Specific Heat Spectroscopy Research Articles

Investigation on specific heat and Raman spectroscopy of YbFe2As2

We hereby present a comprehensive study of polycrystalline YbFe2As2 which includes detailed investigation on the specific heat capacity and Raman spectroscopy measurements at high magnetic fields (up to 16 T) and low temperatures. Low temperature x-ray diffraction investigations at 30 K on YbFe2As2 reveal an orthorhombic unit cell structure, which indicates structural transitions from monoclinic to orthorhombic as temperature decreases. In specific heat capacity measurements, a slope change is observed around 70 K. It may be a signature of spin density wave (SDW) phase transition. The specific heat data has been fitted in the temperature ranging from 20 to 90 K. It has been found that linear coefficient of specific heat (γ) is quite enhanced suggesting the existence of quasi particles with heavy effective mass. Its value changes slowly with magnetic field below 70 K. Raman spectra have been studied in the temperature range of 4 to 300 K. It is observed that there is a trend of shifting the B1G peak position in Raman spectra on the higher side as the temperature decreases. A change of slope in shift of B1G versus temperature plot is noticed around 70 K suggesting phase transition which is consistent with specific heat and previous resistivity measurements. Overall, our results provide new insights into the physical properties and phase transitions of YbFe2As2.

Probing electron–phonon coupling in magnetic van der Waals material NiPS3: A non-magnetic site-dilution study

NiPS3 is a van der Waals antiferromagnet which has been shown to exhibit spin-phonon and spin-charge coupling in the antiferromagnetically ordered state below TN = 155 K. It is also a rare Ni-based negative charge-transfer-type insulator with Ni valence in a linear superposition state ψ= αd8+ βd9L−+ γ d10L−2 , where L− is the ligand hole. Here, we study high-quality single-crystals of Ni1−x Zn x PS3 (0 < x < 0.2) using temperature-dependent specific heat and Raman spectroscopy probes. We show that in pristine NiPS3, the phonon mode at 176 cm−1 (P2), associated with the vibrations of Ni ions, exhibits a distinct Fano asymmetry. The Fano resonance is particularly pronounced in the paramagnetic phase above TN, which was further confirmed by temperature dependent Raman data on the Zn-doped crystals. In the Zn-doped crystals, while the magnetism weakens following the mean-field prediction for site-dilution in a honeycomb lattice, the Fano coupling |1/q| strengthens, increasing monotonically with increasing Zn-doping. The x-ray photoemission spectra suggest an increase in the weight of the d9 and d10 components in the Zn-doped crystals. These observations indicate the presence of strong electron–phonon coupling in Ni1−x Zn x PS3, in addition to the spin-phonon, and spin-charge coupling previously reported.

Possible Partially Disordered Sm State Coexisting with Heavy Quasiparticles in SmAu3Al7

Single crystals of SmAu3Al7 with an anisotropic cage structure have been investigated by single-crystal structural analysis and measurements of magnetization, electrical resistivity, specific heat, and X-ray absorption spectroscopy. The average valence of Sm ions is +2.96, which is close to magnetic trivalent, with no significant temperature dependence. The crystalline-electric-field ground state is a |±3/2〉 doublet with the strong Ising nature with the easy axis along the [0001] direction. In the temperature dependence of specific heat, two clear λ-type peaks appear at 0.9 K (= T*) and 2.8 K (= TN), indicating the existence of multiple ordered phases: phase I (T* < T < TN) and phase II (T < T*) in zero field. In the resistivity for the current along the [0001] direction, a sharp increase appears just below TN, suggesting the development of a superzone gap opening on the Fermi surface perpendicular to the [0001] direction. The temperature dependence of magnetic susceptibility along the magnetic easy [0001] direction exhibits a small cusp at TN, confirming the antiferromagnetic nature of phase I. Below TN, however, a pronounced Curie-type behavior appears, indicating that ∼40% of Sm magnetic moments remain paramagnetic in phase I. The Sommerfeld coefficient is estimated to be over 1 J/mol K2 deep inside the ordered state. These results can be interpreted that disordered Sm ions form a Kondo sublattice through hybridization with conduction electrons (partial Kondo screening state) along with quasiparticle mass enhancement.

Multiple glassy dynamics of a homologous series of triphenylene-based columnar liquid crystals – A study by broadband dielectric spectroscopy and advanced calorimetry

Hexakis(n-alkyloxy)triphenylene) (HATn) consisting of an aromatic triphenylene core and alkyl side chains are model discotic liquid crystal (DLC) systems forming a columnar mesophase. In the mesophase, the molecules of HATn self-assemble in columns, which has one-dimensional high charge carrier mobility along the columns. Here, a homologous series of HATn with different length of the alkyl chain (n = 5,6,8,10,12) is investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques including fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). The investigation of the phase behavior was done utilizing DSC experiments and the influence of the alkyl chain length on the phase behavior was revealed. By the dielectric investigations probing the molecular mobility, a γ-relaxation due to localized fluctuations as well as two glassy dynamics, the αcore- and αalkyl-relaxation, were observed in the temperature range of the plastic crystalline phase. Moreover, the observed glassy dynamics were further studied employing advanced calorimetry. All observed relaxation processes are attributed to the possible specific molecular fluctuations and discussed in detail. From the results a transition at around n = 8 from a rigid constrained (n = 5,6) to a softer system (n = 10,12) was revealed with increasing alkyl chain length. A counterbalance of two competing effects of a polyethylene-like behavior of the alkyl chains in the intercolumnar domains and self-organized confinement is discussed in the context of a hindered glass transition.

Molecular dynamics and electrical conductivity of Guanidinium based ionic liquid crystals: Influence of cation headgroup configuration

Molecular mobility and conductivity of four bent shaped tetramethylated guanidinium based ionic liquid crystals (ILCs) with varying head group configuration (cyclic or acyclic) and alkyl chain length is investigated by a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). Two dielectrically active processes observed in the plastic crystalline phase at low and high temperatures are denoted as γ and α1 relaxation. The former is assigned to localized fluctuations of methyl groups including nitrogen atoms in the guanidinium head groups. SHS investigations reveal one calorimetrically active process termed as α2 relaxation process. The temperature dependencies of the relaxation rates of α1 and α2 are similar for the cyclic ILC while for the acyclic counterpart they are different. Possible molecular assignments for the α1 and α2 relaxation are discussed in detail. Alongside relaxation processes, a significant conductivity contribution was observed for all ILCs, where the absolute value of DC conductivity increases by 4 orders of magnitude at the transition from the crystalline to the hexagonal columnar phase. The increase is traced to the change in the underlying conduction mechanism from the delocalized electrical conduction in the Cry phase to ionic conduction in the quasi 1D ion columns formed in the hexagonal columnar mesophase.

Electrical Conductivity and Multiple Glassy Dynamics of Crown Ether-Based Columnar Liquid Crystals.

The phase behavior of two unsymmetrical triphenylene crown ether-based columnar liquid crystals bearing different lengths of alkyl chains, KAL465 and KAL468, was investigated using differential scanning calorimetry (DSC). A plastic crystalline (Cry), a columnar liquid crystalline (Colh), and an isotropic phase were observed along with two glass transitions in the Cry phase. The molecular mobility of the KAL compounds was further studied by a combination of broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques. By the BDS investigations, three dielectric active relaxation processes were observed for both samples. At low temperatures, a γ-process in the Cry state was detected and is assigned to the localized fluctuations taking place in the alkyl chains. An α2-process takes place at higher temperatures in the Cry phase. An α3-process was found in the Colh mesophase. The advanced calorimetric techniques consist of fast scanning calorimetry (FSC) and specific heat spectroscopy employing temperature-modulated DSC and FSC. The advanced calorimetric investigations revealed that besides the α2-process in agreement with BDS, there is a second dynamic glass transition (α1-process), which is not observed by dielectric spectroscopy. The results are in good agreement with the glass transitions detected by DSC for this process. The temperature dependences of the relaxation rates of the α1-, α2-, and α3-processes are all different. Therefore, different molecular assignments for the relaxation processes are proposed. In addition to the relaxation processes, a conductivity contribution was explored by BDS for both KAL compounds. The conductivity contribution appears in both Cry and Colh phases, where the conductivity increases by ca. 1 order of magnitude at phase transition from the Cry to the hexagonal phase.

Conventional isotropic s -wave superconductivity with strong electron-phonon coupling in Sc5Rh6Sn18

${\mathrm{Sc}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ is a diamagnetic metallic compound (space group: $I{4}_{1}/acd, a\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}13.5\phantom{\rule{0.28em}{0ex}}\AA{}$, $c\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}27.1\phantom{\rule{0.28em}{0ex}}\AA{}$) which becomes superconducting (sc) at ${T}_{\mathrm{c}}=5.15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The sc state was studied on single-crystal samples by temperature- and field-dependent electrical resistivity, specific heat, and muon-spin rotation ($\ensuremath{\mu}\mathrm{SR}$) spectroscopy. No anisotropy of the upper critical field ${B}_{\mathrm{c}2}=7.75\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is observed. Analyses of specific heat indicate enhanced values of the jump $[\mathrm{\ensuremath{\Delta}}{c}_{p}/({\ensuremath{\gamma}}_{\mathrm{tot}}{T}_{\mathrm{c}})=1.58]$ and of the energy gap ratio $[\mathrm{\ensuremath{\Delta}}(0)/({k}_{\mathrm{B}}{T}_{\mathrm{c}})=2.39]$ as well as an exponential decrease of the electronic contribution below ${T}_{\mathrm{c}}/2$. Together with the saturation of the superfluid density for $Tl3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ observed by $\ensuremath{\mu}\mathrm{SR}$ spectroscopy these findings lead us to classify ${\mathrm{Sc}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ as a moderately strong coupled isotropic $s$-wave superconductor. However, above ${T}_{\mathrm{c}}$, the crystals reveal a surprisingly anisotropic and large resistivity of $2.5--4.5\phantom{\rule{0.28em}{0ex}}\mathrm{m}\mathrm{\ensuremath{\Omega}}$ m with metalliclike behavior perpendicular to the $c$ axis and by a factor of $\ensuremath{\sim}2$ larger parallel to $c$, which decreases upon increasing temperature. These observations are discussed in connection to a characteristic intrinsic disorder in the crystal structure of ${\mathrm{Sc}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$.

Magnetotransport and electronic structure of the antiferromagnetic semimetal YbAs

A number of rare-earth monopnictides have topologically non-trivial band structures together with magnetism and strong electronic correlations. In order to examine whether the antiferromagnetic (AFM) semimetal YbAs ($T\rm_N$ = 0.5 K) exhibits such a scenario, we have grown high-quality single crystals using a flux method, and characterized the magnetic properties and electronic structure using specific heat, magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements, together with density functional theory (DFT) calculations. Both ARPES and DFT calculations find no evidence for band inversions in YbAs, indicating a topologically trivial electronic structure. From low-temperature magnetotransport measurements, we map the field-temperature phase diagram, where we find the presence of a field stabilized phase distinct from the AFM phase at low temperatures. An extremely large magnetoresistance (XMR) for both YbAs and the nonmagnetic counterpart LuAs, is also observed, which can consistently be accounted for by the presence of electron-hole compensation. Moreover, an angle-dependent study of the Shubnikov-de Haas effect oscillations reveals very similar Fermi surfaces between YbAs and LuAs, with light effective masses down to at least 0.5 K, indicating that the Yb-$4f$ electrons are well localized, and do not contribute to the Fermi surface. However, the influence of the localized Yb-$4f$ electrons on the magnetotransport of YbAs can be discerned from the distinct temperature dependence of the XMR compared to that of LuAs, which we attribute to the influence of short-ranged spin correlations that appear well above $T\rm_N$.

Electronic band structure and superconducting properties of SnAs

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio $2\Delta/T_c = 3.48 - 3.73$ is very close to the BCS value in the weak coupling limit.

Molecular Dynamics of the Asymmetric Blend PVME/PS Revisited by Broadband Dielectric and Specific Heat Spectroscopy: Evidence of Multiple Glassy Dynamics

The molecular mobility of the highly asymmetric miscible blend poly(vinyl methyl ether)/polystyrene was investigated by broadband dielectric (frequency range 10–1–109 Hz) and specific heat spectroscopy (frequency range 101–104 Hz). The dielectric spectra revealed a complex molecular dynamic behavior, where three different relaxation processes were observed. At temperatures below the glass transition temperature an α′-relaxation was found, with an Arrhenius-like temperature dependence of its relaxation rates. It is assigned to localized fluctuations of the confined PVME segments within a frozen glassy matrix dominated by PS. Above the thermal glass transition temperature two processes with a VFT behavior of their relaxation rates were detected called α1- and α2-relaxation, both originating from PVME dipoles fluctuating in PS-rich environments, however with diverse PS concentrations. The relevant length scales for the processes are assumed to be different, corresponding to the Kuhn segment length for the former relaxation and to the CRR for the latter one. The observed multiple glassy dynamics result from spatial local compositional heterogeneities on a microscopic level. Additionally, SHS investigations were performed for the first time for this system, proving an existence of a fourth relaxation process (α3-relaxation) due to the cooperative fluctuations of both PS and PVME segments. The separation between the thermal α3-relaxation and dielectric α2-relaxation increases dramatically with increasing polystyrene concentration, proving that the thermal response is dominated by PS.

Cu nanoparticles constrain segmental dynamics of cross-linked polyethers: a trade-off between non-fouling and antibacterial properties.

Copper has a strong bactericidal effect against multi-drug resistant pathogens and polyethers are known for their resistance to biofilm formation. Herein, we combined Cu nanoparticles (NPs) and a polyether plasma polymer in the form of nanocomposite thin films and studied whether both effects can be coupled. Cu NPs were produced by magnetron sputtering via the aggregation in a cool buffer gas whereas polyether layers were synthesized by Plasma-Assisted Vapor Phase Deposition with poly(ethylene oxide) (PEO) used as a precursor. In situ specific heat spectroscopy and XPS analysis revealed the formation of a modified polymer layer around the NPs which propagates on the scale of a few nanometers from the Cu NP/polymer interface and then transforms into a bulk polymer phase. The chemical composition of the modified layer is found to be ether-deficient due to the catalytic influence of copper whereas the bulk polymer phase exhibits the chemical composition close to the original PEO. Two cooperative glass transition phenomena are revealed that belong to the modified polymer layer and the bulk phase. The former is characterized by constrained mobility of polymer segments which manifests itself via a 30 K increase of dynamic glass transition temperature. Furthermore, the modified layer is characterized by the heterogeneous structure which results in higher fragility of this layer as compared to the bulk phase. The Cu NPs/polyether thin films exhibit reduced protein adsorption; however, the constrained segmental dynamics leads to the deterioration of the non-fouling properties for ultra-thin polyether coatings. The films are found to have a bactericidal effect against multi-drug resistant Gram-positive Methicillin-Resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa.

La3CrGe3Be2O14 and Nd3CrGe3Be2O14: New magnetic compounds of the langasite family

This communication focuses on recently discovered Ln3CrGe3Be2O14 (Ln = La–Sm) compounds as a new class of low-dimensional magnets. We conducted the first studies of their magnetic properties by means of the magnetic susceptibility, specific heat, and high-resolution optical spectroscopy measurements of the La and Nd representatives. In La3CrGe3Be2O14, which contains only one (chromium) magnetic system, the shortest exchange paths are within isolated chromium chains along the c axis. The compound demonstrates one-dimensional magnetic behavior, i.e., an absence of a long-range order, at least down to 0.4 K, and short-range magnetic correlations with the antiferromagnetic exchange integral along the chains estimated as JCr = 4.0 ± 0.5 K. In Nd3CrGe3Be2O14 containing two magnetic subsystems, a splitting of the neodymium Kramers doublets at low temperatures indicates magnetic activity due to both short-range correlations and possible magnetic ordering at 2 K.

Dynamics and ionic conductivity of ionic liquid crystals forming a hexagonal columnar mesophase.

For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflate ionic liquid crystals (ILCs) having different lengths of alkyl chains was investigated using a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). By self-assembly, these ILCs can form a hexagonal ordered mesophase besides plastic crystalline phases and the isotropic state. Three dielectric active processes were found using BDS for both samples. At low temperatures, a γ-process in the plastic crystalline state is observed which is assigned to localized fluctuations of methyl groups including nitrogen atoms in the guanidinium head. At higher temperatures but still in the plastic crystalline state, an α1-process takes place. An α2-process was detected using SHS but with a completely different temperature dependence of the relaxation times than that of the α1-relaxation. This result is discussed in detail, and different molecular assignments of the processes are suggested. At even higher temperatures, electrical conductivity is detected and an increase in the DC conductivity by four orders of magnitude at the phase transition from the plastic crystalline to the hexagonal columnar mesophase is found. This result is traced to a change in the charge transport mechanism from a delocalized electron hopping in the stacked aromatic systems (in the plastic phase) to one dominated by an ionic conduction in the quasi-1D ion channels formed along the supermolecular columns in the ILC hexagonal mesophases.

Decoupling of Dynamic and Thermal Glass Transition in Thin Films of a PVME/PS Blend.

The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature Tgtherm of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy) and the deduced Vogel temperature (T0). While Tgtherm and T0 showed a monotonous increase, with decreasing film thickness, the dynamic glass transition temperature (Tgdyn) measured at a finite frequency showed a nonmonotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This nonmonotonous thickness dependence of Tgdyn disappears for frequencies characteristic for T0. Further analysis of the fragility parameter showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.

Revealing correlation effect of Co 3d electrons in La3Co4Sn13 and Ce3Co4Sn13 by infrared spectroscopy study

We report resistivity, specific heat and optical spectroscopy measurements on single crystal samples of and . We observed clear temperature-induced spectral weight suppression below 4000 for both compounds in the conductivity spectra , indicating the progressive formation of gap-like features with decreasing temperature. The suppressed spectral weight transfers mostly to the higher energy region. This observation reflects the presence of the correlation effect in the compounds. We attribute the correlation effect to the Co 3d electrons.

Unexpected behavior of ultra-thin films of blends of polystyrene/poly(vinyl methyl ether) studied by specific heat spectroscopy.

Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm-340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO2 substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic Tg of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic Tg with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in Tg with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt. % as well as literature results.

Antiferromagnetism with divalent Eu in EuNi5As3

We have successfully synthesized single crystals of EuNi$_5$As$_3$ using a flux method and we present a comprehensive study of the physical properties using magnetic susceptibility, specific heat, electrical resistivity, thermoelectric power and x-ray absorption spectroscopy (XAS) measurements. EuNi$_5$As$_3$ undergoes two close antiferromagnetic transitions at respective temperatures of $T_{N1}$ = 7.2 K and $T_{N2}$ = 6.4 K, which are associated with the Eu$^{2+}$ moments. Both transitions are suppressed upon applying a field and we map the temperature-field phase diagrams for fields applied parallel and perpendicular to the easy $a$ axis. XAS measurements reveal that the Eu is strongly divalent, with very little temperature dependence, indicating the localized Eu$^{2+}$ nature of EuNi$_5$As$_3$, with a lack of evidence for heavy fermion behavior.

In Situ Nanocalorimetric Investigations of Plasma Assisted Deposited Poly(ethylene oxide)-like Films by Specific Heat Spectroscopy.

In recent years, highly cross-linked plasma polymers have started to unveil their potential in numerous biomedical applications in thin-film form. However, conventional diagnostic methods often fail due to their diverse molecular dynamics conformations. Here, glassy dynamics and the melting transition of thin PEO-like plasma assisted deposited (ppPEO) films (thickness 100 nm) were in situ studied by a combination of specific heat spectroscopy, utilizing a pJ/K sensitive ac-calorimeter chip, and composition analytical techniques. Different cross-linking densities were obtained by different plasma powers during the deposition of the films. Glassy dynamics were observed for all values of the plasma power. It was found that the glassy dynamics slows down with increasing the plasma power. Moreover, the underlying relaxation time spectra broaden indicating that the molecular motions become more heterogeneous with increasing plasma power. In a second set of the experiment, the melting behavior of the ppPEO films was studied. The melting temperature of ppPEO was found to decrease with increasing plasma power. This was explained by a decrease of the order in the crystals due to formation of chemical defects during the plasma process.

Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid.

The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependences of the characteristic time scale associated with the specific heat is compared to the equivalent time scale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two time scales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying "inner clock." Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measured liquid, comply to the isomorph theory.

Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7.

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses TFL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.