arXiv subjects

Bernd Büchner

Publications and source records attributed to Bernd Büchner.

At least 19 recordsLinked to original sources

arXiv2026

Emergent Surface Kondo Flat Band Driven by Competing Interactions in a Topological Ferromagnet

A central goal of modern condensed matter physics is to uncover new quantum states of matter arising from the intertwined effects of strong electron correlations, magnetism, and band topology. Heavy-fermion phases, generated by Kondo interactions, represent one of the most remarkable manifestations of electronic correlations, and topological heavy-fermion states have been identified in several non-magnetic materials. Yet, the consequences of their competition with magnetic order have remained largely unexplored. Here, we reveal a new phenomenon: the spontaneous spatial separation of correlated quantum phases. By showing that magnetism can drive distinct strongly correlated electronic states to coexist in different regions of a single material, our work establishes a previously unknown mechanism for organizing quantum matter and opens a new direction in the study of correlated topological systems. Using \emph{bulk-sensitive} probes, we show that UAsS crystals are, in the bulk, metallic ferromagnets with only moderate correlation-driven band renormalizations. First-principles calculations reveal a topological electronic structure hosting both nodal lines and Weyl points, pointing to a rich underlying topology. Angle-resolved photoemission spectroscopy (ARPES) measurements are consistent with these predictions, resolving the nodal lines and Weyl crossings. In striking contrast, \emph{surface-sensitive} ARPES and scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal a pronounced flat band pinned at the Fermi level, accompanied by a sharp resonance -- hallmarks of an emergent, strongly correlated Kondo state not captured by first-principles calculations.

cond-mat.str-el↗

arXiv2026

Resolving unconventional gap structure in kagome superconductors with hybrid microwave circuits

Unconventional superconductivity is a hallmark of exotic quantum matter, where determining the pairing symmetry is essential for uncovering its microscopic origin. Kagome superconductors provide a fertile landscape for emergent phenomena arising from strong electronic correlations and nontrivial band topology, yet their superconducting pairing symmetry remains elusive. The superconducting gap structure is commonly probed via electrodynamic response, but such measurements are inapplicable to thin flakes due to their small volume and delicate nature. Superconducting microwave resonators offer a coherent and highly sensitive platform for probing electrodynamic responses, with versatile designs that enable incorporation of diverse materials and geometries. Here, we integrate flakes into microwave circuits, enabling noninvasive access to the superfluid response through contactless coupling that preserves structural integrity. By engineering the device geometry to suppress parasitic two-level-system losses that dominate dissipation in microwave circuits, we isolate the intrinsic material response. Remarkably, the temperature-dependent superfluid density exhibits linear behavior at low temperatures, consistent with a nodal gap structure. Our approach establishes a noninvasive platform for probing electrodynamic response of fragile superconducting flakes while advancing hybrid microwave architectures for quantum technologies.

cond-mat.supr-con↗

arXiv2026

Probing bulk superconductivity in centrosymmetric Te-doped PtBi$_2$ single crystals

The Weyl semimetal $γ$-PtBi$_2$ has been shown to be one of the most promising novel materials, recently proposed as a topological i-wave superconductor. A crucial requirement for observing this physics is the absence of inversion symmetry in its trigonal $P31m$ crystal structure. Centrosymmetry has been reported to be readily restored in the $P\overline{3}m1$ structure upon electron doping, partially substituting Bi with as little as 2% Te. In this work, we synthesized Te-doped PtBi$_{2-x}$Te$_{x}$ samples and thoroughly investigated the bulk superconductivity of selected single crystals with nominal composition PtBi$_{1.96}$Te$_{0.04}$, exhibiting the highest superconducting volume fraction of $\sim100$%. Single crystal XRD measurements confirm the centrosymmetric $P\overline{3}m1$ structure in our samples, whereas bulk superconductivity with a critical temperature of $T_\mathrm{c} \approx 2.4\,$K was observed in magnetization and specific heat measurements. The upper critical fields for two different orientations were determined as $H^{\parallel}_{\mathrm{c2}}(0)\approx 6.3\,$kOe for in-plane and $H^{\perp}_{\mathrm{c2}}(0)\approx 4.7\,$kOe for out-of-plane magnetic fields. Robust superconductivity was also found in resistance and point contact measurements, where the latter showed a slight enhancement of the critical temperature up to $T_\mathrm{c} \sim 3\,$K. Finally, ARPES measurements corroborate the centrosymmetric structure of our samples, showing a single surface termination and the absence of Fermi arcs.

cond-mat.supr-con↗

arXiv2026

Power-Law Suppression of Superfluid Stiffness in High-Kinetic-Inductance NbN Films

Disorder is a powerful route to high kinetic inductance in superconducting ultrathin films, enabling compact high-impedance quantum circuits. This functionality, however, comes at the cost of reduced phase rigidity and potentially anomalous electrodynamics. Here, we use NbN microwave resonators with thicknesses down to 2.8 nm and sheet kinetic inductance up to 300 pH per square to probe how this trade-off reshapes the superconducting response. In the thinnest films, transport shows signatures of a Berezinskii-Kosterlitz-Thouless transition, while the microwave response reveals a pronounced low-temperature power-law suppression of the superfluid stiffness, inconsistent with Mattis-Bardeen theory. With increasing thickness, this anomalous regime is progressively suppressed, marking a continuous crossover toward conventional, gap-dominated electrodynamics. Cross-sectional transmission electron microscopy reveals a nanocrystalline twin-domain structure, pointing to oriented microstructural disorder as a crucial factor in the observed response. Overall, the crossover is governed by the ratio of superfluid stiffness to pairing energy, Theta(0)/Tc, identifying this ratio as a parameter governing the boundary between phase-fluctuation-dominated and gap-dominated superconducting electrodynamics in disordered nanofilms.

cond-mat.supr-con↗

arXiv2026

Anomalous field evolution of the mixed-state linewidth in the second superconducting dome of LaFeAsO$_{1-x}M_x$ ($M={\rm F,H}$)

We report a transverse-field muon-spin rotation/relaxation ($μ$SR) study of the internal-field distribution in the mixed state of LaFeAsO$_{0.89}$F$_{0.11}$ and LaFeAsO$_{0.75}$H$_{0.25}$, representative of the first (SC1) and second (SC2) superconducting domes of the LaFeAsO$_{1-x}M_x$ ($M={\rm F,H}$) family, respectively. Below the superconducting transition temperature $T_{\rm c}$, the linewidth of the internal-field distribution increases in both samples, indicating the formation of a vortex lattice. Above $T_{\rm c}$, the linewidth remains field dependent and increases approximately linearly with field, consistent with broadening of the powder spectrum caused by an anisotropic Knight shift. After subtraction of this normal-state contribution, the superconducting linewidth $σ_{\rm sc}$ exhibits qualitatively different field dependences in the two samples. At 4K, the SC1 ($x_{\rm F}=0.11$) sample shows the expected monotonic decrease with increasing field, whereas the SC2 ($x_{\rm H}=0.25$) sample develops a pronounced local maximum near 3T. A contour representation of $σ_{\rm sc}(T,H)$ further reveals a ridge of local maxima whose field position, $H_{σ,\max}(T)$, shifts to lower fields upon warming and disappears near $T_{\rm c}$. The anomalous field evolution observed in the SC2 sample is consistent with an additional field-induced contribution associated with enhanced Pauli-paramagnetic effects, highlighting the distinct electronic character of the two superconducting domes.

cond-mat.supr-con↗

arXiv2026

Rolled-up self-assembly of compact magnetic inductors, transformers and resonators

Three-dimensional self-assembly of lithographically patterned ultrathin films opens a path to manufacture microelectronic architectures with functionalities and integration schemes not accessible by conventional two-dimensional technologies. Among other microelectronic components, inductances, transformers, antennas and resonators often rely on three-dimensional configurations and interactions with electromagnetic fields requiring exponential fabrication efforts when downscaled to the micrometer range. Here, the controlled self-assembly of functional structures is demonstrated. By rolling-up ultrathin films into cylindrically shaped microelectronic devices we realized electromagnetic resonators, inductive and mutually coupled coils. Electrical performance of these devices is improved purely by transformation of a planar into a cylindrical geometry. This is accompanied by an overall downscaling of the device footprint area by more than 50 times. Application of compact self-assembled microstructures has significant impact on electronics, reducing size, fabrication efforts, and offering a wealth of new features in devices by 3D shaping.

physics.app-ph↗

arXiv2026

Disorder-resilient transition of Helical to Conical ground states in M$_{1/3}$NbS$_2$, M=Cr,Mn

The discovery of chiral helical magnetism (CHM) in Cr$_{1/3}$NbS$_2$ and the stabilization of a chiral soliton lattice (CSL) has attracted considerable interest in view of their potential technological applications. However, there is an ongoing debate regarding whether the sister compound, Mn$_{1/3}$NbS$_2$, which shares the same crystal structure, exhibits similar nontrivial properties which rely on the stabilization of the lack of inversion symmetry at the magnetic ion. In this study, we conduct a comprehensive investigation of the magnetically ordered states of both compounds, using $^{53}$Cr, $^{55}$Mn and $^{93}$Nb nuclear magnetic resonance. Our results, supported by density functional calculations, detect in a high-quality single crystal of Cr$_{1/3}$NbS$_2$ all the signatures of the monoaxial CHM in a magnetic field, identifying it as a textbook NMR case. The detailed understanding of this prototypic behavior provides a reference for Mn$_{1/3}$NbS$_2$. Despite the much larger density of specific defects in this second single crystal, we confirm the presence of a CHM phase in the Mn compound, characterized by a very large critical field for the forced ferromagnetic phase ($\approx 5$ T for H$\parallel\hat c$).

cond-mat.mtrl-sci↗

arXiv2026

Electrical and thermal magnetotransport in altermagnetic CrSb

Chromium antimonide has emerged as a key material platform for studying altermagnetism because of its simple binary composition, high Néel temperature, and semimetallic electronic structure. Here, we investigate electrical and thermal magnetotransport in single-crystalline CrSb using steady-and pulsed-magnetic fields up to 65 T, and complement these measurements with neutron diffraction and magnetization data. We confirm the compensated magnetic structure and observe a large nonsaturating magnetoresistance together with a pronounced nonlinear Hall response at low temperatures. Multicarrier modeling, supported by mobility-spectrum analysis, reveals coexisting electron- and hole-like charge carriers with mobilities up to ~3000 cm2/Vs and shows that the number of transport channels that can be resolved strongly depends on the accessible magnetic-field range. Thermal-transport measurements further reveal a nonlinear thermal Hall response and a thermal conductivity substantially exceeding a simple Wiedemann-Franz law. The broadly similar field and temperature evolution of electrical and thermal transport point to a dominant electronic contribution, while the remaining deviations indicate additional heat-carrying channels.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic-field tuning of the spin dynamics in the quasi-2D van der Waals antiferromagnet CuCrP$_{2}$S$_{6}$

The use of antiferromagnets in magnetoelectronic devices as counterparts of ferromagnets is a new, rapidly developing trend in spintronics that leverages antiferromagnetic (AFM) magnons for transmitting of spin currents. Van der Waals (vdW) antiferromagnets are particularly attractive in this respect as they possess tunable magnetic properties and can be easily integrated into spintronic devices. In this work we use electron spin resonance (ESR) spectroscopy to assess the potential of the vdW AFM compound CuCrP$_{2}$S$_{6}$ for magnonic applications by exploring the magnetic field ($H$) dependence of the spectrum of magnon excitations below its AFM ordering temperature $T_{\rm N} \approx 30$ K and the correlated spin dynamics above $T_{\rm N}$. ESR reveals prominent ferromagnetic (FM) spin correlations that persist far above $T_{\rm N}$ suggesting an intrinsically two-dimensional character of the spin dynamics in CuCrP$_{2}$S$_{6}$. Most interestingly, at $T < T_{\rm N}$, CuCrP$_{2}$S$_{6}$ features two non-degenerate, i.e., distinct in energy AFM magnon modes at $H = 0$ which can be tuned to the FM type of collective spin excitations with increasing $H$. These remarkable properties are favorable for the induction and control of unidirectional spin current in CuCrP$_{2}$S$_{6}$ and suggest it as a new functional material for magnetoelectronics.

cond-mat.str-el↗

arXiv2026

Anomalous and Topological Hall Effects in Antiferromagnetic EuSn2As2 Nanostructures

We investigate magnetotransport in exfoliated nanostructures of the candidate magnetic 3D topological insulator $\mathrm{EuSn_{2}As_{2}}$. Similar to macroscopic single crystals, the negative magnetoresistance observed below the Néel temperature ($T_N$ = 24 K) is related to the canted antiferromagnetic state (CAF) of an easy-plane antiferromagnet (AFM), with an increase of the saturation field when tilting the applied magnetic field away from the (ab) plane ($μ_{0} H^{c}_{s}$ = 4.9 T, $μ_{0} H^{ab}_{s}$ = 3.6 T). Higher-accuracy measurements in nanostructures up to 14 T further evidence a non-linear normal Hall response due to several electronic bands. Interestingly, the transverse resistance due to magnetism reveals an anomalous Hall effect in the CAF state, but also a topological Hall effect due to chiral spin textures, as found in AFM or helical magnets. The presence of real-space chiral spin texture, already reported in another magnetic topological insulator, $\mathrm{MnBi_{2}Te_{4}}$, could be a characteristic generally appearing in magnetic 3D topological insulators.

cond-mat.mes-hall↗

arXiv2026

Identifying open-orbit topological surface states in dual topological semimetal TaSb$_2$

TaSb$_2$, a member of the transition metal dipnictide family of materials, hosts the very rare dual topological phase - weak topological insulating state and topological crystalline insulating state along different crystallographic orientations. So far, studies on the electronic structure of transition metal dipnictides have focused on their overall electronic structure and the bulk open-orbit Fermi surfaces. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and transport measurements, we distinguish the intertwined bulk and surface states on the weakly topological $(20\bar{1})$ plane of TaSb$_2$. We identify multiple electron- and hole-like bulk bands, yielding a near-perfect carrier compensation. Crucially, we observe open-orbit FSs parallel to $\bar{L}$-$\bar{Y}$ direction that are entirely of surface origin. Circular-dichroism ARPES reveals $k \rightarrow -k$ spectral reversal, indicating spin-momentum locking and the topological nature of these surface states. Consistent with this, magnetotransport measurements display weak antilocalization, establishing TaSb$_2$ as a platform for spin-polarized topological transport on a weakly topological surface.

cond-mat.mtrl-sci↗

arXiv2026

Transport evidence of surface states in magnetic topological insulator MnBi2Te4

Magnetic topological insulators can host chiral 1D edge channels at zero magnetic field, when a magnetic gap opens at the Dirac point in the band structure of 2D topological surface states, leading to the quantum anomalous Hall effect in ultra-thin nanostructures. For thicker nanostructures, quantization is severely reduced by the co-existence of edge states with other quasi-particles, usually considered as bulk states. Yet, surface states also exist above the magnetic gap, but it remains difficult to identify electronic subbands by electrical measurements due to strong disorder. Here we unveil surface states in MnBi2Te4 nanostructures, using magneto-transport in very-high magnetic fields up to 55 T, giving evidence of Shubnikov-de-Haas oscillations above 40 T. A detailed analysis confirms the 2D nature of these quantum oscillations, thus establishing an alternative method to photoemission spectroscopy for the study of topological surface states in magnetic topological insulators, using Landau level spectroscopy.

cond-mat.mes-hall↗

arXiv2025

Giant universal conductance fluctuations in the antiferromagnetic topological insulator MnBi2Te4

Intrinsic magnetic topological insulators can host quantum states with quantized magneto-electric responses, such as the axion and Chern insulators states evidenced in ultra-thin MnBi2Te4 films. Yet, whereas quantization is investigated thoroughly, transport properties related to the phase of charge carriers remains unexplored. Here, we study quantum coherent transport in mesoscopic Hall bars fabricated from thick exfoliated MnBi2Te4 flakes, and reveal the longest phase-coherence length ever observed in a mesoscopic magnet (about 500nm at 1K), associated to 2D topological surface states. In the fully-coherent regime, significant non-local contributions to quantum interference up to the micron scale lead to giant-amplitude universal conductance fluctuations (about 20e2/h). In the self-averaging regime, the statistical properties of conductance fluctuations confirm the 2D nature of quantum interference and different dephasing mechanisms are identified, as due to either magnetism or magnetic flux through coherent loops. Remarkably, the weak decoherence in magnetic topological insulator nanostructures show their potential to realize novel quantum spin interferometers based on dephasing by local magnetic textures at liquid-helium temperatures.

cond-mat.mes-hall↗

arXiv2025

Non-Hermitian topological devices with Chern insulators

Multi-terminal topological devices are a new generation of electronic devices with quantized properties robust against imperfections. In magnetic topological insulators, dissipationless edge states give functional devices in zero magnetic field, of interest for quantum metrology (resistance standard) or topological electronics (Chern networks). Here we show that a new generation of simple quantum circuits (disk, ring) with non-Hermitian topology, based on the interconnection of 1D Chern states in the quantum anomalous Hall regime, can have a much stronger quantization of their invariant than that of the Chern invariant itself, when measured in a non-metrology grade setup - that is, in industry-relevant conditions. Remarkably, the chirality-related topological skin effect is realized without the need of a magnetic field or an electrical gate, with a record degree of localization for a quantum Hall device. This new type of topological quantum devices based on magnets, with an exponential response that can be switched at small magnetic fields (about 200mT), can operate at liquid-Helium temperature with a good quantization and have some potential as cryogenic sensors for applications in high-precision impedance or magnetic field measurements.

cond-mat.mes-hall↗

arXiv2025

Hysteretic Phonons and Quasielastic Response: A Raman Study of Thermal Memory in Two-dimensional CuCrP2S6

We present a comprehensive temperature and polarization dependent inelastic light scattering (Raman) study on single crystals of two-dimensional CuCrP2S6, a layered van der Waals material exhibiting coupled magnetic and electric degrees of freedom. Raman measurements were performed from 5 to 300 K to probe phonon dynamics across multiple structural and magnetic phase transitions. Our analysis reveals pronounced thermal hysteresis in phonon self-energy parameters and dynamic Raman susceptibility, confirming the first-order nature of the antipolar transition near TC1 ~ 145 K and a second-order transition near TC2 ~ 190 K. Low-frequency modes associated with Cu+ and Cr3+ ions exhibit softening and anomalous linewidth behaviour, in particular phonon mode P2 (~ 37 cm-1) showing non-monotonic temperature dependence and intensity enhancement near 60 K suggesting persistent off-centre Cu+ dynamics in the quasi-antipolar phase. The coexistence and coupling of soft phonon modes and central peaks indicate a crossover from displacive to order-disorder type transition mechanisms. Additionally, phonon anomalies below the Néel temperature (TN ~ 32 K) reflect spin-phonon coupling, linking lattice vibrations to long-range magnetic correlations. Our findings provide critical insight into the lattice instabilities, symmetry evolution, and quasiparticle interactions in CuCrP2S6, offering a deeper understanding of phase transition dynamics in two-dimensional multiferroic systems and guiding future design of magnetoelectric and spintronic devices.

cond-mat.str-el↗

arXiv2025

Physical properties of new delafossite triangular-lattice compounds TlErSe$_2$ and TlTmSe$_2$

Delafossite compounds containing rare-earth ions have been proven to be an ideal platform to investigate frustrated magnetic ground states. Here, we discuss two triangular-lattice antiferromagnets, TlErSe$_2$ and TlTmSe$_2$, as potential candidates for hosting exotic quantum states. Powder X-ray diffraction data analysis of the black-color polycrystalline Tl$RE$Se$_2$ ($RE$: Er and Tm) samples confirms the phase purity. Both materials crystallize in the trigonal $α$-NaFeO$_2$ structure ($R\overline{3}m$) with lattice parameters $a$ = 4.1070(4) Å and $c$ = 23.1472(1) Å for the erbium compound and $a$ = 4.0916(1) Å and $c$ = 23.1483(2) Å for the thulium compound. Magnetic susceptibility measurements show an effective moment of $μ_{\text{eff}} = 9.6(2) μ_B$/f.u. ($7.5(1) μ_B$/f.u.) for TlErSe$_2$ (TlTmSe$_2$) for temperatures above 200 K. While $^3$He specific-heat measurements reveal long-range magnetic order below $T_N = 0.42 $K for TlErSe$_2$, no sign of long-range magnetic order was observed for TlTmSe$_2$. Based on our results, we map out the T-H phase diagram for polycrystalline TlErSe$_2$ and discuss the striking difference in the magnetic behavior of TlTmSe$_2$ based on our ab initio quantum chemical calculations.

cond-mat.str-el↗

arXiv2025

Disentangling the unusual magnetic anisotropy of the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$

In the quest for two-dimensional conducting materials with high ferromagnetic ordering temperature the new family of the layered Fe$_{n}$GeTe$_{2}$ compounds, especially the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$, receives a significant attention. Fe$_{4}$GeTe$_{2}$ features a peculiar spin reorientation transition at $T_\mathrm{SR} \sim 110$ K suggesting a non-trivial temperature evolution of the magnetic anisotropy (MA) - one of the main contributors to the stabilization of the magnetic order in the low-D systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe$_{4}$GeTe$_{2}$. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy-axis type, whose growth below a characteristic temperature $T_{\rm shape} \sim 150$ K renders the sample seemingly isotropic at $T_\mathrm{SR}$. Below one further temperature $T_{\rm d} \sim 50$ K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe$_{4}$GeTe$_{2}$. This finding together with the observed signatures of the intrinsic two-dimensionality should facilitate optimization routes for the use of Fe$_{4}$GeTe$_{2}$ in the magneto-electronic devices, potentially even in the monolayer limit.

cond-mat.str-el↗

arXiv2025

Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Trigonal-PtBi2 has recently garnered significant interest as it exhibits unique superconducting topological surface states due to electron pairing on Fermi arcs connecting bulk Weyl nodes. Furthermore, topological nodal lines have been predicted in trigonal-PtBi2, and their signature was measured in magnetotransport as a dissipationless, i.e. odd under a magnetic field reversal, anomalous planar Hall effect. Understanding the topological superconducting surface state in trigonal-PtBi2 requires unravelling the intrinsic geometric properties of the normal state electronic wavefunctions and further studies of their hallmarks in charge transport characteristics are needed. In this work, we reveal the presence of a strong dissipative, i.e. even under a magnetic field reversal, planar Hall effect in PtBi2 at low magnetic fields and up to room temperature. This robust response can be attributed to the presence of Weyl nodes close to the Fermi energy. While this effect generally follows the theoretical prediction for a planar Hall effect in a Weyl semimetal, we show that it deviates from theoretical expectations at both low fields and high temperatures. We also discuss the origin of the PHE in our material, and the contributions of both the topological features in PtBi2 and its possible trivial origin. Our results strengthen the topological nature of PtBi2 and the strong influence of quantum geometric effects on the electronic transport properties of the low energy normal state.

cond-mat.mes-hall↗