arXiv subjects

G. Zwicknagl

Publications and source records attributed to G. Zwicknagl.

At least 19 recordsLinked to original sources

arXiv2026

Acoustic signatures of the field-induced electronic-topological transitions in YbNi$_4$P$_2$

We investigated the magnetoelastic properties of an YbNi$_4$P$_2$ single crystal at low temperatures under magnetic fields directed along the crystallographic [001] axis. We report a series of strong anomalies in the sound velocity, which is consistent with the cascade of electronic-topological transitions reported previously for this compound. In particular, we identify the vanishing of a small orbit on the Fermi surface, associated with a quantum-oscillation frequency of 34 T. Furthermore, the different transitions are better resolved with acoustic modes of particular symmetry. Using a microscopic model adapted to the strongly correlated electronic structure of YbNi$_4$P$_2$, we describe our results by inspecting realistic electron-phonon couplings in reciprocal space for each acoustic mode. This shows how the $k$ selectivity of ultrasound experiments allows to investigate Fermi-surface reconstructions in strongly correlated electronic systems.

cond-mat.str-el↗

arXiv2024

Origin of the non-Fermi-liquid behavior in CeRh2As2

Unconventional superconductivity in heavy-fermion systems appears often near magnetic quantum critical points (QCPs). This seems to be the case also for CeRh2As2 (Tc $\approx$ 0.31 K). CeRh2As2 shows two superconducting (SC) phases, SC1 and SC2, for a magnetic field along the c axis of the tetragonal unit cell, but only the SC1 phase is observed for a field along the basal plane. Furthermore, another ordered state (phase-I) is observed below T0 $\approx$ 0.48 K whose nature is still unclear: Thermodynamic and magnetic measurements pointed to a non magnetic multipolar state, but recent $μ$SR and NQR/NMR experiments have clearly detected antiferromagnetic (AFM) order below T0 . Also, quasi-two-dimensional AFM fluctuations were observed in NMR and neutron-scattering experiments above T0. The proximity of a QCP is indicated by non-Fermi-liquid (NFL) behavior observed above the ordered states in both specific heat $C(T)/T \propto T^{-0.6}$ and resistivity $ρ(T) \propto T^{0.5}$. These T-dependencies are not compatible with any generic AFM QCP. Because of the strong magnetic-field anisotropy of both the SC phase and phase I, it is possible to study a field-induced SC QCP as well a phase-I QCP by varying the angle $α$ between the field and the c axis. Thus, by examining the behavior of the electronic specific-heat coefficient C(T)/T across these QCPs, we can determine which phase is associated with the NFL behavior. Here, we present low-temperature specific-heat measurements taken in a magnetic field as high as 21 T applied at several angles $α$. We observe that the NFL behavior does very weakly depend on the field and on the angle $α$, a result that is at odd with that observations in standard magnetic QCPs. This suggests a nonmagnetic origin of the quantum critical fluctuations.

cond-mat.str-el↗

arXiv2022

Conventional type-II superconductivity in locally non-centrosymmetric LaRh$_2$As$_2$ single crystals

We report on the observation of superconductivity in LaRh$_2$As$_2$, which is the analogue without $f$-electrons of the heavy-fermion system with two superconducting phases CeRh$_2$As$_2$. A zero-resistivity transition, a specific-heat jump and a drop in magnetic ac susceptibility consistently point to a superconducting transition at a transition temperature of $T_c = 0.28$\,K. The magnetic field-temperature superconducting phase diagrams determined from field-dependent ac-susceptibility measurements reveal small upper critical fields $μ_{\mathrm{0}}H_{c2} \approx 12$\,mT for $H\parallel ab$ and $μ_{\mathrm{0}}H_{c2} \approx 9$\,mT for $H\parallel c$. The observed $H_{c2}$ is larger than the estimated thermodynamic critical field $H_c$ derived from the heat-capacity data, suggesting that LaRh$_2$A$s_2$ is a type-II superconductor with Ginzburg-Landau parameters $κ^{ab}_{GL} \approx 1.9$ and $κ^{c}_{GL}\approx 2.7$. The microscopic Eliashberg theory indicates superconductivity to be in the weak-coupling regime with an electron-phonon coupling constant $λ_{e-ph} \approx 0.4$. Despite a similar $T_c$ and the same crystal structure as the Ce compound, LaRh$_2$As$_2$ displays conventional superconductivity, corroborating the substantial role of the 4$f$ electrons for the extraordinary superconducting state in CeRh$_2$As$_2$.

cond-mat.supr-con↗

arXiv2021

Possible quadrupole density wave in the superconducting Kondo lattice CeRh2As2

CeRh2As2 has recently been reported to be a rare case of multi-phase unconventional superconductor [S. Khim et al., arXiv:2101.09522] close to a quantum critical point (QCP). Here, we present a comprehensive study of its normal state properties and of the phase (I) below To ~ 0.4 K which preempts superconductivity at Tc = 0.26 K. The 2nd-order phase transition at To presents signatures in specific heat and thermal expansion, but none in magnetization and ac-susceptibility, indicating a non-magnetic origin of phase I. In addition, an upturn of the in-plane resistivity at To points to a gap opening at the Fermi level in the basal plane. Thermal expansion indicates a strong positive pressure dependence of To , dTo/dp = 1.5 K/GPa, in contrast to the strong negative pressure coefficient observed for magnetic order in Ce-based Kondo lattices close to a QCP. Similarly, an in-plane magnetic field shifts To to higher temperatures and transforms phase I into another non-magnetic phase (II) through a 1st-order phase transition at about 9 T. Using renormalized band structure calculations, we found that the Kondo effect (TK ~ 30 K) leads to substantial mixing of the excited crystalline-electric-field (CEF) states into the ground state. This allows quadrupolar degrees of freedom in the resulting heavy bands at the Fermi level which are prone to nesting. The huge sensitivity of the quadrupole moment on hybridization together with nesting would cause an unprecedented case of phase transition into a quadrupole-density-wave (QDW) state at a temperature To << TK , which would explain the nature of phase I and II.

cond-mat.str-el↗

arXiv2018

Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures

The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy T_K of the Kondo interaction. Here, we study the evolution of the 4f occupancy with temperature in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In agreement with simple theoretical models, we observe a scaling between the valence at low temperature and T_K obtained from thermodynamic measurements. In contrast, the temperature scale T_v at which the valence increases with temperature is almost the same in all investigated materials while the Kondo temperatures differ by almost four orders of magnitude. This observation is in remarkable contradiction to both naive expectation and precise theoretical predictions of the Kondo model, asking for further theoretical work in order to explain our findings. Our data exclude the presence of a quantum critical valence transition in YbRh2Si2.

cond-mat.str-el↗

arXiv2017

Field induced Lifshitz transition in UPt$_2$Si$_2$: Fermi surface under extreme conditions

We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt$_2$Si$_2$ in magnetic fields up to 60\,T at temperatures down to 50\,mK. For the magnetic field applied along the $c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5$f$ electrons, we find evidence for field induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the Lifshitz type transition is of a discontinuous nature, as predicted for interacting electron systems.

cond-mat.str-el↗

arXiv2015

Competition between Kondo screening and quantum Hall edge reconstruction

We report on a Kondo correlated quantum dot connected to two-dimensional leads where we demonstrate the renormalization of the g-factor in the pure Zeeman case i.e, for magnetic fields parallel to the plane of the quantum dot. For the same system we study the influence of orbital effects by investigating the quantum Hall regime i.e. a perpendicular magnetic field is applied. In this case an unusual behaviour of the suppression of the Kondo effect and of the split zero-bias anomaly is observed. The splitting decreases with magnetic field and shows discontinuous changes which are attributed to the intricate interplay between Kondo screening and the quantum Hall edge structure originating from electrostatic screening. This edge structure made up of compressible and incompressible stripes strongly affects the Kondo temperature of the quantum dot and thereby influences the renormalized g-factor.

cond-mat.mes-hall↗

arXiv2013

Lifshitz transitions and quasiparticle de-renormalization in YbRh$_2$Si$_2$

We study the effect of magnetic fields up to 15 T on the heavy fermion state of YbRh$_2$Si$_2$ via Hall effect and magnetoresistance measurements down to 50 mK. Our data show anomalies at three different characteristic fields. We compare our data to renormalized band structure calculations through which we identify Lifshitz transitions associated with the heavy fermion bands. The Hall measurements indicate that the de-renormalization of the quasiparticles, {\it i.e} the destruction of the local Kondo singlets, occurs smoothly while the Lifshitz transitions occur within rather confined regions of the magnetic field.

cond-mat.str-el↗

arXiv2013

Interplay between Kondo suppression and Lifshitz transitions in YbRh$_2$Si$_2$ at high magnetic fields

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

cond-mat.str-el↗

arXiv2012

Moment screening in the correlated Kondo lattice model

The magnetic correlations, local moments and the susceptibility in the correlated 2D Kondo lattice model at half filling are investigated. We calculate their systematic dependence on the control parameters J_K/t and U/t. An unbiased and reliable exact diagonalization (ED) approach for ground state properties as well as the finite temperature Lanczos method (FTLM) for specific heat and the uniform susceptibility are employed for small tiles on the square lattice. They lead to two major results: Firstly we show that the screened local moment exhibits non-monotonic behavior as a function of U for weak Kondo coupling J_K. Secondly the temperature dependence of the susceptibility obtained from FTLM allows to extract the dependence of the characteristic Kondo temperature scale T* on the correlation strength U. A monotonic increase of T* for small U is found resolving the ambiguity from earlier investigations. In the large U limit the model is equivalent to the 2D Kondo necklace model with two types of localized spins. In this limit the numerical results can be compared to those of the analytical bond operator method in mean field treatment and excellent agreement for the total paramagnetic moment is found, supporting the reliability of both methods.

cond-mat.str-el↗

arXiv2010

Hall effect measurements on YbRh2Si2 and relatives in the light of electronic structure calculations

We report experimental and theoretical investigations of the Hall effect in YbRh2Si2 and its reference compounds LuRh2Si2 and YbIr2Si2. Based on band-structure calculations we identify two bands dominating the Hall coefficient in all these compounds. For the case of LuRh2Si2 - the non-magnetic reference compound of YbRh2Si2 - the temperature dependence of the Hall coefficient is described quantitatively to arise from two hole-like bands. For YbIr2Si2 and YbRh2Si2, renormalized band calculations yield two bands of opposite character. In YbRh2Si2 these two bands almost compensate each other. We present strong indications that the sample dependences of the low-temperature Hall coefficient observed for YbRh2Si2 arise from slight variations of the relative scattering rates of the two bands. Minute changes of the composition appear to be the origin.

cond-mat.str-el↗

arXiv2009

Self-energy effects and electron-phonon coupling in Fe-As superconductors

We report on Raman scattering experiments of the undoped SrFe2As2 and superconducting Sr0.85K0.15Fe2As2 (Tc=28K) and Ba0.72K0.28Fe2As2 (Tc=32K) single crystals. The frequency and linewidth of the B1g mode at 210 cm-1 exhibits an appreciable temperature dependence induced by the superconducting and spin density wave transitions. We give estimates of the electron-phonon coupling related to this renormalization. In addition, we observe a pronounced quasi-elastic Raman response for the undoped compound, suggesting persisting magnetic fluctuations to low temperatures. In the superconducting state the renormalization of an electronic continuum is observed with a threshold energy of 61cm-1.

cond-mat.supr-con↗

arXiv2009

Field dependent mass enhancement in Pr_{1-x}La_xOs_4Sb_12 from aspherical Coulomb scattering

The scattering of conduction electrons by crystalline electric field (CEF) excitations may enhance their effective quasiparticle mass similar to scattering from phonons. A wellknown example is Pr metal where the isotropic exchange scattering from inelastic singlet-singlet excitations causes the mass enhancement. An analogous mechanism may be at work in the skutterudite compounds Pr_{1-x}La_xOs_4Sb_12 where close to x=1 the compound develops heavy quasiparticles with a large linear specific heat coefficient. There the low lying CEF states are singlet ground state and a triplet at 8 K. Due to the tetrahedral CEF the main scattering mechanism must be the aspherical Coulomb scattering. We derive the expression for mass enhancement in this model including also the case of dispersive excitations. We show that for small to moderate dispersion there is a strongly field dependent mass enhancement due to the field induced triplet splitting. It is suggested that this effect may be seen in Pr_{1-x}La_xOs_4Sb_12 with suitably large x when the dispersion is small.

cond-mat.str-el↗

arXiv2008

Feedback spin resonance in superconducting CeCu$_2$Si$_2$ and CeCoIn$_5$

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn$_5$ and CeCu$_2$Si$_2$ are magnetic excitons originating from superconducting quasiparticles. The wave vector ${\bf Q}$ of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet $d_{x^2-y^2}$ state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn$_5$. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from ${\bf Q}$ that should be observed experimentally. Our results suggest a unifying nature of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.

cond-mat.supr-con↗

arXiv2007

Calorimetric Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in the Layered Organic Superconductor κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$

The specific heat of the layered organic superconductor $κ$-% (BEDT-TTF)$_2$Cu(NCS)$_2$, where BEDT-TTF is bisethylenedithio-% tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper critical field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.

cond-mat.supr-con↗

arXiv2006

Spectral functions for strongly correlated 5f-electrons

We calculate the spectral functions of model systems describing 5f-compounds adopting Cluster Perturbation Theory. The method allows for an accurate treatment of the short-range correlations. The calculated excitation spectra exhibit coherent 5f bands coexisting with features associated with local intra-atomic transitions. The findings provide a microscopic basis for partial localization. Results are presented for linear chains.

cond-mat.str-el↗

arXiv2006

Ab initio correlation approach to a ferric wheel-like molecular cluster

We present an ab initio study of electronic correlation effects in a molecular cluster derived from the hexanuclear ferric wheel [LiFe6(OCH3)12-(dbm)6]PF6. The electronic and magnetic properties of this cluster have been studied with all-electron Hartree-Fock, full-potential density functional calculations and multi-reference second-order perturbation theory. For different levels of correlation, a detailed study of the impact of the electronic correlation on the exchange parameter was feasible. As the main result, we found that the influence of the bridge oxygen atoms on the exchange parameter is less intense than the influence of the apical ligand groups, which is due to the geometry of the cluster. With respect to the cluster model approach, the experimental value of the exchange parameter was affirmed.

cond-mat.mtrl-sci↗

arXiv2005

Electronic and magnetic properties of a hexanuclear ferric wheel

The electronic and magnetic properties of the hexanuclear ferric wheel [LiFe6(OCH3)12-(dbm)6]PF6 have been studied with all-electron Hartree-Fock and full-potential density functional calculations. The best agreement for the magnetic exchange coupling is at the level of the B3LYP hybrid functional. Surprisingly, the Hartree-Fock approximation gives the wrong sign for the exchange coupling. The local density approximation and the gradient corrected functional PBE strongly overestimate the exchange coupling due to the too large delocalization of the d-orbitals. These findings are supported by results from the Mulliken population analysis for the magnetic moments and the charge on the individual atoms.

cond-mat.other↗