arXiv subjects

Rolf Lortz

Publications and source records attributed to Rolf Lortz.

At least 19 recordsLinked to original sources

arXiv2025

Point-contact Andreev reflection spectroscopy of layered superconductors with device-integrated diamond anvil cells

Superconductors that can be mechanically exfoliated are an interesting platform for exploring superconducting properties tuned by layer thickness. These layered superconductors are also expected to exhibit sensitivity to applied pressure. While pressure has been demonstrated to be an effective way of tuning bulk superconductors, analogous studies on superconducting thin flakes have been limited due to technical challenges. In particular, spectroscopic measurements under pressure remain insufficiently explored. In this work, we functionalized the diamond anvil cell technique for point-contact Andreev reflection spectroscopy (PCAR) measurement on thin-flake materials under pressure, offering the opportunity to obtain spectroscopic information on superconductivity. To validate the feasibility of this method, we have conducted PCAR measurements on iron-selenide thin flakes to extract temperature-dependent superconducting gap values under ambient and high pressure. Combine with the proven magnetotransport capability, our method provides a conceptually simple tool for a detailed examination of thin-flake superconductors under pressure.

cond-mat.supr-con↗

arXiv2025

Room-temperature superconductivity in ultra-thin carbon nanotube zeolite composites: a conventional or unconventional superconductor?

The recent report of signs of room-temperature superconductivity in ultrathin single-walled carbon nanotubes (CNT) of types (2,1) and (3,0) holds significant promise for energy applications due to their ability to conduct current without dissipation. However, the McMillan Tc formula fails to calculate their superconducting transition temperatures (Tc) accurately, which raises an important question: what is the pairing mechanism driving their room-temperature superconductivity? To explore this further, we first investigate whether the strong curvature of ultrathin CNT leads to exotic phenomena in unconventional superconductors. If no evidence of these exotic characteristics is found and the McMillan formalism indicates that it is not a BCS-type superconductor, could we be observing a new class of unconventional superconductivity that functions independently of phonons and typical exotic features? In this paper, we demonstrate that factors such as the chiral angle of CNT, boron dopants and lattice regularity can be used to tune the theoretical Tc to experimental values. Our finding suggests that combining CNT with a harder substrate could be vital for further enhancing Tc while minimizing lattice distortion under doping. We propose a reconsideration of the common belief regarding whether the McMillan and BCS Tc formulas are adequate for classifying materials as BCS or non-BCS superconductors.

cond-mat.supr-con↗

arXiv2025

Room temperature observation of the anomalous in-plane Hall effect in epitaxial thin films of a Weyl ferromagnet

Topologically nontrivial electronic states can give rise to novel anomalous Hall effects. The potential appearance of these effects at room temperature holds promise for their application in magnetic sensing, spintronics, and energy harvesting technology. The anomalous in-plane Hall effect (IPHE) is predicted to arise in topological magnetic materials when an external magnetic field is applied within the sample plane. Because of stringent symmetry requirements, the conclusive detection of the anomalous IPHE induced by topological electronic states remains challenging, and the study of anomalous Hall effects is often confined to cryogenic conditions. Combining molecular beam epitaxy of the kagome metal Fe$_3$Sn with measurements of the electric Hall effect and theoretical calculations, we propose and experimentally demonstrate that the interplay of the kagome lattice motif with spin-orbit coupling and canted ferromagnetism with large exchange interactions gives rise to the anomalous IPHE at room temperature that is induced by topological Weyl points in the electronic band structure. Synthesizing a topological heterostructure including layers of Fe$_3$Sn and ferromagnetic CoFeB, we further show the enhancement of the anomalous IPHE through the magnetic stray field of the CoFeB layer. Our work establishes a design paradigm for topological magnets and heterostructures to discover and control novel anomalous Hall effects toward their use in technological applications.

cond-mat.mes-hall↗

arXiv2024

Evidence for the novel type of orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual superconducting state, defies high magnetic fields beyond the Pauli paramagnetic limit. It exhibits a spatial modulation of the superconducting order parameter in real space and is exceptionally rare. Recently, an even more exotic variant - the orbital FFLO state - was predicted and identified in the transition metal dichalcogenide superconductor 2H-NbSe2. This state emerges in thin samples with thicknesses below ~40 nm, at the boundary between two and three dimensions. The complex interplay between Ising spin orbit coupling and the Pauli paramagnetic effect can lead to a stabilization of the FFLO state in a relatively large range of the magnetic phase diagram, even well below the Pauli limit. In this study, we present experimental evidence of the formation of this orbital FFLO state in bulk 2H-NbSe2 samples. This evidence was obtained using high-resolution DC magnetization and magnetic torque experiments in magnetic fields applied strictly parallel to the NbSe2 basal plane. Both quantities display a crossover to a discontinuous first-order superconducting transition at the normal state boundary in magnetic fields of 4 T and above. This is usually seen as a sign that Pauli paramagnetic pair breaking effects affect the superconducting state. The magnetic torque reveals a small step-like reversible anomaly, indicating a magnetic field-induced thermodynamic phase transition within the superconducting state. This anomaly bears many similarities to the FFLO transitions in other FFLO superconductors, suggesting the potential existence of an orbital FFLO state in bulk 2H-NbSe2 samples. Additionally, we observe a pronounced in-plane 6-fold symmetry of the upper critical field in the field range above this phase transition, which has previously been interpreted as a hallmark of the orbital FFLO state in thin 2H-NbSe2.

cond-mat.supr-con↗

arXiv2024

Thermodynamic signatures of a potential Fulde-Ferrell-Larkin Ovchinnikov state in the isotropic superconductor Ti4Ir2O

This study presents an investigation of the isotropic superconductor Ti4Ir2O, focusing on its magnetic field vs. temperature phase diagram near and above the Pauli limit for superconductivity. The data exhibits characteristic features that align with the formation of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, including a deviation of the upper critical field line from the standard WHH model upon approaching the Pauli limiting field and a phase transition line separating the ordinary low-field superconducting phase from an unusual high field superconducting state above the Pauli limit. We discuss why the upper critical field line in this isotropic superconductor can approach the Pauli limit, necessitating a particularly high orbital limit for superconductivity. The research also identifies peculiar features in the specific heat data set taken at 0.6 K, which may be an example of the segmentation of the FFLO phase with multiple q phases. The findings provide an explanation for the violation of the Pauli limit in this superconductor, adding a fully isotropic superconductor to the list of potential FFLO superconductors and providing new insights for contemporary theories of the FFLO state.

cond-mat.supr-con↗

arXiv2024

Intra-Family Transformation of The Bi-Te Family via in-situ Chemical Interactions

The Bi-Te binary system, characterized by the homologous series of the (Bi2)m(Bi2Te3)n, has always attracted research interest for its layered structures and potential in advanced materials applications. Despite Bi2Te3 has been extensively studied, exploration of other compounds has been constrained by synthesis challenges. This study reports the molecular beam epitaxy (MBE) growth of FeTe on Bi2Te3, demonstrating that varying growth conditions can turn the Bi2Te3 layer into different Bi-Te phases and form corresponding FeTe/Bi-Te heterostructures. Our combined analysis using reflection high-energy electron diffraction (RHEED), high-resolution X-ray diffraction (HRXRD), and high-resolution scanning transmission electron microscopy (HR-STEM), indicates that specific growth conditions used for the growth of the FeTe layer can facilitate the extraction of Te from Bi2Te3, leading to the formation of Bi4Te3 and Bi6Te3. Additionally, by lowering the FeTe growth temperature to 230 oC, Te extraction from the Bi2Te3 layer could be avoided, preserving the Bi2Te3 structure. Notably, all the three FeTe/Bi-Te structures exhibit superconductivity with the FeTe/Bi2Te3 heterostructure enjoying the highest superconductivity quality. These findings introduce a novel method for realizing Bi4Te3 and Bi6Te3 through Te extraction by growing FeTe on Bi2Te3, driven by the high reactivity between Fe and Te. This approach holds promise for synthesizing other members of the Bi-Te series, expanding the functional potential of these materials.

cond-mat.supr-con↗

arXiv2024

Synergistic interplays between the selective electron-phonon coupling, antiferromagnetic fluctuations and charge density wave in the YBa2Cu3Ox cuprate superconductor

This research aims to investigate the synergistic effect between charge density wave, selective electron-phonon coupling under antiferromagnetic fluctuations, as well as the unusual electron distribution observed in ARPES data in YBa2Cu3Ox superconductors (YBCO). By considering their synergistic impact, our model can calculate the superconducting transition temperature Tc of YBa2Cu3Ox as a function of pressure for x = 6.5 and 7 at a reasonable level. Moreover, we have identified a specific antiferromagnetic phonon that contributes significantly to the high Tc observed in YBCO. This finding highlights the significance of these effects in achieving high Tc values. Our study not only identifies an imbalanced charge-density wave effect for triggering selective electron-phonon coupling but also explains why the charge density wave usually occurs around the magnetic copper atoms. Furthermore, our research reveals limitations in the conventional mean-field ab-initio approximation used for antiferromagnetic fluctuations in YBCO. It shows that the dynamic behavior of electrons in YBCO might not be accurately captured by this approximation, as non-uniform magnetic fields under antiferromagnetic fluctuations induce an additional electric potential on electrons across the boundary between non-magnetic to magnetic sites instantaneously. This instantaneous electric potential, in turn, suggest that the influence of the antiferromagnetic phonon-based pairing mechanism might not have been optimized in theory

cond-mat.supr-con↗

arXiv2024

Exploring the Unconventional Electron Distribution Patterns in Iron-based Superconductors

For more than a decade, the unusual distribution of electrons observed in ARPES (angle-resolved photoemission spectroscopy) data within the energy range of ~30meV to ~300meV below the Fermi level, known as the ARPES range, has remained a puzzle in the field of iron-based superconductivity. However, in this study, we have made a noteworthy observation: although the electron-phonon coupling alone is insufficient to account for the observed ARPES pattern, our analysis reveals that when the instantaneous electron-phonon coupling occurring in selective phonon channel is enhanced by the coexistence of antiferromagnetic spin density wave and charge density wave phenomena, the amplified interaction becomes comparable to the ARPES range. This finding suggests that the instantaneous interplay between these intricate phenomena should play a crucial role in generating the observed energy range in ARPES. Our work may provide a valuable clue towards achieving a deeper understanding of the complex relationship between electronic correlations, lattice structure, and superconductivity in iron-based materials for uncovering the origin of the unconventional ARPES pattern.

cond-mat.supr-con↗

arXiv2023

Decoding 122-Type Iron-Based Superconductors: A Comprehensive Simulation of Phase Diagrams and Transition Temperatures

Iron-based superconductors, a cornerstone of low-temperature physics, have been the subject of numerous theoretical models aimed at deciphering their complex behavior. In this study, we present a comprehensive approach that amalgamates several existing models and incorporates experimental data to simulate the superconducting phase diagrams of the principal 122-type iron-based compounds. Our model considers a multitude of factors including the momentum dependence of the superconducting gap, spin-orbital coupling, antiferromagnetism, spin density wave, induced XY potential on the tetrahedral structure, and electron-phonon coupling. We have refined the electron-phonon scattering matrix using experimental angle-resolved photoemission spectroscopy (ARPES) data, ensuring that all electrons pertinent to iron-based superconductivity are accounted for. This innovative approach allows us to calculate theoretical critical temperature Tc values for Ba1-xKxFe2As2, CaFe2As2 and SrFe2As2 as functions of pressure. These calculated values exhibit remarkable agreement with experimental findings. Furthermore, our model predicts that MgFe2As2 remains non-superconducting irrespective of the applied pressure. Given that 122-type superconductivity at low pressure or low doping concentration has been experimentally validated, our combined model serves as a powerful predictive tool for generating superconducting phase diagrams at high pressure. This study underscores that the high transition temperatures and the precise doping and pressure dependence of iron-based superconductors are intrinsically linked to an intertwined mechanism involving a strong interplay between structural, magnetic and electronic degrees of freedom.

cond-mat.supr-con↗

arXiv2023

Preliminary Tc calculations for iron-based superconductivity in NaFeAs, LiFeAs, FeSe and nanostructured FeSe/SrTiO3 Superconductors

Many theoretical models of iron-based superconductors have been proposed but Tc calculations based on the models are usually missing. We have chosen two models of iron-based superconductors in the literature and then compute the Tc values accordingly: Recently two models have been announced which suggest that superconducting electron concentration involved in the pairing mechanism of iron-based superconductors may have been underestimated, and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. We use bulk FeSe, LiFeAs and NaFeAs data to calculate the Tc based on these models and test if the combined model can predict the superconducting transition temperature (Tc) of the nanostructured FeSe monolayer well. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately superimpose the dynamics of the electron in the upper and lower tetrahedral plane. The results of our two-channel model support the literature data. Our computational model takes into account this amplifying effect of antiferromagnetism and the correction of the electron-phonon scattering matrix together with the abnormal soft out-of-plane lattice vibration of the layered structure, which allows us to calculate theoretical Tc values of LiFeAs, NaFeAs and FeSe as a function of pressure that correspond reasonably well to the experimental values. More importantly, by taking into account the interfacial effect between an FeSe monolayer and its SrTiO3 substrate as an additional gain factor, our calculated Tc value is up to 91 K high, and provides evidence that the strong Tc enhancement recently observed in such monolayers with Tc reaching 100 K may be contributed from the electrons within the ARPES range.

cond-mat.supr-con↗

arXiv2023

Proximity-induced quasi-one-dimensional superconducting quantum anomalous Hall state: a promising scalable top-down approach towards localized Majorana modes

In this work, ~100 nm wide quantum anomalous Hall insulator (QAHI) nanoribbons are etched from a two-dimensional QAHI film. One part of the nanoribbon is covered with superconducting Nb, while the other part is connected to an Au lead via two-dimensional QAHI regions. Andreev reflection spectroscopy measurements were performed, and multiple in-gap conductance peaks were observed in three different devices. In the presence of an increasing magnetic field perpendicular to the QAHI film, the multiple in-gap peak structure evolves into a single zero-bias conductance peak (ZBCP). Theoretical simulations suggest that the measurements are consistent with the scenario that the increasing magnetic field drives the nanoribbons from a multi-channel occupied regime to a single channel occupied regime, and that the ZBCP may be induced by zero energy Majorana modes as previously predicted [24]. Although further experiments are needed to clarify the nature of the ZBCP, we provide initial evidence that quasi-1D QAHI nanoribbon/superconductor heterostructures are new and promising platforms for realizing zero-energy Majorana modes.

cond-mat.supr-con↗

arXiv2023

Simulation of environmental impacts on the synthesis of carbyne with more than 6000 atoms for emerging continuously tunable energy barriers in CNT-based transistors

Transistors made up of carbon nanotubes CNT have demonstrated excellent current-voltage characteristics which outperform some high-grade silicon-based transistors. A continuously tunable energy barrier across semiconductor interfaces is desired to make the CNT-based transistors more robust. Despite the direct band gap of carbyne inside a CNT can be widely tuned by strain, the size of carbyne cannot be controlled easily. The production of a monoatomic chain with more than 6000 carbon atoms is an enormous technological challenge. To predict the optimal chain length of a carbyne in different molecular environments, we have developed a Monte Carlo model in which a finite-length carbyne with a size of 4000-15000 atoms is encapsulated by a CNT at finite temperatures. Our simulation shows that the stability of the carbyne@nanotube is strongly influenced by the nature and porosity of the CNT, the external pressure, the temperature and the chain length. We have observed an initiation of chain-breaking process in a compressed carbyne@nanotube. Our work provides much needed input for optimising the carbyne length to produce carbon chains much longer than 6000 atoms at ~300K. Design rules are proposed for synthesizing ~1% strained carbyne@(6,5)CNT as a component in CNT-based transistors to tune the energy barriers continuously.

cond-mat.mtrl-sci↗

arXiv2022

Interfacial superconductivity and zero bias peak in quasi-one-dimensional Bi2Te3/Fe1+yTe heterostructure nanostructures

Bi2Te3/Fe1+yTe heterostructures are known to exhibit interfacial superconductivity between two non-superconducting materials: Fe1+yTe as the parent compound of Fe-based superconducting materials and the topological insulator Bi2Te3. Here, we present a top-down approach starting from two-dimensional (2D) heterostructures to fabricate one-dimensional (1D) Bi2Te3/Fe1+yTe nanowires or narrow nanoribbons. We demonstrate that the Bi2Te3/Fe1+yTe heterostructure remains intact in nanostructures of widths on the order of 100 nm and the interfacial superconductivity is preserved, as evidenced by electrical transport and Andreev reflection point contact spectroscopy experiments measured at the end of the nanowire. The differential conductance shows a similar superconducting twin-gap structure as in two-dimensional heterostructures, but with enhanced fluctuation effects due to the lower dimensionality. A zero-bias conductance peak indicates the presence of an Andreev bound state and given the involvement of the topological Bi2Te3 surface state, we discuss a possible topological nature of superconductivity with strong interplay with an emerging ferromagnetism due to the interstitial excess iron in the Fe1+yTe layer, developing in parallel with superconductivity at low temperatures.

cond-mat.supr-con↗

arXiv2022

Discovery of Superconductivity in Nb$_4$SiSb$_2$ with a V$_4$SiSb$_2$-Type Structure and Implications of Interstitial Doping on its Physical Properties

We report on the discovery, structural analysis, and the physical properties of Nb$_4$SiSb$_2$ -- a hitherto unknown compound crystallizing in the V$_4$SiSb$_2$-type structure with the tetragonal space group $I4/mcm$ and unit cell parameters $a$ = 10.3638(2) $\mathring{\mathrm{A}}$ and $c$ = 4.9151(2) $\mathring{\mathrm{A}}$. We find Nb$_4$SiSb$_2$ to be a metal undergoing a transition to a superconducting state at a critical temperature of $T_{\rm c} \approx$ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of $ΔC(T_{\rm c})/γT_{\rm c} = 1.33\, {\rm mJ}\, {\rm mol}^{-1}\, {\rm K}^{-2}$. We find that for Nb$_4$SiSb$_2$, the unoccupied sites on the $4b$ Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at $T_{\rm c} \approx\, 1.2\, {\rm K}$ for Nb$_4$Cu$_{0.2}$SiSb$_2$, and $T_{\rm c} \approx\, 0.8\, {\rm K}$ for Nb$_4$Pd$_{0.2}$SiSb$_2$ as well as for Nb$_4$Pt$_{0.14}$SiSb$_2$. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.

cond-mat.supr-con↗

arXiv2022

Thermal transport properties and some hydrodynamic-like behavior in 3D topological semimetal ZrTe5

Hydrodynamic fluidity in condensed matter physics has been experimentally demonstrated only in a limited number of compounds due to the stringent conditions that must be met. Herein, we performed thermal and electrical transport experiments in three-dimensional topological semimetal ZrTe5. By measuring the thermal properties in a wide temperature range, two representative experimental evidences of the hydrodynamics are observed in temperature window between the ballistic and diffusive regimes: a faster evolution of the thermal conductivity than in the ballistic regime and the non-monotonic temperature-dependent effective quasiparticle mean-free-path. In addition, magneto-thermal conductivity results indicate that charged quasiparticles, as well as phonons, may also play an important role in this hydrodynamic-like flow in ZrTe5.

cond-mat.str-el↗

arXiv2022

Nodal and nematic superconducting phases in NbSe2 monolayers from competing superconducting channels

Transition metal dichalcogenides like 2H-NbSe2 in their two-dimensional (2D) form exhibit Ising superconductivity with the quasiparticle spins are firmly pinned in the direction perpendicular to the basal plane. This enables them to withstand exceptionally high magnetic fields beyond the Pauli limit for superconductivity. Using field-angle-resolved magnetoresistance experiments for fields rotated in the basal plane we investigate the field-angle dependence of the upper critical field (Hc2), which directly reflects the symmetry of the superconducting order parameter. We observe a six-fold nodal symmetry superposed on a two-fold symmetry. This agrees with theoretical predictions of a nodal topological superconducting phase near Hc2, together with a nematic superconducting state. We demonstrate that in NbSe2 such unconventional superconducting states can arise from the presence of several competing superconducting channels.

cond-mat.supr-con↗

arXiv2022

Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors

We compare the upper critical field of bulk single-crystalline samples of the two intrinsic transition metal dichalcogenide (TMD) superconductors, 2H-NbSe2 and 2H-NbS2, in high magnetic fields where their layer structure is aligned strictly parallel and perpendicular to the field, using magnetic torque experiments and a high-precision piezo-rotary positioner. While both superconductors show that orbital effects still have a significant impact when the layer structure is aligned parallel to the field, the upper critical field of NbS2 rises above the Pauli limiting field and forms a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while orbital effects suppress superconductivity in NbSe2 just below the Pauli limit. From the out-of-plane anisotropies, the coherence length perpendicular to the layers of 31 Å in NbSe2 is much larger than the interlayer distance, leading to a significant orbital effect suppressing superconductivity before the Pauli limit is reached, in contrast to the more 2D NbS2.

cond-mat.supr-con↗

arXiv2021

Evidence for Isotropic s-Wave Superconductivity in High-Entropy Alloys

High-entropy alloys (HEA) form through the random arrangement of five or more chemical elements on a crystalline lattice. Despite the significant amount of resulting compositional disorder, a subset of HEAs enters a superconducting state below critical temperatures $T_c<10$ K. The superconducting properties of the known HEAs seem to suffice a Bardeen-Cooper-Schrieffer (BCS) description, but little is known about their superconducting order parameter and the microscopic role of disorder. We report on magnetic susceptibility measurements on films of the superconducting HEA (TaNb)$_{1-x}$(ZrHfTi)$_x$ for characterizing the lower and upper critical fields $H_{c1}(T)$ and $H_{c2}(T)$, respectively as a function of temperature $T$. Our resulting analysis of the Ginzburg-Landau coherence length and penetration depth demonstrates that HEAs of this type are single-band isotropic s-wave superconductors in the dirty limit. Despite a significant difference in the elemental composition between the $x=0.35$ and $x=0.71$ films, we find that the observed $T_c$ variations cannot be explained by disorder effects.

cond-mat.supr-con↗