Valence Instability Research Articles

Valence instability and collapse of ferromagnetism in EuB[formula omitted] at high pressures

Despite the simplicity of their cubic crystal lattice, rare-earth hexaborides display complex physical properties including a (long debated) onset of metallization via magnetic polaron formation at Tc1≈ 15 K preceding ferromagnetic ordering at Tc2≈ 12 K. In this work, we used applied pressure to tune the interplay between electronic structure and magnetism in EuB6. We probed the magnetism, valence, and structure of EuB6 under quasi-hydrostatic pressures up to 30 GPa using X-ray techniques. Our findings show evidence for collapse of ferromagnetism above 20 GPa following a monotonic increase of mean Eu valence. While X-ray diffraction measurements in the paramagnetic state at room temperature show that the lattice retains cubic symmetry, a measurable quadrupole interaction seen by time-domain synchrotron Mössbauer spectroscopy suggests a lowering of symmetry associated with magnetic ordering, becoming more prominent across the magnetic transition. The interplay between conduction band electron count and magnetism observed under applied pressure in EuB6 opens possibilities for fine-tuning metallization and magnetic properties of similar Eu-based semi-metal systems.

Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m-Pr2Co3Ge5.

For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4f1 systems accessible using Pr4+-based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr4+ state in the polymorphic system Pr2Co3Ge5. The structure-valence transition from a nearly full Pr4+ electronic state to a typical Pr3+ state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

Appearance of a Photoinduced Hidden State in the Electron Donor–Acceptor Type Metal–Organic Framework (NPr4)2[Fe2(Cl2An)3

AbstractElectron donor–acceptor (DA)‐type metal–organic frameworks (MOFs) with valence instability are promising molecular materials for the design of photo‐responsive and electronic/magnetic functional materials. Here, ultrafast photoinduced dynamics in a DA‐type layered MOF that exhibits a charge‐transfer (CT)‐type phase transition are reported: (NPr4)2[Fe2(Cl2An)3], where NPr4+ = tetra‐n‐propylammonium and Cl2An2− = 2,5‐dichloro‐3,6‐dihydroxo‐1,4‐benzoquinonate. At room temperature (300 K: RT), ultrafast photoinduced CT between the Fe and Cl2An ions induces a sensitive change in the state associated with the valence instability from a single‐chain, electron‐correlated state to a new photoinduced, structurally modulated state. In the photoinduced state, two absorption bands are observed, one on the higher‐energy side of the CT band and the other in the mid‐IR range. This strongly implies that the local inversion center on the Cl2An ion that exists in the initial state disappears instantly upon photoexcitation, causing an ultrafast change in the lattice structure due to the softening of rigid bonds. This has never been realized in thermal excitation. These findings demonstrate that a new electronic state with a unique lattice structure—i.e., a photoinduced hidden state—appears in this MOF system at ultra‐high speed (within 110 fs) upon photoexcitation at RT.

Investigating the electronic states of UTe2 using X-ray spectroscopy

The recent discovery of superconductivity in paramagnetic UTe2 turns spotlight on a serious candidate for spin-triplet state. To draw a complete picture of the superconducting state in UTe2 precise knowledge of the electronic properties of the 5 f states of Uranium is missing. We report on x-ray absorption and magnetic circular dichroism experiments performed at the U M4,5 edges at 2.7 K. At ambient pressure the 5 f electron count is found to be in-between 2.6 and 2.8. Partial delocalization of the 5 f electrons is further confirmed by the reduced value of the U orbital to spin magnetic moment ratio. The 5 f count is reduced by as large as 7 percent at the transition to a magnetically ordered state at Pc ≈ 1.5 GPa. At pressures above 4 GPa, the 5 f count increases back towards U3+ in the tetragonal phase. The observed “valence instabilities” and their interplay with magnetism seem to be important ingredients to understand the electronic structure in UTe2 in different phases.

Maltese cross-type magnetic phase diagrams in Tm1-xYbxB12 antiferromagnets with Yb-valence instability and dynamic charge stripes

Precise X-ray diffraction, heat capacity, magnetoresistance and magnetization measurements have been carried out on high quality Tm1-xYbxB12 single crystals with the goal to reconstruct the H-T and angular H-φ magnetic phase diagrams in the (110) plane of these antiferromagnets (AF) with dynamic charge stripes and Yb-ion valence instability. The analysis developed here allowed us to conclude in favor of essential changes in the filamentary structure of fluctuating charges, and the emergence and variation of stripe-induced magnetic and charge transport anisotropy that are controlled by lowering the temperature and ytterbium doping. It was found that local charge and spin fluctuations on Yb-sites suppress strongly the complicated AF state. However, the magnetic anisotropy is conserved, and the only moderate modifications of the Maltese Cross –type magnetic phase diagrams are detected in the range x < 0.2. We argue that the AF ordering of Tm3+ magnetic moments is the main factor, which determines the anisotropy in the Néel phase of Tm1-xYbxB12 with carrier-mediated magnetic indirect RKKY exchange renormalized significantly by quantum fluctuations of the electron density along 〈110〉 directions.PACS: 73.22.-f, 75.47.-m, 71.27. + a.

Valence and magnetism in EuPd3S4 and (Y,La)xEu1−xPd3S4

The double Dirac candidate EuPd${}_{3}$S${}_{4}$ appears to exhibit a roughly 50:50 mixture of Eu${}^{2+}$ and Eu${}^{3+}$. Substituting yttrium rapidly drives the remaining europium divalent, whereas lanthanum has the opposite effect. Analysis shows that average site volumes play a dominant role in the valence distribution suggesting that local strains matter more than electron count. Remarkably, lanthanum substitution also promotes valence instability leading to the appearance of intermediate valence ``Eu${}^{2.5+}$'', the fraction of which increases with both lanthanum content and temperature. Extrapolation of the onset temperature to zero lanthanum predicts ${T}_{\text{onset}}$ \ensuremath{\sim} 340 K matching recent reports of a structural transition attributed to a loss of valence order.

Pressure-induced valence fluctuation in CsEuF3: From divalent Eu valence to trivalent Eu valence state

The ternary rare-earth metal fluoride CsEuF3 adopts an ideal cubic perovskite structure [ABX3] under ambient conditions. The B-cation site is occupied by the rare-earth Eu ion and EuF6 octahedra are formed. In the present study, magnetic susceptibility and synchrotron X-ray absorption spectroscopy (XAS) analysis at the Eu-L3 edge confirmed that Eu is in a divalent oxidation state under ambient conditions. Temperature-dependent magnetic susceptibility data revealed that the average Eu valence increased below 20 K due to a partial transition from the Eu2+ state to the Eu3+ state, thereby resulting in a mixed valence state with an average valence of +2.23. Direct evidence for valence fluctuation by the Eu ions in CsEuF3 was obtained using the high pressure high energy resolution fluorescence detection-XAS technique, where continuous changes in valence were observed from 2.15+ at ambient pressure up to 2.5+ at 10.5 GPa. These findings indicate the possibility of discovering interesting physical properties associated with valence instabilities by rare-earth metals in similar systems.

Structural Phase Transition and Possible Valence Instability of Ce-4f Electron Induced by Pressure in CeCoSi

X-ray powder diffraction and electrical resistivity measurements were performed on the tetragonal compound CeCoSi under pressure to elucidate the phase boundary of the pressure-induced structural transition and the change in the 4$f$ electronic state. The temperature-pressure phase diagram has been determined from the shift of the Bragg peaks and from the anomaly in the resistivity. The critical pressure, $P_{\rm s}$ $\sim$ 4.9 GPa at 300 K, decreases to $P_{\rm s}$ $\sim$ 3.6 GPa at 10 K. The decrease of $P_{\rm s}$ is due not only to the decrease in volume of the unit cell but also to an anisotropic shrinkage by cooling. When crossing the boundary to the high-pressure phase, the resistivity shows a significant drop to exhibit a metallic temperature dependence. The results of this study strongly suggest that the structural phase transition can be ascribed to valence instability of Ce-$4f$ electron.

Magnetovolume Effect on the First-Order Metamagnetic Transition in UTe2

The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe$_2$ was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at $\mu_0H_{\rm m}\approx 35$~T for the applied magnetic fields along the crystallographic $b$ axis in the orthorhombic structure. The resultanting volume magnetostriction of $\Delta V/V \approx-5.9\times 10^{-4}$ gives the initial pressure dependence of $H_{\rm m}$ by employing the Clausius-Clapeyron's equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the $a$ axis, those along the $b$ and $c$ axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Gr\"uneisen parameters compared to the other $f$-electron systems. The volume shrinkage in UTe$_2$ at $H_{\rm m}$, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.

Anisotropy-driven quantum criticality in an intermediate valence system

Intermetallic compounds containing f-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic f-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass (m*) and a corresponding decrease of the Fermi temperature. However, the presence of f-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate m*. Such systems are typically isotropic, with a characteristic energy scale T0 of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show the discovery of a quantum critical behaviour and a Lifshitz transition under low magnetic field in an intermediate valence compound α-YbAlB4. The QC origin is attributed to the anisotropic hybridization between the conduction and localized f-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

Magnetic Properties under Pressure in Novel Spin-Triplet Superconductor UTe2

We report the magnetic susceptibility and the magnetization under pressures up to 1.7GPa above the critical pressure, Pc ~ 1.5GPa, for H // a, b, c-axes in the novel spin triplet superconductor UTe2. The anisotropic magnetic susceptibility at low pressure with the easy magnetization a-axis changes to the quasi-isotropic behavior at high pressure, revealing a rapid suppression of the susceptibility for a-axis, and a gradual increase of the susceptibility for the b-axis. At 1.7GPa above Pc, magnetic anomalies are detected at T_MO ~ 3K and T_WMO ~ 10K. The former anomaly corresponds to long-range magnetic order, most likely antiferromagnetism, while the latter shows a broad anomaly, which is probably due to the development of short range order. The unusual decrease and increase of the susceptibility below T_WMO for H // a and b-axes, respectively, indicate the complex magnetic properties at low temperatures above Pc. This is related to the interplay between multiple fluctuations dominated by antiferromagnetism, ferroamgnetism, valence and Fermi surface instabilities.

Quantitatively determining the degree of spin fluctuations in actinide metals and compounds

Actinide materials are well-documented for nuclear properties, but their 5f electrons that produce exotic phenomena are not, due to the complexity of a dual localized and itinerant nature that remains a mystery. Particular interest is given to the electronic correlations at the localized and itinerant boundary where strong spin fluctuations are present. We report the identification of an intensity defined by integrating the normalized resistivity that approximately provides a quantitative measure of spin fluctuations. The intensity is very sensitive to the tuning of non-thermal parameters such as pressure and chemical doping, probing the anomalies in the evolution of spin fluctuations close to a valence or magnetic instability. In this way, our results are not only connected with the long-standing controversy on the anomalous low-temperature resistivity behavior of actinide metals, but also highlight an unconventional type of superconducting pairing, mediated by valence and/or spin fluctuations, for a wealth of 4f and 5f-electron systems. In an unified picture proposed, it is helpful to determine the degree of spin fluctuations for understanding the origin of the emergent superconductivity in systems with correlated electrons.

Anomalous thermal expansion in intermediate valence compound Ce2Ni3Si5

The linear thermal expansion coefficients in the intermediate valence system Ce2Ni3Si5 were determined along the main crystallographic directions by means of neutron diffraction in the temperature range 10–300 K. The anomalous contribution to thermal expansion due to valence instabilities of cerium was estimated by employing the virtual mix approach. The magnitude of the anomalous magnetic contribution to the volume coefficient of thermal expansion in this intermetallic system is about 1.2× 10−5 K−1. The observed effect is caused by a strong temperature dependence of the degree of the partial delocalization of the cerium 4f electrons in the intermediate valence regime. The effective Kondo temperature of Ce2Ni3Si5 TK = 140 К obtained in the present work is in a very good agreement with the earlier estimates based on the temperature dependences of the macroscopic magnetic and thermodynamic properties.

Electric and Thermosalient Properties of a Charge-Transfer Complex Exhibiting a Minor Valence Instability Transition

The charge transfer (CT) complex 3,3’,5,5’-tetramethylbendizine-7,7,8,8-tetracyanoquinodimethane (TMB-TCNQ) was found to display the thermosalient effect, in which crystals exhibit mechanical respo...

Magnetic Phase Diagram of Tm0.96Yb0.04B12 Antiferromagnet with Dynamic Charge Stripes and Yb Valence Instability

Magnetic Phase Diagram of Tm_0.96Yb_0.04B₁₂ Antiferromagnet with Dynamic Charge Stripes and Yb Valence Instability

Valence instability across the magnetostructural transition in USb2

We have performed pressure dependent X-ray diffraction and resonant X-ray emission spectroscopy experiments on USb$_2$ to further characterize the AFM-FM transition occurring near 8 GPa. We have found the magnetic transition coincides with a tetragonal to orthorhombic transition resulting in a 17% volume collapse as well as a transient $\textit{f}$-occupation enhancement. Compared to UAs$_2$ and UAsS, USb$_2$ shows a reduced bulk modulus and transition pressure and an increased volume collapse at the structural transition. Except for an enhancement across the transition region, the $\textit{f}$-occupancy decreases steadily from 1.96 to 1.75.

Phase Stability of Ce-Substituted Magnetoplumbite-Type Sr Ferrite

We studied the phase stability of Ce-substituted magnetoplumbite-type Sr ferrite under various oxygen pressures. We have successfully obtained Ce-substituted samples as revealed from X-ray diffraction and wavelength dispersion X-ray analyses. The solubility limit of Ce in Sr1-xCexFe12O19 was estimated to be $x = 0.24$ at ambient pressure, which is reduced to 0.05-0.10 under higher oxygen pressures. The variation of the $c$ -axis against $x$ and the reduction of the solubility by increasing oxygen pressure suggests that the valence of Ce is not 4+ but 3+. The solubility limit of Ce is dominated not only by the ionic radius but also by the valence instability of Ce.

X-ray absorption study on the origin of the deviation from Vegard's law for U(Al1−xGex)3 solid solution

The cell parameter evolution along the cubic U(Al1−xGex)3 solid solution shows a significant deviation from Vegard's law. The origin of such deviation has been investigated by X-ray powder and electron diffractions, EXAFS and HERFD XANES at the U L3 and Ge K edges and specific heat measurements. The results of such investigations show that, from the structural point of view, U(Al1−xGex)3 does not present any local or long range ordering and the deviation from Vegard's law is caused by valence instability of U with an actual decrease of the U–Ge bond length with increasing Al-content and cell parameter.

Anomalous connection between antiferromagnetic and superconducting phases in the pressurized noncentrosymmetric heavy-fermion compound CeRhGe3

Unconventional superconductivity frequently emerges as the transition temperature of a magnetic phase, typically antiferromagnetic, is suppressed continuously toward zero temperature. Here, we report contrary behavior in pressurized CeRhGe3, a non-centrosymmetric heavy fermion compound. We find that its pressure-tuned antiferromagnetic transition temperature (TN) appears to avoid a continuous decrease to zero temperature by terminating abruptly above a dome of pressure-induced superconductivity. Near 21.5 GPa, evidence for TN suddenly vanishes, the electrical resistance becomes linear in temperature and the superconducting transition reaches a maximum. In light of X-ray absorption spectroscopy measurements, these characteristics appear to be related to a pressured-induced Ce valence instability, which reveals as a sharp increase in the rate of change of Ce valence with applied pressure.

Erratum: Towards first-principles prediction of valence instabilities in mixed stack charge-transfer crystals [Phys. Rev. B 95 , 155125 (2017)

Received 21 October 2018DOI:https://doi.org/10.1103/PhysRevB.98.199901©2018 American Physical SocietyPhysics Subject Headings (PhySH)TechniquesDensity functional theoryHubbard modelCondensed Matter, Materials & Applied Physics