Low-temperature Specific Heat Measurements Research Articles

Majorana-fermion origin of the planar thermal Hall effect in the Kitaev magnet α-RuCl3.

The field-induced quantum-disordered state of layered honeycomb magnet α-RuCl3 is a prime candidate for Kitaev spin liquids hosting Majorana fermions and non-Abelian anyons. Recent observations of anomalous planar thermal Hall effect demonstrate a topological edge mode, but whether it originates from Majorana fermions or bosonic magnons remains controversial. Here, we distinguish these origins from combined low-temperature measurements of high-resolution specific heat and thermal Hall conductivity with rotating magnetic fields within the honeycomb plane. A distinct closure of the low-energy bulk gap is observed for the fields in the Ru-Ru bond direction, and the gap opens rapidly when the field is tilted. Notably, this change occurs concomitantly with the sign reversal of the Hall effect. General discussions of topological bands show that this is the hallmark of an angle rotation-induced topological transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall effect in α-RuCl3.

Charge density wave and superconductivity in 6R-TaS[formula omitted

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 660K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

Magnetic phase diagram in the three-dimensional triangular-lattice antiferromagnet Sr3CoTa2O9 with small easy-axis anisotropy

We report the results of low-temperature magnetization and specific heat measurements of ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ powder, in which ${\mathrm{Co}}^{2+}$ ions with effective spin-1/2 form a uniform triangular lattice in the $ab$ plane. It was found that ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ undergoes successive antiferromagnetic transitions at ${T}_{\mathrm{N}1}=0.97\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=0.79\phantom{\rule{4pt}{0ex}}\mathrm{K}$ at zero magnetic field. As the magnetic field increases, both ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{s}}=2.3$ T indicates that ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ is described as a spin-1/2 three-dimensional triangular-lattice antiferromagnet with a weak easy-axis anisotropy. We discuss the characteristics of the magnetic phase diagram, which approximates the phase diagram for the magnetic field perpendicular to the $c$ axis.

Anisotropic spin-glass and magnetic behavior in single-crystalline U2PtSi3

The results of ac and dc susceptibilities, magnetization, magnetic relaxation, electrical resistivity, and specific heat measurements performed on a hexagonal AlB2-type single-crystalline U2PtSi3 are reported. The previously reported spin-glass transition in polycrystalline U2PtSi3 was confirmed to also occur in the single-crystalline sample with spin freezing temperature Tf=12.4 K. It is newly found that the spin-glass effect and magnetic behavior of U2PtSi3 measured along and perpendicular to the hexagonal c-axis show different magnetic field responses. Along the hard magnetization axis (c-axis), both the rate of change of Tf with the magnetic field and the detected magnetic viscosity coefficient are much smaller than those obtained in the c-plane. On the other hand, the low-temperature specific heat measurement yields an enhanced coefficient of the linear temperature term, and the temperature dependence of electrical resistivity illustrates a shallow minimum at the temperature much larger than Tf. The possible mechanisms for these phenomena are discussed by comparing with the corresponding properties observed in other ternary 2:1:3 uranium intermetallic compounds.

Spin excitations in the quantum dipolar magnet Yb(BaBO3)3

We report results of magnetization, specific-heat and muon-spin relaxation measurements on single crystals of disorder-free Yb$^{3+}$ triangular lattice Yb(BaBO$_3$)$_3$. The magnetization experiments show anisotropic magnetic properties with Curie-Weiss temperatures $\theta_{\perp}=-1.40$~K ($H \perp c$) and $\theta_{\parallel}=-1.16$~K ($H \parallel c$) determined from low temperature data. The absence of both long-range antiferromagnetic order and spin freezing is confirmed down to 0.27 K at zero field. A two-level Schottky anomaly due to the opening of the ground-state Kramers doublet is observed from the low-temperature specific-heat measurements when the applied magnetic fields $\mu_0H >0.7$~T. At zero field, the increase of both $C_{\rm mag}/T$ and the muon spin relaxation rate $\lambda$ below 1~K is due to the electronic spin excitations, which often exist in quantum magnets where dipole-dipole interaction creates an anisotropy of magnetic properties. The spin excitation is also supported by the unusual maximum of field dependence of $\lambda$ due to the field-induced increase of the density of excitations. We argue that dipolar interaction is dominant and induces the spin dynamics in the quantum magnet Yb(BaBO$_3$)$_3$.

Superconductivity in Ce3PdIn11 single crystals – intrinsic phenomenon or parasitic effect?

Single crystals of the compound Ce3PdIn11 that belongs to a large family of superconducting cerium indides with the general formula Ce m T n In3m +2n (T stands for d-electron transition metal) were grown from indium flux and characterized by means of low-temperature specific heat and electrical resisitivity measurements. The collected data revealed significant sample-dependent divergence in their magnetic and superconducting properties which raises the question of whether these two cooperative phenomena are intrinsic properties of Ce3PdIn11.

Investigations on superconductivity in an equi-atomic disordered Hf-Nb-Ta-Ti-V high entropy alloy

High entropy alloys (HEA) are expected to show unusual superconducting behavior due to chemical disorder along with highly distorted lattice. Herein, we report the synthesis, structure, and detailed superconducting properties of a new and nearly equi-atomic single phase disordered superconducting high entropy alloy with nominal composition Hf20Nb20Ta20Ti20V20. Rietveld refinement of the Synchrotron X-ray diffraction (SXRD) confirms the DFT predicted single-phase BCC structure in the HEA with lattice parameter a = 3.32 Å. Reverse monte carlo (RMC) modelling of the diffraction data evince the local atomic disorder and a considerable lattice distortion overriding the crystal structure due to significant variations in bond length and co-ordination numbers. Microstructural characterization using energy-dispersive X-ray spectroscopy (XEDS) equipped SEM shows dendritic morphology of the as solidified alloy with incomplete partition of the constituent elements. Transport, magnetization and low temperature specific heat measurement established that the current HEA is a moderately coupled type-II superconductor with a Tc of 5.0 K, lower critical field Hc1(0)=19mT and upper critical field Hc2(0) = 6.63T.

Synthesis, structures and physical properties of new transition metal fluoroselenides Ba3F2MSe3 (M = Zn, Cd)

Two new transition metal fluoroselenides Ba3F2MSe3 (M ​= ​Zn, Cd) have been successfully synthesized by solid-state reaction. Both compounds adopt the same crystal structure with orthorhombic space group Pnma (No. 62), consisting of an infinite corner-sharing [MSe3]4- anion chain separated by [Ba3F2]4+ cationic chain of the fluorite type. Ba3F2ZnSe3 and Ba3F2CdSe3 show paramagnetic behaviors from 1.8 to 300 ​K due to the absence of magnetic transition metals. Their Debye temperatures are 170 ​K and 163 ​K, respectively, determined from the low-temperature specific heat measurements. The diffuse-reflectance spectroscopy measurements reveal that these new compounds are semiconductors with optical band gaps of 1.80 ​eV and 1.69 ​eV for Ba3F2ZnSe3 and Ba3F2CdSe3, which are consistent with the band gaps obtained from the first-principle calculations.

Probing the superconducting ground state of noncentrosymmetric high-entropy alloys using muon-spin rotation and relaxation

Recently, high entropy alloys (HEAs) have emerged as a new platform for discovering superconducting materials and offer avenues to explore exotic superconductivity. The highly disordered nature of HEA suggests regular phonon required for BCS superconductivity may be unlikely to occur. Therefore understanding the microscopic properties of these superconducting HEA is important. We report the first detailed characterization of the superconducting properties of the noncentrosymmetric ($\alpha$-Mn structure) HEA {(HfNb)}$_{0.10}${(MoReRu)}$_{0.90}$, and {(ZrNb)}$_{0.10}${(MoReRu)}$_{0.90}$ by using magnetization, specific heat, AC transport, and muon-spin relaxation/rotation ($\mu$SR). Despite the disordered nature, low temperature specific heat and transverse-field muon spin rotation measurements suggest nodeless isotropic superconducting gap and Zero-field $\mu$SR measurements confirm that time reversal symmetry is preserved in the superconducting ground state.

Twisting of 2D Kagomé Sheets in Layered Intermetallics.

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal, and superconducting properties. Here, we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ∼4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measurements, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ∼300 to 100 K, no evidence for a phase transition was found in physical property measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modeling of the displacive disorder in the crystal structure shows the presence of a second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits interlayer twisting, but with a different pattern, consistent with the change in electron count due to the removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.

Specific heat and NMR evidence for the low Fermi-level density of states in semimetallic ScSb

We report the electronic and thermal properties of scandium monoantimonide ScSb by means of the Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance measurements. The experimental Seebeck coefficient exhibits a strong temperature dependence, and the theoretical calculation based on the two-band model provides a realistic description of the observed feature. The analysis of the thermal conductivity reveals that the lattice thermal conductivity dominates at low temperatures while electronic thermal conductivity makes a major contribution at high temperatures. A small value of the Sommerfeld coefficient of $0.38\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\text{--}2}$ was extracted from the low-temperature specific heat measurement, indicative of a low electronic Fermi-level density of states (DOS) in ScSb. Furthermore, we have deduced the Sc $3d$ and Sb $5s$ partial Fermi-level DOSs based on the Korringa behavior in the $^{45}\mathrm{Sc}$ and $^{121}\mathrm{Sb}$ NMR spin-lattice relaxation rates. The determined values of the DOS are quite low, giving strong evidence for the semimetallic character in ScSb.

Controllable structural ordering via chemical substitution to the efficient and thermally stable luminescence in NASICON-type phosphor Series: Na1+xHf2−xScx(PO4)3:Eu

Exploring NASICON-type materials emerges the fully ordered NaHf2(PO4)3 compound. Its cation/vacancy distribution forms an absolutely structural ordering three-dimensional arrangement. Further preparing a solid solution following Na1+xHf2−xScx(PO4)3:Eu (NHSx:Eu; 0.0 ≤ x ≤ 1.6) confirms all compounds are isostructural. Densely atomic-connected framework guarantees the efficient blue emission, which yields > 90% photoluminescence quantum yield (PLQY) for all scandium-substituted analogues, yet the existence of heavy amount of luminescence killer (Eu3+) limits the performance of NaHf2(PO4)3:Eu as a luminescent phosphor. Moreover, a controllable structural ordering/disordering is achieved by varying x value, which is confirmed by Raman and 31P MAS NMR spectra. The structural ordering/disordering is crucial in determining structural rigidity. Consequently, despite the voids in the framework filled with sodium ions as x increases has positive contribution to the structural robustness, cation/vacancy ordered form still builds the most rigid structure for NHS0.3, as established by the NHS0.3 composition possessing the highest Debye temperature within the scandium-involved analogues, which are extracted experimentally by the low temperature specific heat measurements. As such, NHS0.3:Eu has the best thermal stability across the x range, which keeps 90% of integrated intensity up to 203 ℃ compared with room temperature value. This work provides insight into the structural specialty of NASICON structure.

Specific heat of CeRhIn5 in high magnetic fields: Magnetic phase diagram revisited

${\mathrm{CeRhIn}}_{5}$ is a prototypical antiferromagnetic heavy-fermion compound, whose behavior in a magnetic field is unique. A magnetic field applied in the basal plane of the tetragonal crystal structure induces two additional phase transitions. When the magnetic field is applied along, or close to, the $c$ axis, a new phase characterized by a pronounced in-plane electronic anisotropy emerges at ${B}^{*}\ensuremath{\approx}$ 30 T, well below the critical field, ${B}_{c}\ensuremath{\simeq}$ 50 T, to suppress the antiferromagnetic order. The exact origin of this new phase, originally suggested to be an electronic-nematic state, remains elusive. Here we report low-temperature specific heat measurements in ${\mathrm{CeRhIn}}_{5}$ in high static magnetic fields up to 36 T applied along both the $a$ and $c$ axes. For fields applied along the $a$ axis, we confirmed the previously suggested phase diagram and extended it to higher fields. This allowed us to observe a triple point at $\ensuremath{\sim}30$ T, where the first-order transition from an incommensurate to commensurate magnetic structure merges into the onset of the second-order antiferromagnetic transition. For fields applied along the $c$ axis, we observed a small but distinct anomaly at ${B}^{*}$, which we discuss in terms of a possible field-induced transition, probably weakly first-order. We further suggest that the transition corresponds to a change of magnetic structure. We revise magnetic phase diagrams of ${\mathrm{CeRhIn}}_{5}$ for both principal orientations of the magnetic field based entirely on thermodynamic anomalies.

Reduced phase space of heat-carrying acoustic phonons in single-crystalline InTe

Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity κph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of κph(T) in single-crystalline InTe along the c axis show that κph exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of In+ cations in tunnels formed by edge-sharing (In3+Te42−)− tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the In+-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low κph value of 1.2Wm−1K−1 at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the In+ site occupancy.

Low-energy magnetic excitations in the mixed spin-( 12,52 ) chain

We present studies on the low-energy magnetic excitations in the Lieb-Mattis (LM) ferrimagnetic phase. We investigate the low-energy magnetic excitations in the mixed spin-($\frac{1}{2},\frac{5}{2}$) chain $(4\ensuremath{-}\mathrm{Br}\ensuremath{-}o\ensuremath{-}\mathrm{Me}\mathrm{Py}\ensuremath{-}\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_{4}$ based on low-temperature specific heat and multifrequency electron spin resonance (ESR) measurements. A clear $\sqrt{T}$ term is observed in the specific heat, which indicates a contribution of the low-energy magnetic excitation with a ferromagnetic quadratic dispersion expected in the LM ferrimagnetic state. Through the ESR mode analysis, we examine the effects of magnetic anisotropy on the excited states in the LM plateau phase.

Dimensional crossover in Cr-doped Co3BO5

The crystal structure and the magnetic properties of the ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$ ludwigite have been investigated by x-ray diffraction, magnetization, and specific heat experiments. Cr ions mainly occupy sites 4 of the ludwigite lattice, substituting approximately half of the ${\mathrm{Co}}^{3+}$. It changes the interatomic distances and favors a high spin state for the remaining ${\mathrm{Co}}^{3+}$ ions. Doping the homometallic ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ with magnetic Cr drastically increases the magnetic transition temperature from 42 K, for the undoped compound, to 76 K for ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$. The latter contains three different magnetic ions, ${\mathrm{Co}}^{2+}, {\mathrm{Co}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ in high spin states, as evidenced by magnetization measurements. The specific position of Cr in the structure (sites 4) gives rise, according to calculations of the exchange interactions and the Goodenough-Kanamori rules, to a few frustrated magnetic interactions. This is responsible for the high ferrimagnetic ordering temperature of 76 K, when compared to that of the pure compound. The experimental results imply a magnetic structure for the system that we show is consistent with the known interactions and the Goodenough-Kanamori rules. Low temperature specific heat and magnetization measurements reveal that the two-dimensional nature of the magnetism ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ when doped with Cr acquires a three-dimensional character with a high temperature ferrimagnetic order.

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.

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.

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.

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.