arXiv subjects

D. Graf

Publications and source records attributed to D. Graf.

At least 19 recordsLinked to original sources

arXiv2026

Anisotropic metamagnetism and magnetotransport of heavy rare-earth orthorhombic single-crystal TbAlGe

We report a comprehensive investigation of the anisotropic magnetism and magnetic field-induced transitions in single crystals of the orthorhombic system TbAlGe, a member of the topological RAlGe (R = rare-earth) family with the highest ordering temeprature in the RAlX (X = Si, Ge) series. With a single rare earth site with triangular coordination in its Cmcm orthorhombic unit cell, TbAlGe harbors complex magnetic interactions that yield two antiferromagnetic transitions at 40 K and 8 K in zero field, and a rich cascade of metamagnetic transitions that only appear for fields directed along the crystallographic a-axis. Combining electrical resistivity, magnetization and heat capacity measurements with magnetotransport experiments performed up to 41.5 T, we construct a magnetic phase diagram mapping the multiple magnetic phases of TbAlGe, and discuss the complex interplay between localized 4f magnetism and itinerant electronic topology, establishing TbAlGe as a compelling platform for exploring tunable magnetic semimetal physics.

cond-mat.str-el↗

arXiv2025

Heterometallic spin-1/2 quantum magnet under hydrostatic pressure

We investigate the properties of CuVOF$_4$(H$_2$O)$_6$$\cdot$H$_2$O, in which two different spin species, Cu(II) and V(IV), form antiferromagnetic spin-1/2 dimers with weak interdimer coupling provided via hydrogen bonding. Using radio-frequency susceptometry and electron-spin resonance (ESR), we show how the temperature-magnetic field spin-dimer phase diagram evolves as a function of applied hydrostatic pressure and correlate this with pressure-induced changes to the crystal structure. These results, coupled with pressure-tuned DFT calculations, confirm the prior prediction that the primary exchange interaction is mediated via an unusual mechanism in which the V(IV) ions provide considerable spin density to the oxygen that joins the two spins in each dimer and which lies along the Jahn-Teller axis of the Cu(II) ion. In addition, the dissimilarity in the spins that make up each dimer unit leads to a non-linear field dependence of the electronic energy levels as detected in the ESR measurements.

cond-mat.str-el↗

arXiv2025

Quantum critical origin of strange-metals at the end of a pseudogap phase in infinite-layer nickelates

The quantum-critical origin of strange metals remains debated, particularly in cuprates where $T$-linear resistivity emerges at the end of the pseudogap phase, a regime without long-range order whose nature remains one of the largest mysteries of quantum materials~\cite{Michon2019Thermodynamic, zhong_2022}. Superconducting infinite-layer nickelates provide a new platform to revisit this issue, given their close similarities to cuprates. Here too, $T$-linear resistivity onsets at a critical doping $x^\star$ near the middle of the superconducting dome. Establishing whether $x^\star$ is a quantum critical point (QCP) -- a zero-temperature phase transition -- typically relies on the electronic specific heat $C_{\rm el}$, which follows $C_{\rm el}/T \propto \log(T)$ at a QCP, rather than the constant behaviour of a conventional metal. However, the thin-film form of infinite-layer nickelates precludes calorimetry. We therefore use the Seebeck coefficient as a low-temperature proxy for specific heat per carrier. In La$_{\rm 1-x}$Sr$_{\rm x}$NiO$_2$ at $x^\star$, the high-temperature Seebeck response is quantitatively captured by the ARPES-measured band structure, indicating well-defined quasiparticles. Below 60 kelvin, however, $S/T$ develops a logarithmic divergence, $S/T \propto \log(T)$, persisting to the lowest temperature once superconductivity is suppressed by $B=41.5$~T. This identifies $x^\star$ as a QCP terminating the underdoped phase. Finally, we find that the carrier density $n_{\rm d}$ of the Ni-$d_{\rm x^2-y^2}$ pocket drops from $1+x$ above $x^\star$ to $x$ below, mirroring the hallmark of the pseudogap phase in cuprates and iridates. These results point to a pseudogap-like underdoped regime ending at $x^\star$, from which strange-metal behaviour emerges.

cond-mat.str-el↗

arXiv2025

Electronic dimensionality of UTe2

Superconductivity in the heavy-fermion metal UTe2 survives the application of very high magnetic fields, presenting both an intriguing puzzle and an experimental challenge. The strong, non-perturbative influence of the magnetic field complicates the determination of superconducting order parameters in the high-field phases. Here, we report electronic transport anisotropy measurements in precisely aligned microbars in magnetic fields to 45 T applied along the b-axis. Our results reveal a highly directional vortex pinning force in the field-reinforced phase. The critical current is significantly suppressed for currents along the c direction, whereas the flux-flow voltage is reduced with slight angular misalignments--hallmarks of vortex lock-in transitions typically seen in quasi-2D superconductors like cuprates and pnictides. These findings challenge the assumption of nearly isotropic charge transport in UTe2 and point to enhanced two-dimensionality in the high-field state, consistent with a change in the order parameter. A pair-density-wave-like state at high fields could naturally induce a layered modulation of the superfluid density, forming planar structures that confine vortices and guide their sliding in the flux-flow regime.

cond-mat.supr-con↗

arXiv2025

Application of Correlated-Wavefunction and Density-Functional Theories to Endofullerenes: A Cautionary Tale

A recent study by Panchagnula et al. [J. Chem. Phys. 161, 054308 (2024)] illustrated the non-concordance of a variety of electronic structure methods at describing the symmetric double-well potential expected along the anisotropic direction of the endofullerene Ne@C$_{70}$. In this article we present new correlated-wavefunction data from coupled cluster theory for this system, and scrutinise a variety of state-of-the-art density-functional approximations (DFAs) and dispersion corrections (DCs). We identify rigorous criteria for the double-well potential and compare the shapes, barrier heights, and minima positions obtained with the DFAs and DCs to the correlated wavefunction data. We show that many of the DFAs are extremely sensitive to the numerical integration grid used, the dispersion damping function, and the extent of exact-exchange mixing. We pose the Ne@C$_{70}$ system as a challenge to functional developers and as a diagnostic system for testing dispersion corrections, and reiterate the need for more experimental data for comparison.

physics.chem-ph↗

arXiv2024

Unveiling the quasiparticle behaviour in the pressure-induced high-$T_c$ phase of an iron-chalcogenide superconductor

Superconductivity of iron chalocogenides is strongly enhanced under applied pressure yet its underlying pairing mechanism remains elusive. Here, we present a quantum oscillations study up to 45 T in the high-$T_c$ phase of tetragonal FeSe$_{0.82}$S$_{0.18}$ up to 22 kbar. Under applied pressure, the quasi-two dimensional multiband Fermi surface expands and the effective masses remain large, whereas the superconductivity displays a three-fold enhancement. Comparing with chemical pressure tuning of FeSe$_{1-x}$S$_x$, the Fermi surface enlarges in a similar manner but the effective masses and $T_c$ are suppressed. These differences may be attributed to the changes in the density of states influenced by the chalcogen height, which could promote stronger spin fluctuations pairing under pressure. Furthermore, our study also reveals unusual scattering and broadening of superconducting transitions in the high-pressure phase, indicating the presence of a complex pairing mechanism.

cond-mat.supr-con↗

arXiv2024

Targeting spectroscopic accuracy for dispersion bound systems from ab initio techniques: translational eigenstates of Ne@C$_{70}$

We investigate the endofullerene system Ne@C$_{70}$, by constructing a three-dimensional Potential Energy Surface (PES) describing the translational motion of the Ne atom. We compare a plethora of electronic structure methods including: MP2, SCS-MP2, SOS-MP2, RPA@PBE, C(HF)-RPA, which were previously used for He@C$_{60}$ in J. Chem. Phys. 160, 104303 (2024), alongside B86bPBE-25X-XDM and B86bPBE-50X-XDM. The reduction in symmetry moving from C$_{60}$ to C$_{70}$ introduces a double well potential along the anisotropic direction, which forms a test of the sensitivity and effectiveness of the methods. Due to the large cost of these calculations, the PES is interpolated using Gaussian Process Regression due to its effectiveness with sparse training data. The nuclear Hamiltonian is diagonalised using a symmetrised double minimum basis set outlined in J. Chem. Phys. 159, 164308 (2023), with translational energies having error bars $\pm 1$ and $\pm 2$ cm$^{-1}$. We quantify the shape of the ground state wavefunction by considering its prolateness and kurtosis, and compare the eigenfunctions between electronic structure methods from their Hellinger distance. We find no consistency between electronic structure methods as they find a range of barrier heights and minima positions of the double well, and different translational eigenspectra which also differ from the Lennard-Jones (LJ) PES given in J. Chem. Phys. 101, 2126,2140 (1994). We find that generating effective LJ parameters for each electronic structure method cannot reproduce the full PES, nor recreate the eigenstates and this suggests that the LJ form of the PES, while simple, may not be best suited to describe these systems. Even though MP2 and RPA@PBE performed best for He@C$_{60}$, due to the lack of concordance between all electronic structure methods we require more experimental data in order to properly validate the choice.

physics.chem-ph↗

arXiv2024

Translational eigenstates of He@C$_{60}$ from four-dimensional \textit{ab initio} Potential Energy Surfaces interpolated using Gaussian Process Regression

We investigate the endofullerene system $^3$He@C$_{60}$ with a four-dimensional Potential Energy Surface (PES) to include the three He translational degrees of freedom and C$_{60}$ cage radius. We compare MP2, SCS-MP2, SOS-MP2, RPA@PBE and C(HF)-RPA to calibrate and gain confidence in the choice of electronic structure method. Due to the high cost of these calculations, the PES is interpolated using Gaussian Process Regression (GPR), owing to its effectiveness with sparse training data. The PES is split into a two-dimensional radial surface, to which corrections are applied to achieve an overall four-dimensional surface. The nuclear Hamiltonian is diagonalised to generate the in-cage translational/vibrational eigenstates. The degeneracy of the three-dimensional harmonic oscillator energies with principal quantum number $n$ is lifted due to the anharmonicity in the radial potential. The $(2l+1)$-fold degeneracy of the angular momentum states is also weakly lifted, due to the angular dependence in the potential. We calculate the fundamental frequency to range between 96cm$^{-1}$ and 110cm$^{-1}$ depending on the electronic structure method used. Error bars of the eigenstate energies were calculated from the GPR and are on the order of approximately $\pm$ 1.5cm$^{-1}$. Wavefunctions are also compared by considering their overlap and Hellinger distance to the one-dimensional empirical potential. As with the energies, the two \textit{ab initio} methods MP2 and RPA@PBE show the best agreement. While MP2 has better agreement than RPA@PBE, due to its higher computational efficiency and comparable performance, we recommend RPA as an alternative electronic structure method of choice to MP2 for these systems.

physics.chem-ph↗

arXiv2023

Asymmetric phase diagram and dimensional crossover in a system of spin-1/2 dimers under applied hydrostatic pressure

We present the magnetic and structural properties of [Cu(pyrazine)$_{0.5}$(glycine)]ClO$_4$ under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other $S = 1/2$ dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero-field at a critical pressure of $p_{\rm c} = 15.7(5)$ kbar. Pressure-dependent single-crystal X-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram.

cond-mat.str-el↗

arXiv2023

Fermi surface of magnetic kagome compound GdV6Sn6 investigated using de Haas van Alphen Oscillations

The shape of the Fermi surface, and the cyclotron effective mass of the kagome magnet GdV6Sn6 charge carriers are investigated using de Haas van Alphen (dHvA) oscillations measurements and electronic band structure calculations. The temperature and angle-dependent torque magnetometry measurements revealed at least nine different frequencies ranging from ~10 T up to ~9000 T. These frequencies correspond to extremal areas of the Fermi surface ranging from ~0.2 % up to 50% of the first Brillouin zone, qualitatively consistent with the electronic band structure calculations. The angle dependent dHvA oscillation frequencies indicate that the smaller pockets of the Fermi surface have almost 3D character whereas the bigger pockets of the Fermi surface are mostly two-dimensional. We also find evidence of the presence of light (0.28(1) m0) as well as heavy (2.37(18) m0) charge carriers through the analysis of the temperature dependence of dominant frequencies. The comparison of the observed frequencies with the electronic band structure calculations indicates that the heavy masses correspond to saddle-point-like features of electronic band structure at the M point. The observation of the multiple low frequencies and the calculated contributions from various bands to such low frequencies prevent the estimation of topological nature of bands containing lighter fermions. In conclusion, our work reveals the features of a Fermi surface containing enhanced mass fermions originated from saddle points in the electronic band structure at the M point, which is inherent to kagome lattices.

cond-mat.mtrl-sci↗

arXiv2023

Multi-k magnetic structure and large anomalous Hall effect in candidate magnetic Weyl semimetal NdAlGe

The magnetic structure, magnetoresistance, and Hall effect of non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this non-centrosymmetric compound. The magnetization and magnetoresistance measurements are both highly anisotropic and indicate an Ising-like magnetic system. The magnetic structure is complex in that it involves three magnetic ordering vectors including an incommensurate spin density wave and commensurate ferrimagnetic state in zero field. We have discovered a large anomalous Hall conductivity that reaches = 430 {\Omega}-1cm-1 implying that it originates from an intrinsic Berry curvature effect stemming from Weyl nodes found in the electronic structure. These electronic structure calculations indicate the presence of nested Fermi surface pockets with nesting wave vectors similar to the measured magnetic ordering wavevector and the presence of Weyl nodes in proximity to the Fermi surface. We associate the incommensurate magnetic structure with the large anomalous Hall response to be the result of the combination of Fermi surface nesting and the Berry curvature associated with Weyl nodes.

cond-mat.mtrl-sci↗

arXiv2022

Correlated states of 2D electrons near the Landau level filling $\nu=1/7$

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors ($\nu\lesssim1/6$) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultra-high-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density $n=6.1\times10^{10}$ /cm$^2$ and mobility $\mu=25\times10^6$ cm$^2$/Vs, we find a deep minimum in the longitudinal magnetoresistance ($R_{xx}$) at $\nu=1/7$ when $T\simeq104$ mK. There is also a clear sign of a developing minimum in the $R_{xx}$ at $\nu=2/13$. While insulating phases are still predominant when $\nu\lesssim1/6$, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence $\nu=p/(2mp\pm1)$ even in the very low Landau level filling limit. The magnetic field dependent activation energies deduced from the relation $R_{xx}\propto e^{E_A/2kT}$ corroborate this view, and imply the presence of pinned Wigner solid states when $\nu\neq p/(2mp\pm1)$. Similar results are seen in another sample with a lower density, further generalizing our observations.

cond-mat.mes-hall↗

arXiv2021

Anomalous magnetic exchange in a dimerized quantum-magnet composed of unlike spin species

We present here a study of the magnetic properties of the antiferromagnetic dimer material CuVOF$_4$(H$_2$O)$_6\cdot$H$_2$O, in which the dimer unit is composed of two different $S = 1/2$ species, Cu(II) and V(IV). An applied magnetic field of $\mu_0H_{\rm c1} = 13.1(1)~\rm T$ is found to close the singlet-triplet energy gap, the magnitude of which is governed by the antiferromagnetic intradimer, $J_0 \approx 21~\rm K$, and interdimer, $J' \approx 1~\rm K$, exchange energies, determined from magnetometry and electron-spin resonance measurements. The results of density functional theory (DFT) calculations are consistent with the experimental results and predicts antiferromagnetic coupling along all nearest-neighbor bonds, with the magnetic ground state comprising spins of different species aligning antiparallel to one another, while spins of the same species are aligned parallel. The magnetism in this system cannot be accurately described by the overlap between localized V orbitals and magnetic Cu orbitals lying in the Jahn-Teller (JT) plane, with a tight-binding model based on such a set of orbitals incorrectly predicting that interdimer exchange should be dominant. DFT calculations indicate significant spin density on the bridging oxide, suggesting instead an unusual mechanism in which intradimer exchange is mediated through the O atom on the Cu(II) JT axis.

cond-mat.str-el↗

arXiv2020

Effect of pressure on the pseudogap and charge-density-wave phases of the cuprate La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ probed by thermopower measurements

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO), at low-temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to $H = 31$ T. Using a newly developed AC thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient $S$. At ambient pressure and low temperature, $S/T$ was recently found to suddenly increase in Nd-LSCO at the pseudogap critical doping $p^{\star} = 0.23$, consistent with a drop in carrier density $n$ from $n = 1 + p$ above $p^{\star}$ to $n = p$ below. Under a pressure of 2.0 GPa, we observe that this jump in $S/T$ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect which found $dp^{\star}/dP \simeq - 0.01$ holes/GPa, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ in the charge density-wave phase near $p \sim 1/8$, which reveals a weakening of this phase under pressure.

cond-mat.supr-con↗

arXiv2020

High Fermi velocities and small cyclotron masses in LaAlGe

We report quantum oscillation measurements of LaAlGe, a Lorentz-violating type-II Weyl semimetal with tilted Weyl cones. Very small quasiparticle masses and very high Fermi velocities were detected at the Fermi surface. Whereas three main frequencies have been observed, angular dependence of two Fermi surface sheets indicates possible two-dimensional (2D) character despite the absence of the 2D structural features such as van der Waals bonds. Such conducting states may offer a good platform for low-dimensional polarized spin current in magnetic RAlGe (R = Ce, Pr) materials.

cond-mat.mes-hall↗

arXiv2020

Linear-in temperature resistivity from an isotropic Planckian scattering rate

A variety of "strange metals" exhibit resistivity that decreases linearly with temperature as $T\rightarrow 0$, in contrast with conventional metals where resistivity decreases as $T^2$. This $T$-linear resistivity has been attributed to charge carriers scattering at a rate given by $\hbar/\tau=\alpha k_{\rm B} T$, where $\alpha$ is a constant of order unity. This simple relationship between the scattering rate and temperature is observed across a wide variety of materials, suggesting a fundamental upper limit on scattering---the "Planckian limit"---but little is known about the underlying origins of this limit. Here we report a measurement of the angle-dependent magnetoresistance (ADMR) of Nd-LSCO---a hole-doped cuprate that displays $T$-linear resistivity down to the lowest measured temperatures. The ADMR unveils a well-defined Fermi surface that agrees quantitatively with angle-resolved photoemission spectroscopy (ARPES) measurements and reveals a $T$-linear scattering rate that saturates the Planckian limit, namely $\alpha = 1.2 \pm 0.4$. Remarkably, we find that this Planckian scattering rate is isotropic, i.e. it is independent of direction, in contrast with expectations from "hot-spot" models. Our findings suggest that $T$-linear resistivity in strange metals emerges from a momentum-independent inelastic scattering rate that reaches the Planckian limit.

cond-mat.str-el↗

arXiv2020

Topologically-Driven Linear Magnetoresistance in Helimagnetic FeP

The helimagnet FeP is part of a family of binary pnictide materials with the MnP-type structure which share a nonsymmorphic crystal symmetry that preserves generic band structure characteristics through changes in elemental composition. It shows many similarities, including in its magnetic order, to isostructural CrAs and MnP, two compounds that are driven to superconductivity under applied pressure. Here we present a series of high magnetic field experiments on high quality single crystals of FeP, showing that the resistance not only increases without saturation by up to several hundred times its zero field value by 35 T, but that it also exhibits an anomalously linear field dependence over the entire field range when the field is aligned precisely along the crystallographic c-axis. A close comparison of quantum oscillation frequencies to electronic structure calculations links this orientation to a semi-Dirac point in the band structure which disperses linearly in a single direction in the plane perpendicular to field, a symmetry-protected feature of this entire material family. We show that the two striking features of MR-large amplitude and linear field dependence-arise separately in this system, with the latter likely due to a combination of ordered magnetism and topological band structure.

cond-mat.str-el↗

arXiv2020

Three-dimensional Fermi surface and small effective masses in Mo8Ga41

We report Fermi surface characteristics of Mo8Ga41, a two-gap superconductor with critical temperature Tc = 10 K, obtained from quantum oscillation measurements. Four major frequencies have been observed with relatively small quasiparticle masses. Angular dependence of major frequencies indicates three-dimensional Fermi surface sheets. This argues for a relatively isotropic superconducting state and, given its relatively high Tc, shows that a search for materials in this class could be of interest for superconducting wire applications.

cond-mat.supr-con↗