electron-phonon interaction and semimetallic ground state in Lu2Ir2O7.

The low-temperature specific heat of thermal reduced graphene oxide

Measurements of heat capacity at constant pressure of thermally reduced graphene oxide (trGO) and graphite oxide (GtO) were performed in the temperature range from 1.8 to 275 K by thermal relaxation method. Above 15 K, the specific heat values of trGO and GtO are close to each other within 20%. Below 10 K, the ratio of the specific heat of trGO to the specific heat of GtO increases with decreasing temperature and reaches 5 at 2 K. It was found that the temperature dependence of the specific heat of trGO is of a complex character. There are temperature ranges in which of the specific heat of trGO is close to either linear or T2.5.

Crystal growth, low-temperature specific heat, and electronic structure of the filled skutterudite compound ThOs4As12

ABSTRACTSingle crystals of the filled skutterudite compound ThOsAs were grown by a Cd:As flux method. Low-temperature specific-heat measurement reveals a very small Sommerfeld coefficient of the electronic term mJ/molK, indicative of a semiconducting or semimetallic character of ThOsAs. A low electronic density of states at the Fermi level is consistent with anticipations based on the Zintl concept for the filled skutterudites as well as with our results of band structure calculations. Investigations of a nonmagnetic material ThOsAs can be relevant to unmask puzzling strong electron correlations in the putative topological Kondo insulator CeOsAs.

Disorder-driven glasslike thermal conductivity in colusite Cu26V2Sn6S32 investigated by Mössbauer spectroscopy and inelastic neutron scattering

The influence of structural disorder on the thermal transport in the colusite $\mathrm{C}{\mathrm{u}}_{26}{\mathrm{V}}_{2}\mathrm{S}{\mathrm{n}}_{6}{\mathrm{S}}_{32}$ has been investigated by means of low-temperature thermal conductivity and specific heat measurements (2--300 K), $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy and temperature-dependent powder inelastic neutron scattering (INS). Variations in the high-temperature synthesis conditions act as a key parameter for tuning the degree of disorder in colusite compounds. Intriguingly, we find that all synthesized samples are disordered, the degree of which varies with the synthesis conditions used. M\"ossbauer data clearly evidence that Sn atoms do not solely occupy the $6c$ site of the crystal lattice but are present on possibly both the Cu and V sites, leading to a random distribution of these three cations within the unit cell. Increasing the disorder in these materials tends to lead to a smearing out of the main features in the phonon density of states measured by INS. Although the evolution of the inelastic signal upon warming is well described by a quasiharmonic approximation, elastic properties calculations indicate large average Gr\"uneisen parameters, consistent with those determined experimentally from thermodynamic data. Increasing the level of disorder results in a decreased average Gr\"uneisen parameter suggesting that the lowered lattice thermal conductivity is not driven by enhanced anharmonicity. These results provide experimental evidence to support that the remarkable changeover in the lattice thermal conductivity from crystalline to glasslike is solely driven by enhanced disorder accompanied by local lattice distortions.

Correlation between glasses forming ability and density of states for the micro-alloying Al-based metallic glasses

Through study of the effect of micro-alloying on density of states at the Fermi level in the Al-based metallic glasses, we provide compelling experimental evidence that the density of electronic energy states at the Fermi level indeed is closely correlated with the glass forming ability (GFA) of metallic glasses, as predicted by the nearly free electron model. The values of the density of states at the Fermi level, N(EF), for the Al86Ni9Y5, Al86Ni6.75Co2.25Y5 and Al86Ni6.75Co2.25Y3.25La1.75 alloys are estimated from the measurements of low-temperature specific heat (LTSH) and upper critical field (UCF). It is found that the value of N(EF) decreases with a micro-alloying induced GFA improvement. And the decreasing of N(EF) with Co and La alloying is due to the increasing hybridization between s and d states. Our results highlight the significance of electronic structural effects on improving GFA by micro-alloying.

Effect of quenched disorder on the quantum spin liquid state of the triangular-lattice antiferromagnet 1T−TaS2

To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient $\gamma$ and the finite residual linear term of the thermal conductivity in the zero temperature limit $\kappa_0/T=\kappa/T(T\rightarrow 0)$ are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses $\gamma$, whereas it enhances $\kappa_0/T$. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new insights into the influence of quenched disorder on the QSL.

Low temperature specific heat anomaly with boson peak in isotope-enriched boron carbides B4·3C – B10C

The low temperature specific heat of boron carbide exhibits characteristic deviations from the classical Debye model (c ∝ T3). Including α-rhombohedral boron, the degree of deviations from this model is proportional to the concentration of B(3) voids in the structure and to the content of B11C icosahedra, respectively. The observed effects of specific structural defects can be described in accordance with the model of voids in amorphous Si proposed by Nakhmanson and Drabold. Results of β-rhombohedral boron, largely corresponding to the Debye model, are used for comparison.

Single crystal growth, structural and transport properties of bad metal RhSb2**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674375, 11634015, 11925408, and 11674369), the National Basic Research Program of China (Grant Nos. 2016YFA0300600, 2016YFA030240, 2017YFA0302901, and 2018YFA0305700), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 1018 cm−3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb2. In addition, a large thermopower S(T) about –140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.

Kinetic Characteristics of Phonons and the Structural Heterogeneities of the Monoaluminate Y1 –xErxAlO3 Solid Solutions

The low-temperature specific heat, the transport characteristics of phonons at the helium temperatures, and the nanostructural heterogeneities in the erbium monoaluminate YAlO3:Er solid solution single crystals synthesized by directional solidification are studied.

Magnetic studies of monoclinic Cu4O(SeO3)3, a copper-oxo-selenite derivative

Cu4O(SeO3)3 is a copper-oxo-selenite belonging to the CuxO(SeO3)(x–1) family of the topological chiral magnet Cu2OSeO3. We report magnetometry and specific heat data measured in a monoclinic Cu4O(SeO3)3 single crystal grown through a Chemical Vapour Transport (CVT) reaction. Our study shows a typical antiferromagnetic behaviour, with a Néel temperature TN = 58 K, similar to that of the Cu2OSeO3 and an additional transition at 13 K. The effective magnetic moment per Cu atom is 1.84 μB, close to the expected theoretical value for Cu2+. The low-temperature M(H) curves, show a transition starting at Hc1 ~ 400 Oe at 1.8 K shifting to a lower value of ~ 280 Oe at 30 K, likely from a helical into a conical intermediate phase, and a second transition at Hc2 ~ 1 kOe, above which the net moment increases linearly with applied field. The magnetisation moment value in a 90 kOe field is 0.053 μB/Cu at 1.8 K and attains a maximum value of 0.061 μB at 13 K. Low-temperature specific heat measurements confirm the presence of the magnetic transition at 13 K, slightly shifting to lower temperatures under an applied magnetic field.

Change of electronic properties on transition from high-entropy to Ni-rich (TiZrNbCu)[formula omitted]Nix alloys

We present results of comprehensive study of electronic properties of (TiZrNbCu)1−xNix metallic glasses performed in broad composition range x encompassing both, high entropy (HE) range, and conventional Ni-base alloy concentration range, x ≥ 0.35. The electronic structure studied by photoemission spectroscopy and low temperature specific heat (LTSH) reveal a split-band structure of density of states inside valence band with d-electrons of Ti, Zr, Nb and also Ni present at Fermi level N(EF), whereas LTSH and magnetoresistivity results show that variation of N(EF) with x changes in Ni-base regime. The variation of superconducting transition temperatures with x closely follows that of N(EF). The electrical resistivities of all alloys are high and decrease with increasing temperature over most of explored temperature range, and their temperature dependence seems dominated by weak localization effects over a broad temperature range (10–300 K). The preliminary study of Hall effect shows positive Hall coefficient that decreases rapidly in Ni-base alloys.

Consequences of magnetic ordering in chiral Mn1/3NbS2

We have investigated the structural, magnetic, thermodynamic, and charge transport properties of Mn1/3NbS2 single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall effect measurements. Mn1/3NbS2 displays a magnetic transition at TC ~ 45 K with highly anisotropic behavior expected for a hexagonal structured material. Below TC, neutron diffraction reveals increased scattering near the structural Bragg peaks having a wider Q-dependence along the c-axis than the nuclear Bragg peaks. This indicates helimagnetism with a long pitch length of ~250 nm (or a wavevector q~0.0025 {\AA}-1) along the c-axis. This q is substantially smaller than that found for the helimagnetic state in isostructural Cr1/3NbS2 (0.015 {\AA}-1). Specific heat capacity measurements confirm a second-order magnetic phase transition with a substantial magnetic contribution that persists to low temperature. The large low-temperature specific heat capacity is consistent with a large density of low-lying magnetic excitations that are likely associated with topologically interesting magnetic modes. Changes to the magnetoresistance, the magnetization, and the magnetic neutron diffraction, which become more apparent below 20 K, imply a modification in the character of the magnetic ordering corresponding to the magnetic contribution to the specific heat capacity. These observations signify a more complex magnetic structure both at zero and finite fields for Mn1/3NbS2 than for the well-investigated Cr1/3NbS2.

Gapless spin-liquid state in the structurally disorder-free triangular antiferromagnet NaYbO2

We present the structural characterization and low-temperature magnetism of the triangular-lattice delafossite NaYbO$_2$. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness, whereas heat-capacity measurements and muon spectroscopy reveal the absence of magnetic order and persistent spin dynamics down to at least 70\,mK. Continuous magnetic excitations with the low-energy spectral weight accumulating at the $K$-point of the Brillouin zone indicate the formation of a novel spin-liquid phase in a triangular antiferromagnet. This phase is gapless and shows a non-trivial evolution of the low-temperature specific heat. Our work demonstrates that NaYbO$_2$ practically gives the most direct experimental access to the spin-liquid physics of triangular antiferromagnets.