dHvA Data Research Articles

Effect of Nonlocal Correlations on the Electronic Structure of LiFeAs.

We investigate the role of nonlocal correlations in LiFeAs by exploring an abinitio-derived multiorbital Hubbard model for LiFeAs via the two-particle self-consistent (TPSC) approach. The multiorbital formulation of TPSC approximates the irreducible interaction vertex to be an orbital-dependent constant, which is self-consistently determined from local spin and charge sum rules. Within this approach, we disentangle the contribution of local and nonlocal correlations in LiFeAs and show that in the local approximation one recovers the dynamical mean field theory result. The comparison of our theoretical results to most recent angular-resolved photoemission spectroscopy and de Haas-van Alphen data shows that nonlocal correlations in LiFeAs are decisive to describe the measured spectral function A(k[over →],ω), Fermi surface, and scattering rates. These findings underline the importance of nonlocal correlations and benchmark different theoretical approaches for iron-based superconductors.

Fermi surface investigation of the semimetal TaAs2

The transversal magnetoresistance associated with the semimetal ${\mathrm{TaAs}}_{2}$ shows a parabolic field dependence that rises unrestrictedly to 2800 at 14 T and 1.8 K. Here, we report the results of a comprehensive quantum-oscillation study. Angular-dependent de Haas--van Alphen (dHvA) data were obtained with the method of cantilever-torque magnetometry. These were compared with the results of density-functional theory calculations, which predict a Fermi surface with two kinds of electron pockets, as well as two types of hole pockets. Only the electron pockets could be experimentally verified, whereas no evidence for the hole pockets is present in the measured dHvA frequencies.

Electronic band structure and proximity to magnetic ordering in the chiral cubic compound CrGe

CrGe belongs to the family of cubic B20 intermetallics. From experimental investigations by susceptibility and de Haas-van Alphen (dHvA) measurements and from calculations of its electronic band structure by density-functional theory (DFT), CrGe is found to form a metallic paramagnetic ground state. Combining dHvA and DFT data, a detailed picture of the Fermi surface of CrGe is provided. The proximity to a magnetic long-range ordering in CrGe is suggested from a prominent thermal magnetic susceptibility. The possibility to induce magnetic long-range order in CrGe is discussed based on calculated properties for CrGe substituting Ge by As or Sn, and from a comparison with MnGe and the alloy series ${\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Ge}$. Owing to the noncentrosymmetric and nonsymmorphic crystal structure of CrGe, in absence of broken time reversal symmetry, its band structure is marked by forced nodal lines at the Fermi edge. Moreover, this material hosts degenerate unconventional electronic quasiparticles. In particular, CrGe exhibits a sixfold degeneracy of fermions crossing within about 5 meV of the Fermi energy at the $R$ point of the Brillouin zone.

Electronic states in antiferromagnet UCd11 and reference compound ThCd11: studied by the de Haas–van Alphen effect

AbstractWe carried out the de Haas–van Alphen (dHvA) experiment for an antiferromagnet UCd11 and its non‐5f reference compound ThCd11 to clarify the Fermi surface properties and the true electronic specific‐heat coefficient γ of UCd11. We observed dHvA signals of both compounds and compared the experimental results with the result of theoretical band calculations. The Fermi surfaces of both compounds are found to be small and closed. The cyclotron masses obtained from the dHvA data of UCd11 are compared with those of reference compounds ThCd11, NpCd11, and LaCd11, and the γ value of UCd11 is estimated to be about 100 mJ K−2 mol−1, which is much smaller than the 840 mJ K−2 mol−1 in the previous reports. It is concluded that this large value is mainly not due to heavy quasiparticles but due to the magnetic contribution to the specific heat.

Isotope Effect at the Fano Resonance in Superconducting Gaps for Multiband Superconductors at a 2.5 Lifshitz Transition

The doping dependent isotope effect in cuprates is explained in the framework of shape resonances in the superconducting gaps (belonging to the class of Fano resonances) in multicondensate superconductors. This new paradigm for high temperature superconductivity is based on the recent Fermiology scenario emerging from dHvA and quantum oscillation data showing a 2.5 Lifshitz topological transition due to the appearance of new small Fermi surface in the underdoped regime. The isotope effect is calculated for an electronic system near a band edge for a superlattice of stripes. The model reproduces the doping dependence of the isotope exponent behavior in cuprates and allows to identify the relative role of the intraband Cooper pairing and the configuration interaction between pairing channels from experimental data.

De Haas–van Alphen Effect and Fermi Surface Properties in High-Quality Single Crystals YbCu2Si2 and YbCu2Ge2

We succeeded in growing high-quality single crystals of a valence fluctuating compound YbCu 2 Si 2 and a divalent compound YbCu 2 Ge 2 . The magnetic susceptibility of YbCu 2 Si 2 follows the Curie–Weiss law with Yb 3+ at high temperatures, but reveals a broad peak around 40 K for H ∥[100], which is due to the formation of a 4 f -itinerant heavy fermion state at lower temperatures. This is also reflected in the temperature dependence of Hall coefficient, thermoelectric power and thermal expansion. The corresponding de Haas–van Alphen (dHvA) branches are approximately explained by the 4 f -itinerant LDA band model, and the 4 f -itinerant LDA+ U model is found to be much applicable to the dHvA data. The cyclotron effective masses of main Fermi surfaces are relatively large, being 30–40 m 0 , which is consistent with the electronic specific heat coefficient γ=150 mJ/(K 2 ·mol). These results indicate that the localized 4 f electrons at high temperatures become itinerant at low temperatures, forming a narrow ...

Fermi surface in the superconductingβ-pyrochlore oxideCsOs2O6

We report de Haas--van Alphen effect (dHvA) measurements and band-structure calculations for $\mathrm{Cs}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$. Sixteen dHvA frequency branches are observed in the $(\overline{1}10)$ plane. The Fermi surface consists of hole and electron surfaces. The topology of the measured Fermi surface can be described by the band-structure calculations if slight rigid band shifts are incorporated. The electron surface has through holes in the ⟨111⟩ directions, indicating that the van Hove singularity is above the Fermi level unlike previously thought. Using band-structure results, we have estimated that the Stoner enhancement factor $S\ensuremath{\sim}3.1$ and that the Wilson ratio $R\ensuremath{\sim}0.86$. The dHvA effective masses are mostly between three and four times the corresponding band masses, which is consistent with the specific-heat mass enhancement of 3.6. We have also estimated the orbital-specific Stoner enhancement factor for one orbit from dHvA data: the most probable value of the product $Sg$, where $g$ is the $g$ factor, for that orbit is found to be 9.374. The small Wilson ratio and Stoner enhancement indicate rather limited importance of electron-electron interactions in $\mathrm{Cs}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$.

High pressure de Haas–van Alphen studies of Sr2RuO4 using an anvil cell

Abstract We have developed a novel way to perform de Haas–van Alphen (dHvA) measurements under high pressure using a microcoil system in an anvil cell, allowing us to track the electronic structure of strongly correlated electronic systems as a function of pressure. The unconventional superconductor Sr2RuO4 is chosen to illustrate the feasibility of this technique. The dHvA frequencies are obtained from the oscillatory data and they are compared with the values reported earlier in the literature. The quasiparticle effective masses are extracted from our high resolution dHvA data.

K-Space tomography of the Fermi surface by spatially resolved photoemission spectroscopy with variable photon energy

When mapping spatially resolved photoemission intensity for electron kinetic energy corresponding to Fermi energy, one obtains a spherical cut through a three-dimensional Fermi surface of a metal. At the APE-INFM beamline at Elettra, we developed an automated method of measuring the Fermi surface cuts for a large number of photon energies and putting them together in order to reconstruct the three-dimensional Fermi surface. The energy and k ∥ (momentum parallel to the surface) resolution are set by the monochromator and energy analyzer properties, while the k ⊥ (momentum perpendicular to the surface) resolution depends on the photon energy step chosen for obtaining subsequent data sets. By 3D interpolation of the data, one obtains the k-space tomography of the constant initial state surfaces (e.g. of the Fermi surface). From the tomography, one can retrieve true k x , k y , k z resolved maps. Besides de Haas van Alphen (dHvA) oscillations, this method is the most direct way for the determination of the Fermi surfaces. A set of data on the Fermi surface of Be(0 0 0 1) is presented and compared to dHvA data and theory.

Electronic structure and anisotropic superconductivity in diborides and borocarbides

We compare calculated Fermi surface sheet (FSS) areas F and masses of MgB 2 and ZrB 2 with dHvA data. Deviations in F in MgB 2 can be removed by a small mutual shift of σ- and π bands. The dHvA masses lead to orbit averaged el–ph coupling constants λ ̄ σ ≈1.03 and λ ̄ π ≈0.32 , while for ZrB 2 only λ ̄ <0.1 holds. The anisotropy of the Fermi velocities and the orbital character of various FSS of rare earth (R) transition metal (T) borocarbides RT 2B 2C, their relationship to the H c2-anisotropy, and the coexistence of magnetism and superconductivity are discussed.

Conventional Electronic Structure of MgB2 and ZrB2: LDA vs. de Haas-v. Alphen & ARPES Data

We compare full potential LDA band calculations of the Fermi surfaces areas and band masses of MgB2 and ZrB2 previously reported and new dHvA data. Discrepancies in areas in MgB2 can be removed by a small shift of σ bands relative to π bands. Comparison of effective masses lead to orbit averaged el-ph coupling constants λσ=1.3 and λπ=0.5, whereas for ZrB2 only weak el-ph coupling with λ<0.3 is found. The ARPES data can be also well described by the LDA showing the presence of surface states.

Quadrupolar Phase Diagram and Fermi Surface Properties in the Localized 5f2-Electron System UPd3

We measured the electrical resistivity, specific heat, magnetostriction, magnetization and de Haas-van Alphen (dHvA) oscillation in UPd 3 with the hexagonal structure in order to clarify the quadrupolar and magnetic properties as well as the nature of conduction electrons. The high-field magnetization measurement at 4.2 K in magnetic fields up to about 50 T reveals two-successive metamagnetic transitions at 10.2 and 17.9 T for the field along the [0001] direction, and one transition at 27.6 T along [11\bar20]. From the results of magnetization as well as the magnetostriction measurements at various temperatures, we determined the magnetic phase diagram. Phase boundaries are discussed on the basis of the crystalline electric field scheme of the 5 f 2 configuration and the quadrupolar interaction of O zx . Reflecting the localized 5 f -electrons, the conduction electrons have small cyclotron effective masses ranging from 0.24 to 3.62 m 0 , which were determined by the dHvA experiment. The dHvA data are comp...

Indications of non-Fermi liquid behavior at the metamagnetic transition of UPt3

The specific heat and resistivity of high purity needle crystals and the magnetization of a 300mg piece of a Czochralski-process single crystal of UPt3 aligned so that B⊥c were measured at 0, 18, 19, 20, 22, and 24T applied fields between 0.5 and 10K, where Bmetamag.≈20T. The data for C/T, ρ, and χ at 18T and for C/T and ρ at 24T indicate that there is still Fermi liquid behavior at low temperatures for B≠Bmetamag.. (For B≥21T,χ displays apparent ferromagnetic behavior.) However, atBmetamag.C/T is found to be proportional to −logT, or characteristic of non-Fermi liquid (nFl) behavior, over more than a decade in temperature while ρ=ρ0+AT1.2 at 22T (0.5K≤T≤3K) and χ=χ0−aT at 20.5T (0.5K≤T≤20K)—also both characteristic of nFl behavior. This C/T∝−logT behavior is a different nFl temperature dependence than previously found for CeRu2Si2 near its Bmetamag., where C/T was found to decrease linearly with increasing temperature. Thus, although metamagnetism in these, and other, yet to be studied, highly correlated f-electron systems may be a route for attaining long range magnetic interactions that prohibit Fermi-liquid behavior, already the first two systems studied show varying behavior. dHvA data at Bmetamag. for CeRu2Si2 and Shubnikov–de Haas (SdH) data at Bmetamag. for UPt3, and theories thereof, will be compared. The SdH data shed light on the present χ(H) data for UPt3 for B>Bmetamag..

Heavy Fermions in YbAl3Studied by the de Haas-van Alphen Effect

We carried out the de Haas-van Alphen (dHvA) experiment in YbAl 3 with the cubic crystal structure. One of the six dHvA branches, named β with a dHvA frequency of 4.5×10 7 Oe, is nearly spherical in topology and has a large cyclotron mass of 23.9 m 0 . The other branches were observed only in small angular regions centered at symmetrical directions. Their cyclotron masses are in the range from 13.5 to 17.4 m 0 . The topology of the Fermi surface cannot be explained by the conventional band model because the fully occupied 4 f -levels in YbAl 3 are below the Fermi level. A small alteration so as to shift upwards the 4 f -levels is necessary to fit to the experimental dHvA data. These give a direct evidence that the 4 f -electrons form heavy fermions at low temperatures, which is most likely related to the many-body Kondo effect.

Electronic structure and superconductivity of nonmagnetic transition metal borocarbides

The electronic structure of tetragonal (RC(N))m(TB)2, m=1−3, compounds, with R= Y,Lu,La, T=Ni,Pt is studied. Total and partial densities of states N(E) are calculated and compared with orbital resolved x-ray absorption data. Special attention is paid to the structure of N(E) near the Fermi-level and its consequences for thermodynamic properties in the superconducting and the normal states. A medium el-ph coupling constant 0.5≤λ≤ 1.2 is found. There is no simple correlation between N(0) and Tc. The analysis of the upper critical field reveals the presence of at least two groups of electrons with quite different Fermi velocities vFin accord with dHvA data, and with calculated distributions of vFaround the Fermi surface

Fermi surface studies of?-phase organic metals and superconductors

We present first de Haas-van Alphen (dHvA) experiments on the quasi-two-dimensional (quasi-2D) organic metals α-(ET)2MHg(SeCN)4(M = Tl or K) and compare the results with dHvA data of the isostructural organic superconductor α-(ET)2NH4Hg(SCN)4. The angular dependences of the single found dHvA frequency and the field-independent effective cyclotron mass, mc, perfectly follow the 2D 1/cosΘ behavior. From the observed spin-splitting zeros we unambiguously could extract the values of gμb. Comparison with mc strongly suggests a reduced electron-phonon coupling constant in the non-superconducting salts.

Quasiparticles in heavy-fermion systems with nearly integer valence

The theory describing the electron mass enhancement in Kondo-lattice heavy-fermion systems is presented. The description is based on the picture of a spin Fermi liquid of RVB type which strongly interacts with conduction electrons. It is shown that the electron - spinon scattering results in large phase shifts for conduction electrons near the Fermi level which mimic the resonance renormalization but does not involve f-electron states. The results are compared with dHvA data for several Ce-based compounds.

The electronic structure of h.c.p. Ytterbium

The energy bands and Fermi surface of h.c.p. Ytterbium have been calculated by the relativistic APW method. The results are in good agreement with recent dHvA data. A new procedure for the calculation of cross-sectional areas and other geometrical properties of the Fermi surface is presented.

De Haas-van Alphen effect in rare-earth hexaborides (RE=Pr, Nd, Gd)

The de Haas-van Alphen (dHvA) effect has been measured for antiferromagnetic PrB6, NdB6, and GdB6. The dHvA data of PrB6 and NdB6 show paramagnetic Fermi surfaces which arise from magnetic breakdown through the small antiferromagnetic gaps in high fields very similar to those of LaB6. In contrast, the antiferromagnetic Fermi surfaces which arise from new magnetic Brillouin zone boundaries introduced by the magnetic order seem to be very different in PrB6 and NdB6, respectively. In GdB6 dHvA oscillations could be observed for the first time. In all three compounds magnetic phase transitions have been observed in the dHvA measurements in the field range of 10–12 T.

Itinerant magnetism and electronic properties of FeGe2

The electronic and magnetic structure of tetragonal FeGe2 is determined using the local spin-density functional approximation and the ASW method to solve the band-structure problem self-consistently. The calculations reveal that the collinear antiferromagnetic structure-in contrast to a non-collinear one-has the lowest total energy and seems the most preferable candidate for the low-temperature ground-state phase of FeGe2 in agreement with recent neutron diffraction experiments. The calculated Fermi surface cross-sections and cyclotron masses are in good agreement with the DHVA experimental data. From the DHVA and heat capacity data together with the results of band structure calculations the average value of the electron-phonon interaction constant lambda equivalent to 0.5 has been deduced for FeGe2. For the alternative non-collinear model the authors obtained a higher value of the total energy and poor agreement with the DHVA experiment.