arXiv subjects

Christian Hess

Publications and source records attributed to Christian Hess.

At least 19 recordsLinked to original sources

arXiv2026

Surface-extended Andreev bound states emerging from intrinsic surface superconductivity of PtBi$_2$

Intrinsic topological superconductivity remains one of the central open questions in condensed-matter physics. The three-dimensional Weyl semimetal PtBi$_2$ has recently been shown by angle-resolved photoemission spectroscopy to host a superconducting gap that opens exclusively on its Fermi-arc surface states and exhibits a nodal structure, making it a promising platform for exploring unconventional and potentially topological superconductivity. However, despite these momentum-resolved insights, the real-space spectroscopic signatures of unconventional superconductivity in PtBi$_2$ remain unclear. Here, using scanning tunneling microscopy and spectroscopy, we reveal a sizable and spatially homogeneous gap ($Δ> 10\,\mathrm{meV}$) on atomically pristine terraces of PtBi$_2$. Within this gap, we detect surface-extended in-gap states exhibiting pronounced atomic-scale spatial modulation. Analysis of the tunneling spectra indicates that these states are most consistently interpreted as Andreev bound states within a superconducting gap. Further phenomenological modeling shows that their characteristic spectral features are naturally captured by a chiral pairing scenario. Our results establish a direct real-space connection between the superconducting gap and surface-extended quasiparticle states in PtBi$_2$, and provide new insight into the nature of its unconventional surface superconductivity. These findings highlight PtBi$_2$ as a versatile platform for exploring exotic quasiparticle excitations and potential Majorana physics.

cond-mat.supr-con↗

arXiv2025

Nematic Fluctuations and Electronic Correlations in Heavily Hole-Doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ Probed by Elastoresistance

This work investigates nematic fluctuations and electronic correlations in the hole-doped iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by means of longitudinal and transverse elastoresistance measurements over a wide doping range ($0.63 < x < 0.98$). For this purpose, the orbital character of the electronic response was revealed by decomposition of the elastoresistance into the $A_{1g}$ and $B_{2g}$ symmetry channels. It was shown that at lower doping levels nematic fluctuations in the $B_{2g}$ channel dominate, while for $x > 0.68$ the $A_{1g}$ channel becomes dominant and reaches a pronounced maximum at $x \approx 0.8$ which indicates strong orbital-selective electronic correlations. Despite the dominance of the $A_{1g}$ signal at high doping, a weak contribution in the $B_{2g}$ channel persists, which can be interpreted as a remnant of nematic fluctuations. Model calculations based on a five-orbital tight-binding Hamiltonian with interactions attribute the observed enhancement in the $A_{1g}$ channel to an orbital-selective Kondo-like resonance, predominantly involving the $d_{xy}$ orbital. We discuss our results in relation to the evolution of the Sommerfeld coefficient reported in the literature and a reported change of the superconducting order parameter. All this indicates that for $x > 0.68$ qualitatively new physics emerges. Our findings suggest that electronic correlations in the strongly hole-doped regime play an important role in superconductivity, while the detectable weak nematic fluctuations may also be of relevance.

cond-mat.str-el↗

arXiv2025

Direct probe of magnetic field effects on phonons by ultrasound propagation in a quasi-two-dimensional honeycomb magnet Na$_2$Co$_2$TeO$_6$

We study the phonon behavior of a Co-based honeycomb frustrated magnet Na$_2$Co$_2$TeO$_6$ under magnetic field applied perpendicular to the honeycomb plane. The temperature and field dependence of the sound velocity and sound attenuation unveil prominent spin-lattice coupling in this material, promoting ultrasound as a sensitive probe for magnetic properties. An out-of-plane ferrimagnetic order is determined below the Néel temperature $T_N=27$~K. A comprehensive analysis of our data further supports a triple-Q ground state of Na$_2$Co$_2$TeO$_6$. Furthermore, the ultrasound data were systematically compared to the thermal transport results from literature, to unveil the importance of phononic contribution to the observed transport behaviors.

cond-mat.str-el↗

arXiv2025

A Cryogenic Uniaxial Strain Cell for Elastoresistance Measurements

We present the design, implementation, and validation of a cryo-compatible strain cell for elastoresistance measurements in quantum materials. The cell is actuated by three large-format piezoelectric stacks and enables both compressive and tensile strain up to approximately $\pm5\%$. The relative displacement of the sample holders is measured in situ using a high-resolution capacitive displacement sensor, ensuring precise strain control throughout measurement. To operate the strain cell over a broad temperature range from 4.2\,K to 300\,K, a modular measurement probe was developed, allowing efficient thermal coupling and integration into cryogenic environments. The functionality and precision of the setup were demonstrated by elastoresistance measurements on the iron-based superconductor BaFe$_2$As$_2$ along the [110] direction. The results were validated against conventional techniques based on glued samples and strain gauges, showing excellent agreement in amplitude and temperature dependence of the elastoresistance coefficient. Additional tests revealed that the system remains robust under mechanical vibrations introduced by vacuum pumps, enabling stable operation even with liquid nitrogen cooling. This highlights the high mechanical and thermal reliability of the developed platform for low-temperature transport measurements under controlled uniaxial strain.

cond-mat.supr-con↗

arXiv2025

Temperature dependence of surface superconductivity in t-PtBi$_2$

The Weyl semimetal trigonal PtBi$_2$ has recently been identified as a promising candidate material for intrinsic topological surface superconductivity emerging from the Fermi arc states of the material with a sizeable superconducting gap. We report the temperature evolution of the superconducting excitation spectrum using scanning tunneling spectroscopy in the range of $8-45\,$K. A large low-temperature gap in the order of $Δ\approx 9\,$meV and a closing of the gap around $T_c \approx 45\,$K is observed. Thus, our results confirm the previously indicated high $T_c$-like superconductivity in t-PtBi$_2$.

cond-mat.supr-con↗

arXiv2024

Bulk and surface electron scattering in disordered Bi$_{2}$Te$_{3}$ probed by quasiparticle interference

We investigated the electronic properties of the topological insulator Bi$_{2}$Te$_{3}$ by scanning tunneling microscopy and spectroscopy at low temperature. We obtained high-resolution quasiparticle interference data of the topological surface Dirac electrons at different energies. Spin-selective joint density of states calculations were performed for surface and bulk electronic states to interpret the observed quasiparticle interference data. The topological properties of our crystals are demonstrated by the absence of backscattering along with the linear energy dispersion of the dominant scattering vector. In addition, we detect non-dispersive scattering modes which we associate with bulk-surface scattering and, thus, allow an approximate identification of the bulk energy gap range based on our quasiparticle interference data. Measurements of differential conductance maps in magnetic fields up to 15 T have been carried out, but no strong modifications could be observed.

cond-mat.mes-hall↗

arXiv2024

Surface superconductivity in the topological Weyl semimetal t-PtBi$_2$

The advancement of quantum computation is eager on generating fault tolerant qubits, and topological superconductivity is a very promising concept for reaching this goal. Early experimental achievements study hybrid systems as well as doped intrinsic topological or superconducting materials presenting the phenomena at very low temperatures. However, higher critical temperatures are indispensable for technological exploitation. Promising very recent angle-resolved photoemission spectroscopy results reveal that superconductivity of the type-I Weyl semimetal trigonal PtBi$_2$ (t-PtBi$_2$) is located at the Fermi arcs surface states which renders t-PtBi$_2$ a candidate for intrinsic topological superconductivity. Here we show, using scanning tunnelling microscopy and spectroscopy (STM/STS) that t-PtBi$_2$ presents surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spatially inhomogeneous and spans from 0 to 20 meV. In particular, the large gap value and the shape of the quasiparticle excitation spectrum resemble the phenomenology of high-Tc superconductors. To our knowledge, this is the largest superconducting gap so far measured in a topological material. Moreover, we show that the superconducting state at 5 K persists up to 12 T magnetic field. Thus, we show that t-PtBi2 is a prime candidate for intrinsic topological superconductivity at technologically relevant temperatures, fields and gap magnitudes.

cond-mat.supr-con↗

arXiv2023

Spinon heat transport in the three-dimensional quantum magnet PbCuTe$_2$O$_6$

Quantum spin liquids (QSL) are novel phases of matter which remain quantum disordered even at the lowest temperature. They are characterized by emergent gauge fields and fractionalized quasiparticles. Here we show that the sub-Kelvin thermal transport of the three-dimensional $S=1/2$ hyper-hyperkagome quantum magnet PbCuTe$_2$O$_6$ is governed by a sizeable charge-neutral fermionic contribution which is compatible with the itinerant fractionalized excitations of a spinon Fermi surface. We demonstrate that this hallmark feature of the QSL state is remarkably robust against sample crystallinity, large magnetic field, and field-induced magnetic order, ruling out the imitation of QSL features by extrinsic effects. Our findings thus reveal the characteristic low-energy features of PbCuTe$_2$O$_6$ which qualify this compound as a true QSL material.

cond-mat.str-el↗

arXiv2023

Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na$_2$Co$_2$TeO$_6$

The recent report of a half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ has sparked a strong debate on whether it is generated by Majorana fermion edge currents or whether other more conventional mechanisms involving magnons or phonons are at its origin. A more direct evidence for Majorana fermions which could be expected to arise from a contribution to the longitudinal heat conductivity $κ_{xx}$ at $T\rightarrow0$ is elusive due to a very complex magnetic field dependence of $κ_{xx}$. Here, we report very low temperature (below 1~K) thermal conductivity ($κ$) of another candidate Kitaev material, Na$_2$Co$_2$TeO$_6$. The application of a magnetic field along different principal axes of the crystal reveals a strong directional-dependent magnetic-field ($\bf B$) impact on $κ$. We show that no evidence for mobile quasiparticles except phonons can be concluded at any field from 0~T to the field polarized state. In particular, severely scattered phonon transport is observed across the $B-T$ phase diagram, which is attributed to prominent magnetic fluctuations. Cascades of phase transitions are uncovered for all $\bf B$ directions by probing the strength of magnetic fluctuations via a precise record of $κ$($B$). Our results thus rule out recent proposals for itinerant magnetic excitations in Na$_2$Co$_2$TeO$_6$, and emphasise the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.

cond-mat.str-el↗

arXiv2023

Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$

Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the thermal Hall signal in $α$-RuCl$_3$ rather stems from phonons or topological magnons than from Majorana fermions. Here we investigate the thermal Hall effect of the closely related Kitaev quantum material Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the transversal heat conductivity $κ_{xy}$ carries clear signatures of the phononic $κ_{xx}$, but changes sign upon entering the low-temperature, magnetically ordered phase. We systematically resolve the two components by considering the detailed temperature and field dependence of both $κ_{xy}$ and $κ_{xx}$. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in a Kitaev topological quantum spin liquid requires to disentangle phonon vs. magnetic contributions where the latter include potentially fractionalized excitations such as the expected Majorana fermions.

cond-mat.str-el↗

arXiv2023

Berezinskii-Kosterlitz-Thouless transition in the type-I Weyl semimetal PtBi$_2$

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi$_2$ lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with $T_c \sim 275-400$~mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition with $T_\text{BKT} \sim 310$~mK is revealed in up to 60 nm thick flakes, which is nearly an order of magnitude thicker than the rare examples of two-dimensional superconductors exhibiting such a transition. This makes PtBi$_2$ an ideal platform to study low dimensional and unconventional superconductivity in topological semimetals.

cond-mat.supr-con↗

arXiv2022

Absence of hexagonal to square structural transition in LiFeAs vortex matter

We investigated magnetic vortices in two stoichiometric LiFeAs samples by means of scanning tunneling microscopy and spectroscopy. The vortices were revealed by measuring the local electronic density of states (LDOS) at zero bias conductance of samples in magnetic fields between 0.5 and 12 T. From single vortex spectroscopy we extract the Ginzburg-Landau coherence length of both samples as $4.4\pm0.5$ nm and $4.1\pm0.5$ nm, in accordance with previous findings. However, in contrast to previous reports, our study reveals that the reported hexagonal to square-like vortex lattice transition is absent up to 12 T both in field-cooling and zero-field-cooling processes. Remarkably, a highly ordered zero field cooled hexagonal vortex lattice is observed up to 8 T. We argue that several factors are likely to determine the structure of the vortex lattice in LiFeAs such as (i) details of the cooling procedure (ii) sample stoichiometry that alters the formation of nematic fluctuations, (iii) details of the order parameter and (iv) magnetoelastic coupling.

cond-mat.supr-con↗

arXiv2022

Heat transport of the kagomé Heisenberg quantum spin liquid candidate YCu$_3$(OH)$_{6.5}$Br$_{2.5}$: localized magnetic excitations and spin gap

The spin-1/2 kagomé Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we turn to YCu$_3$(OH)$_{6.5}$Br$_{2.5}$, recently recognized as another host of a globally undistorted kagomé Cu$^{2+}$ lattice free from the orphan spins, thus a more feasible system for studying the intrinsic kagomé quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishing small residual linear term of $κ/T$ ($T\rightarrow 0$), and thus clearly rule out itinerant gapless fermionic excitations. Unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a $\mathbb{Z}_2$ quantum spin liquid ground state. Additionally, the analysis of magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized.

cond-mat.str-el↗

arXiv2022

Elastoresistivity of heavily hole doped 122 iron pnictides superconductors

Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, we conclude none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism how elastoresistivity is enhanced at a Lifshitz transition.

cond-mat.supr-con↗

arXiv2022

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped $BaFe_{2}As_{2}$ and Co-doped $LaFeAsO$. Our data thus provide fresh evidence that in these systems nematic fluctuations foster the superconductivity throughout the phase diagram.

cond-mat.supr-con↗

arXiv2022

Absence of nematic instability in LiFeAs

The relationship between unconventional superconductivity, antiferromagnetism and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K, without undergoing a structural or magnetic transition. However, some recent experimental studies suggested the existence of finite nematic fluctuations and even a nematic superconducting state. In this study, we employ elastoresistance as a measure of nematic fluctuations in pristine LiFeAs and compare the findings with the elastoresistance of LiFeAs at low Co and V doping levels as well with that of magnetically and nematically ordering NaFeAs. We find LiFeAs and cobalt-doped LiFeAs far away from a nematic instability.

cond-mat.supr-con↗

arXiv2021

Thermal transport of the frustrated spin-chain mineral linarite: Magnetic heat transport and strong spin-phonon scattering

The mineral linarite (PbCuSO$_4$(OH)$_2$) forms a monoclinic structure where a sequence of Cu(OH)$_2$ units forms a spin-$\frac{1}{2}$ chain. Competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin structure imply magnetic frustration and lead to magnetic ordering below $T_N =$2.8 K in a mutliferroic elliptical spin-spiral ground state. Upon the application of a magnetic field along the spin-chain direction, distinct magnetically ordered phases can be induced. We studied the thermal conductivity $κ$ in this material across the magnetic phase diagram as well as in the paramagnetic regime in the temperature ranges 0.07-1 K and 9-300 K. We found that in linarite the heat is carried mainly by phonons but shows a peculiar non-monotonic behavior in field. In particular, $κ$ is highly suppressed at the magnetic phase boundaries, indicative of strong scattering of the phonons off critical magnetic fluctuations. Even at temperatures far above the magnetically ordered phases, the phononic thermal conductivity is reduced due to scattering off magnetic fluctuations. The mean free path due to spin-phonon scattering ($l_{\text{spin-phonon}}$) was determined as function of temperature. A power law behavior was observed mainly above 0.5 K indicating the thermal activation of spin fluctuations. In the critical regime close to the saturation field, $l_{\text{spin-phonon}}$ shows a $1/T$ dependence. Furthermore, a magnon thermal transport channel was verified in the helical magnetic phase. We estimate a magnon mean free path which corresponds to about 1000 lattice spacings.

cond-mat.str-el↗

arXiv2021

Quartic metal: Spontaneous breaking of time-reversal symmetry due to four-fermion correlations in Ba$_{1-x}$K$_x$Fe$_2$As$_2$

Discoveries of ordered quantum states of matter are of great fundamental interest, and often lead to unique applications. The most well known example -- superconductivity -- is caused by the formation and condensation of pairs of electrons. A key property of superconductors is diamagnetism: magnetic fields are screened by dissipationless currents. Fundamentally, what distinguishes superconducting states from normal states is a spontaneously broken symmetry corresponding to long-range coherence of fermion pairs. Here we report a set of experimental observations in hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ which are not consistent with conventional superconducting behavior. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, for $x \sim 0.8$, instead of the standard for superconductors, zero resistance and diamagnetic screening, for a range of temperatures, we observe the opposite effect: the generation of self-induced magnetic fields measured by spontaneous Nernst effect and muon spin rotation experiments. The finite resistance and the lack of any detectable diamagnetic screening in this state exclude the spontaneously broken symmetry associated with superconducting two-fermion correlations. Instead, combined evidence from transport and thermodynamic measurements indicates that the formation of fermionic bound states leads to spontaneous breaking of time-reversal symmetry above the superconducting transition temperature. These results demonstrate the existence of a broken-time-reversal-symmetry bosonic metal state. In the framework of a multiband theory, such a state is characterized by quartic correlations: the long-range order exists only for {\it pairs} of fermion pairs.

cond-mat.supr-con↗