TR 288 - Elasto-Q-Mat: Research Data

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  • Research Data
    Theoretical data for: Orbital occupancy and hybridization in strained SrVO3 epitaxial films
    These data were produced for an experimental paper of M. Mirjolet [1]. To generate them we used the density functional theory FPLO [2]. This gave us the integrated density of states, with whom we combined the different orbitals and atoms to obtain the results depicted in the publication [1]. The data of the post processing part are the data given in this upload. To reproduce the density functional part one needs to repeat the calculation. The input parameters are given in the publication [1]. For further information on the experimental data, please contact: [1] M. Mirjolet et al. Phys. Rev. Mater. 5, 095002 (2021) [2]
      86  5
  • Research Data
    Data for: Influence of magnetism, strain and pressure on the band topology of EuCd2As2
    Motivated by the wealth of proposals and realizations of nontrivial topological phases in EuCd2As2, such as a Weyl semimetallic state and the recently discussed semimetallic versus semiconductor behavior in this system, we analyze in this work the role of the delicate interplay of Eu magnetism, strain and pressure on the realization of such phases. For that we invoke a combination of a group theoretical analysis with ab initio density functional theory calculations and uncover a rich phase diagram with various non-trivial topological phases beyond a Weyl semimetallic state, such as axion and topological crystalline insulating phases, and discuss their realization.
      26  3
  • Research Data
    Microscopic analysis of the valence transition in tetragonal EuPd2Si2
    Under temperature or pressure tuning, tetragonal EuPd2Si2 is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by ab initio density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd2Si2 when approaching the valence transition shows an enhanced c−f hybridization between localized Eu 4f states and itinerant conduction states (Eu 5d, Pd 4d, and Si 3p) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd2Si2 are reflected in a simultaneous observation of divalent and trivalent signals from the Eu 4f shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition.
      17  4
  • Research Data
    Moment canting and domain effects in antiferromagnetic DyRh2Si2
    A combined experimental and theoretical study of the layered antiferromagnetic compound DyRh2Si2 in the ThCr2Si2-type structure is presented. The heat capacity shows two transitions upon cooling: The first one at the Néel temperature TN = 55 K and a second one at TN2 = 12 K. Using magnetization measurements, we study the canting process of the Dy moments upon changing the temperature and can assign TN2 to the onset of the canting of the magnetic moments towards the [100] direction away from the c axis. Furthermore, we found that the field dependence of the magnetization is highly anisotropic and shows a two-step process for H || 001. We used a mean-field model to determine the crystalline electric field as well as the exchange interaction parameters. Our magnetization data together with the calculations reveal a moment orientation close to the [101] direction in the tetragonal structure at low temperatures and fields. Applying photoemission electron microscopy, we explore the (001) surface of the cleaved DyRh2Si2 single crystal and visualize Si- and Dy-terminated surfaces. Our results indicate that the Si-Rh-Si surface protects the deeper lying magnetically active Dy layers and is thus attractive for investigation of magnetic domains and their properties in the large family of LnT2Si2 materials.
      29  7
  • Research Data
    Colossal magnetoresistance in EuZn2P2 and its electronic and magnetic structure
    We investigate single crystals of the trigonal antiferromagnet EuZn2P2 (P-3m1) by means of electrical trans- port, magnetization measurements, x-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES), and ab initio band structure calculations (DFT + U ). We find that the electrical resistivity of EuZn2P2 increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (colossal magnetoresistance effect). Resonant magnetic scattering reveals a magnetic ordering vector of q = (0 0 1/2 ), corresponding to an A-type antiferromagnetic order, below TN = 23.7 K. We find that the moments are canted out of the a-a plane by an angle of about 40° ± 10° and aligned along the [100] direction in the a-a plane. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below ≈150 K, the temperature below which also the field dependence of the material’s resistivity starts to increase. An analysis of the infrared reflectivity spectrum at T = 295 K allows us to resolve the main phonon bands and intraband and interband transitions, and estimate indirect and direct band gaps of E_opt,i = 0.09 and E_opt,d = 0.33 eV, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly T independent above and below T_N . The comparison of the theoretical and experimental data shows a weak intermixing of the Eu 4f states close to the point with the bands formed by the phosphorous 3p orbitals leading to an induction of a small magnetic moment at the P sites.
      40  4
  • Research Data
    From magnetic order to valence-change crossover in EuPd2(Si1−xGex)2 using He-gas pressure
    We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd2(Si1−xGex )2 for 0 ≤ x ≤ 0.2 and temperatures 2 K ≤ T ≤ 300 K. Data were taken at ambient pressure and finite He-gas pressure p ≤ 0.5 GPa. For x = 0 and ambient pressure we observe a pronounced valence-change crossover centred around T′_V ≈ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of dT′_V /dp = (80 ±10) K/GPa. We observe a shift of T′_V to lower temperatures with increasing Ge-concentration, reaching T′_V ≈ 90 K for x = 0.1, while still showing a non-magnetic ground state. Remarkably, on further increasing x to 0.2 we find a stable Eu(2+δ)+ valence with long-range antiferromagnetic order below T_N = (47.5 ± 0.1) K, reflecting a close competition between two energy scales in this system. In fact, by the application of hydrostatic pressure as small as 0.1 GPa, the ground state of this system can be changed from long-range antiferromagnetic order for p < 0.1 GPa to an intermediate-valence state for p ≥ 0.1 GPa.
      1  41
  • Research Data
    Pressure study on the interplay between magnetic order and valence crossover in EuPd2(Si1−xGex)2
    We present results of the magnetic susceptibility on high-quality single crystals of EuPd2(Si1−xGex )2 for Ge concentrations 0 ≤ x ≤ 0.105 performed under varying hydrostatic (He-gas) pressure 0 ≤ p ≤ 0.5 GPa. The work extends recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-crossover behavior for x = 0 and 0.058, to long-range antiferromagnetic (AFM) order below T_N = 47 K for x = 0.105. The valence-crossover temperature T_V shows an extraordinarily strong pressure dependence of dT'_V/dp= +(80 ± 10) K/GPa. In contrast, a very small pressure dependence of dT_N/dp ≤ +(1 ± 0.5) K/GPa is found for the AFM order upon pressurizing the x = 0.105 crystal from p = 0 to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from AFM order to valencecrossover behavior is observed. Estimates of the electronic entropy related to the Eu 4f electrons, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between AFM order and valence crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical end point of the first-order transition for x = 0.105 at p_cr ≈ 0.06 GPa and T_cr ≈ 39 K where intriguing strong-coupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.
      39  3
  • Research Data
    Field-induced effects in the spin liquid candidate PbCuTe2O6
    PbCuTe2O6 is considered to be one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below T_FE ≈ 1 K, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic, and dielectric properties of single-crystalline PbCuTe2O6 in magnetic fields B ≤ 14.5 T. The combination of these various probes allows us to construct a detailed B-T phase diagram including a ferroelectric phase for B ≤ 8 T and a B-induced magnetic phase at B ≥ 11 T. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted noncubic low-temperature state. The phase diagram discloses a ferroelectric quantum critical point at B_c1 = 7.9 T, where the second-order phase transition line associated with ferroelectric order is suppressed to zero. In addition, a magnetic quantum phase transition is revealed at B_c2 = 11 T. The corresponding phase transition to a fieldinduced magnetic order at B > B_c2 is likely to be of first order. Field-induced lattice distortions, observed in the state at T > 1 K and which are assigned to the effect of spin-orbit interaction of the Cu2+ ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material.
      3  20
  • Research Data
    From valence fluctuations to long-range magnetic order in EuPd2(Si1−xGex)2 single crystals, original data from the figuresPaper
    EuPd2Si2 is a valence-fluctuating system undergoing a temperature-induced valence crossover at T_V=160 K. We present the successful single-crystal growth using the Czochralski method for the substitution series EuPd2(Si1−xGex)2, with substitution levels x < 0.15. A careful determination of the germanium content revealed that only half of the nominal concentration is built into the crystal structure. From thermodynamic measurements it is established that T_V is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for x ~ 0.10. The valence transition is accompanied by a pronounced change of the lattice parameter a of order 1.8%. In the antiferromagnetically ordered state below T_N = 47 K, we find sizable magnetic anisotropy with an easy plane perpendicular to the crystallographic c direction. An entropy analysis revealed that no valence fluctuations are present for the magnetically ordered materials. Combining the obtained thermodynamic and structural data, we construct a concentration-temperature phase diagram demonstrating a rather abrupt change from a valence-fluctuating to a magnetically ordered state in EuPd2(Si1−xGex)2.
      66  11
  • Research Data
    Epitaxial EuPd2Si2 thin films
    Bulk EuPd2Si2 show a temperature-driven valence transisition of europium from ~+2 above 200 K to~+3 below 100 K, which is correlated with a shrinking by approximatly 2 % of the crystal lattice along the two a-axes. Due to this interconnection between lattice and electronic degrees of freedom the influence of strain in epitaxial thin films is particularly interesting. Ambient X-ray diffraction (XRD) confirms an epitaxial relationship of tetragonal EuPd2Si2 on MgO(001) with an out-of plane c-axis orientation for the thin film, whereby the a-axes of both lattices align. XRD at low temperatures reveals a strong coupling of the thin film lattice to the substrate, showing no abrupt compression over the temperature range from 300 to 10 K. Hard X-ray photoelectron spectroscopy at 300 and 20 K reveals a temperature-independent valence of +2.0 for Eu. The evolving biaxial tensile strain upon cooling is suggested to suppress the valence transition. Instead low temperature transport measurements of the resistivity and the Hall effect in a magnetic field up to 5 T point to a film thickness independent phase transition at 16-20 K, indicating magnetic ordering.
      110  2
  • Research Data
    A j_eff 12 Kitaev material on the triangular lattice: The case of NaRuO2
    Motivated by recent reports of a quantum disordered ground state in the triangular lattice compound NaRuO$_2$, we derive a $j_{\rm eff}=1/2$ magnetic model for this system by means of first-principles calculations. The pseudospin Hamiltonian is dominated by bond-dependent off-diagonal $\Gamma$ interactions, complemented by a ferromagnetic Heisenberg exchange and a notably \emph{antiferromagnetic} Kitaev term. In addition to bilinear interactions, we find a sizable four-spin ring exchange contribution with a \emph{strongly anisotropic} character, which has been so far overlooked when modeling Kitaev materials. The analysis of the magnetic model, based on the minimization of the classical energy and exact diagonalization of the quantum Hamiltonian, points toward the existence of a rather robust easy-plane ferromagnetic order, which cannot be easily destabilized by physically relevant perturbations.
      292  31