arXiv subjects

L. W. Molenkamp

Publications and source records attributed to L. W. Molenkamp.

At least 19 recordsLinked to original sources

arXiv2026

Unconventional incommensurate epitaxy of superconducting FeSe films on SrTiO$_3$

We present a combined X-ray diffraction and transmission electron microscopy study of superconducting FeSe/FeTe multilayers grown by molecular beam epitaxy on SrTiO$_3$(001) substrates. While X-ray diffraction confirms perfect in-plane epitaxial alignment between FeSe, FeTe, and the substrate, scanning transmission electron microscopy reveals a surprising lack of atomic registry at the FeSe/SrTiO$_3$ interface. Instead of adapting to the substrate lattice, FeSe retains its own in-plane lattice spacing. A periodic lateral shift between the atomic positions of FeSe and SrTiO$_3$ is observed, with a registry recurrence length that matches the lattice mismatch determined by X-ray diffraction. No misfit dislocations or other relaxation features are detected at the interface. This coexistence of directional alignment and registry-free growth suggests an unconventional regime of epitaxy in which crystallographic orientation is maintained without atomic matching. The findings offer insight into strain accommodation in layered systems and may have implications for interface engineering in Fe-based superconductors.

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arXiv2025

Zero external magnetic field quantum standard of resistance at the 10-9 level

The quantum anomalous Hall effect holds promise as a disruptive innovation in condensed matter physics and metrology, as it gives access to Hall resistance quantization in terms of the von-Klitzing constant RK = h/e2 at zero external magnetic field. In this work, we study the accuracy of Hall resistance quantization in a device based on the magnetic topological insulator material (V,Bi,Sb)2Te3. We show that the relative deviation of the Hall resistance from RK at zero external magnetic field is (4.4 +/- 8.7) nohm/ohm when extrapolated to zero measurement current, and (8.6 +/- 6.7) nohm/ohm when extrapolated to zero longitudinal resistivity (each with combined standard uncertainty, k = 1), which sets a new benchmark for the quantization accuracy in topological matter. This precision and accuracy at the nohm/ohm level (or 10-9 of relative uncertainty) achieve the thresholds for relevant metrological applications and establish a zero external magnetic field quantum standard of resistance - an important step towards the integration of quantum-based voltage and resistance standards into a single universal quantum electrical reference.

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arXiv2023

Quantum transport and mobility spectrum of topological carriers in (001) SnTe/PbTe heterojunctions

Measurements of magnetotransport in SnTe/PbTe heterojunctions grown by the MBE technique on (001) undoped CdTe substrates were performed. At low magnetic fields, quantum corrections to conductivity were observed that may be attributed to the presence of topological states at the junction interface. For a sample with 5 nm thick SnTe layer, the data analysis suggests that midgap states are actually gapped. However, the phase coherence effects in 10 nm and 20 nm SnTe/PbTe samples are fully explained assuming existence of gapless Dirac cones. Magnetotransport at higher magnetic fields is described in the framework of mobility spectrum analysis (MSA). We demonstrate that the electron- and hole-like peaks observed simultaneously for all SnTe/PbTe heterojunctions may originate from the concave and convex parts of the energy isosurface for topological states -- and not from the existence of quasiparticles both carrying negative and positive charges. This interpretation is supported by numerical calculations of conductivity tensor components for gapless (100) Dirac cones, performed within a classical model and based on the solutions of Boltzmann transport equation. Our approach shows the feasibility of MSA in application to magnetotransport measurements on topological matter.

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arXiv2022

Bulk-like magnetic properties in MBE-grown unstrained, antiferromagnetic CuMnSb

A detailed study of the influence of molecular beam epitaxial growth conditions on the structural and magnetic characteristics of CuMnSb films on lattice matched GaSb is presented. For a set of nine 40~nm thick layers, the Mn and Sb fluxes are varied to produce material with different elemental compositions. It is found that the layers grown under a relative Mn to Sb flux ratio of $Φ_{\text{Mn}}$/$Φ_{\text{Sb}}=1.24\pm0.02$ are closest to the stoichiometric composition, for which the Néel temperature ($T_\text{N}$) attains its maximum values. Mn-related structural defects are believed to be the driving contribution to changes in the vertical lattice parameter. Having established the optimum growth conditions, a second set of samples with CuMnSb layer thickness varied from 5 to 510 nm is fabricated. We show that for sufficiently large thicknesses, the magnetic characteristics ($T_\text{N}\simeq62$ K, Curie-Weiss temperature $Θ_\text{CW} = -100$ K) of the stoichiometric layers do correspond to the parameters reported for bulk samples. On the other hand, we observe a reduction of $T_\text{N}$ as a function of the CuMnSb thickness for our thinnest layers. All findings reported here are of particular relevance for studies aiming at the demonstration of Néel vector switching and detection in this noncentrosymmetric antiferromagnet, which have been recently proposed.

cond-mat.mtrl-sci↗

arXiv2021

Antiferromagnetic order in MnBi2Te4 films grown on Si(111) by molecular beam epitaxy

MnBi2Te4 has recently been predicted and shown to be a magnetic topological insulator with intrinsic antiferromagnetic order. However, it remains a challenge to grow stoichiometric MnBi2Te4 films by molecular beam epitaxy (MBE) and to observe pure antiferromagnetic order by magnetometry. We report on a detailed study of MnBi2Te4 films grown on Si(111) by MBE with elemental sources. Films of about 100 nm thickness are analyzed in stoichiometric, structural, magnetic and magnetotransport properties with high accuracy. High-quality MnBi2Te4 films with nearly perfect septuple-layer structure are realized and structural defects typical for epitaxial van-der-Waals layers are analyzed. The films reveal antiferromagnetic order with a Neel temperature of 19 K, a spin-flop transition at a magnetic field of 2.5 T and a resistivity of 1.6 mOhm cm. These values are comparable to that of bulk MnBi2Te4 crystals. Our results provide an important basis for realizing and identifying single-phase MnBi2Te4 films with antiferromagnetic order grown by MBE.

cond-mat.mtrl-sci↗

arXiv2021

Any Axion Insulator Must be a Bulk Three-Dimensional Topological Insulator

In recent attempts to observe axion electrodynamics, much effort has focused on trilayer heterostructures of magnetic topological insulators, and in particular on the examination of a so-called zero Hall plateau, which has misguidedly been overstated as direct evidence of an axion insulator state. We investigate the general notion of axion insulators, which by definition must contain a nontrivial volume to host the axion term. We conduct a detailed magneto-transport analysis of Chern insulators comprised of a single magnetic topological insulator layer of varying thickness as well as trilayer structures, for samples optimized to yield a perfectly quantized anomalous Hall effect. Our analysis gives evidence for a topological magneto-electric effect quantized in units of e$^2$/2h, allowing us to identify signatures of axion electrodynamics. Our observations may provide direct experimental access to electrodynamic properties of the universe beyond the traditional Maxwell equations, and challenge the hitherto proclaimed exclusive link between the observation of a zero Hall plateau and an axion insulator.

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arXiv2021

Unidirectional magnetoresistance and spin-orbit torque in NiMnSb

Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced non-equilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetiresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field -- current indices which complicates its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb.

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arXiv2020

Survival of the quantum anomalous Hall effect in orbital magnetic fields as a consequence of the parity anomaly

Recent experimental progress in condensed matter physics enables the observation of signatures of the parity anomaly in two-dimensional Dirac-like materials. Using effective field theories and analyzing band structures in external out-of-plane magnetic fields (orbital fields), we show that topological properties of quantum anomalous Hall (QAH) insulators are related to the parity anomaly. We demonstrate that the QAH phase survives in orbital fields, violates the Onsager relation, and can be therefore distinguished from a quantum Hall (QH) phase. As a fingerprint of the QAH phase in increasing orbital fields, we predict a transition from a quantized Hall plateau with $σ_\mathrm{xy}= -\mathrm{e}^2/\mathrm{h}$ to a not perfectly quantized plateau, caused by scattering processes between counterpropagating QH and QAH edge states. This transition can be especially important in paramagnetic QAH insulators, such as (Hg,Mn)Te/CdTe quantum wells, in which exchange interaction and orbital fields compete.

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arXiv2020

Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility

It is well established that topological insulators sustain Dirac fermion surface states as a consequence of band inversion in the bulk. These states have a helical spin polarization and a linear dispersion with large Fermi velocity. In this article we report on a set of experimental observations indicating the existence of massive surface states. These states are confined at the interface and dominate equilibrium and transport properties at high energy and/or high electric field. By monitoring the AC admittance of HgTe topological insulator field-effect capacitors, we access the compressibility and conductivity of surface states in a broad range of energy and electric fields. The Dirac surface states are characterized by a compressibility minimum, a linear energy dependence and a high mobility persisting up to energies much larger than the transport bandgap of the bulk. New features are revealed at high energies with signatures such as conductance peaks, compressibility bumps, a strong charge metastability and a Hall resistance anomaly. These features point to the existence of excited massive surface states, responsible for a strong intersubband scattering with the Dirac states and the nucleation of metastable bulk carriers. The spectrum of excited states agrees with predictions of a phenomenological model of the topological-trivial semiconductor interface. The model accounts for the finite interface depth and the effect of electric fields. The existence of excited topological states is essential for the understanding of topological phases and opens a route for engineering and exploiting topological resources in quantum technology.

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arXiv2018

Residual strain in free-standing CdTe nanowires overgrown with HgTe

We investigate the crystal properties of CdTe nanowires overgrown with HgTe. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) confirm, that the growth results in a high ensemble uniformity and that the individual heterostructures are single-crystalline, respectively. We use high-resolution X-ray diffraction (HRXRD) to investigate strain, caused by the small lattice mismatch between the two materials. We find that both CdTe and HgTe show changes in lattice constant compared to the respective bulk lattice constants. The measurements reveal a complex strain pattern with signatures of both uniaxial and shear strains present in the overgrown nanowires.

cond-mat.mtrl-sci↗

arXiv2018

Magnetoresistance in the in-plane magnetic field induced semi-metallic phase of inverted HgTe quantum wells

In this study we have measured the magnetoresistance response of inverted HgTe quantum wells in the presence of a large parallel magnetic field up to 33 T is applied. We show that in quantum wells with inverted band structure a monotonically decreasing magnetoresistance is observed when a magnetic field up to order 10 T is applied parallel to the quantum well plane. This feature is accompanied by a vanishing of non-locality and is consistent with a predicted modification of the energy spectrum that becomes gapless at a critical in-plane field $B_{c}$. Magnetic fields in excess of $B_c$ allow us to investigate the evolution of the magnetoresistance in this field-induced semi-metallic region beyond the known regime. After an initial saturation phase in the presumably gapless phase, we observe a strong upturn of the longitudinal resistance. A small residual Hall signal picked up in non-local measurements suggests that this feature is likely a bulk phenomenon and caused by the semi-metallicity of the sample. Theoretical calculations indeed support that the origin of these features is classical and a power law upturn of the resistance can be expected due to the specifics of two-carrier transport in thin (semi-)metallic samples subjected to large magnetic fields.

cond-mat.mes-hall↗

arXiv2017

Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction

We study the electronic transport across an electrostatically-gated lateral junction in a HgTe quantum well, a canonical 2D topological insulator, with and without applied magnetic field. We control carrier density inside and outside a junction region independently and hence tune the number and nature of 1D edge modes propagating in each of those regions. Outside the 2D gap, magnetic field drives the system to the quantum Hall regime, and chiral states propagate at the edge. In this regime, we observe fractional plateaus which reflect the equilibration between 1D chiral modes across the junction. As carrier density approaches zero in the central region and at moderate fields, we observe oscillations in resistance that we attribute to Fabry-Perot interference in the helical states, enabled by the broken time reversal symmetry. At higher fields, those oscillations disappear, in agreement with the expected absence of helical states when band inversion is lifted.

cond-mat.mes-hall↗

arXiv2017

Epitaxy and Structural Properties of (V,Bi,Sb)$_2$Te$_3$ Layers Exhibiting the Quantum Anomalous Hall Effect

The influence of Sb content, substrate type and cap layers on the quantum anomalous Hall effect observed in V-doped (Bi,Sb)$_2$Te$_3$ magnetic topological insulators is investigated. Thin layers showing excellent quantization are reproducibly deposited by molecular beam epitaxy at growth conditions effecting a compromise between controlled layer properties and high crystalline quality. The Sb content can be reliably determined from the in-plane lattice constant measured by X-ray diffraction, even in thin layers. This is the main layer parameter to be optimized in order to approach charge neutrality. Within a narrow range at about 80% Sb content, the Hall resistivity shows a maximum of about 10 k$Ω$ at 4 K and quantizes at mK temperatures. Under these conditions, thin layers grown on Si(111) or InP(111) and with or without a Te cap exhibit quantization. The quantization persists independently of the interfaces between cap, layer and substrate, the limited crystalline quality, and the degradation of the layer proving the robustness of the quantum anomalous Hall effect.

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arXiv2017

Quantum anomalous Hall effect in Mn doped HgTe quantum wells

Magnetotransport measurements are presented on paramagnetic (Hg,Mn)Te quantum wells (QWs) with an inverted band structure. Gate-voltage controlled density dependent measurements reveal an unusual behavior in the transition regime from n- to p-type conductance: A very small magnetic field of approximately 70 mT is sufficient to induce a transition into the nu = -1 quantum Hall state, which extends up to at least 10 Tesla. The onset field value remains constant for a unexpectedly wide gate-voltage range. Based on temperature and angle-dependent magnetic field measurements we show that the unusual behavior results from the realization of the quantum anomalous Hall state in these magnetically doped QWs.

cond-mat.mes-hall↗

arXiv2017

Scaling of the Quantum Anomalous Hall Effect as an Indicator of Axion Electrodynamics

We report on the scaling behavior of V-doped (Bi,Sb)$_2$Te$_3$ samples in the quantum anomalous Hall regime for samples of various thickness. While previous quantum anomalous Hall measurements showed the same scaling as expected from a two-dimensional integer quantum Hall state, we observe a dimensional crossover to three spatial dimensions as a function of layer thickness. In the limit of a sufficiently thick layer, we find scaling behavior matching the flow diagram of two parallel conducting topological surface states of a three-dimensional topological insulator each featuring a fractional shift of $\frac{1}{2} e^2/h$ in the flow diagram Hall conductivity, while we recover the expected integer quantum Hall behavior for thinner layers. This constitutes the observation of a distinct type of quantum anomalous Hall effect, resulting from $\frac{1}{2} e^2/h$ Hall conductance quantization of three-dimensional topological insulator surface states, in an experiment which does not require decomposition of signal to separate the contribution of two surfaces. This provides a possible experimental link between quantum Hall physics and axion electrodynamics.

cond-mat.mes-hall↗

arXiv2017

CdTe-HgTe core-shell nanowire growth controlled by RHEED

We present results on the growth of CdTe-HgTe core-shell nanowires, a realization of a quasi one-dimensional heterostructure of the topological insulator HgTe. The growth is a two step process consisting of the growth of single crystalline zinc blende CdTe nanowires with the vapor-liquid-solid method and the overgrowth of these wires with HgTe such that a closed shell is formed around the CdTe core structure. The CdTe wire growth is monitored by RHEED allowing us to infer information on the crystal properties from the electron diffraction pattern. This information is used to find and control the optimal growth temperature. High quality single crystal CdTe nanowires grow with a preferred orientation. For the growth of the conductive HgTe shell structure we find that the supplied Hg:Te ratio is the crucial parameter to facilitate growth on all surface facets.

cond-mat.mtrl-sci↗

arXiv2017

Josephson junction dynamics in the presence of $2π$- and $4π$-periodic supercurrents

We investigate theoretically the dynamics of a Josephson junction in the framework of the RSJ model. We consider a junction that hosts two supercurrrent contributions: a $2π$- and a $4π$-periodic in phase, with intensities $I_{2π}$ and $I_{4π}$ respectively. We study the size of the Shapiro steps as a function of the ratio of the intensity of the mentioned contributions, i.e. $I_{4π}/I_{2π}$. We provide detailed explanations where to expect clear signatures of the presence of the $4π$-periodic contribution as a function of the external parameters: the intensity AC-bias $I_\text{ac}$ and frequency $ω_\text{ac}$. On the one hand, in the low AC-intensity regime (where $I_\text{ac}$ is much smaller than the critical current, $I_\text{c}$), we find that the non-linear dynamics of the junction allows the observation of only even Shapiro steps even in the unfavorable situation where $I_{4π}/I_{2π}\ll 1$. On the other hand, in the opposite limit ($I_\text{ac}\gg I_\text{c}$), even and odd Shapiro steps are present. Nevertheless, even in this regime, we find signatures of the $4π$-supercurrent in the beating pattern of the even step sizes as a function of $I_\text{ac}$.

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arXiv2016

Anisotropic and strong negative magneto-resistance in the three-dimensional topological insulator Bi2Se3

We report on high-field angle-dependent magneto-transport measurements on epitaxial thin films of Bi2Se3, a three-dimensional topological insulator. At low temperature, we observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. The magneto- resistance of Bi2Se3 is found to be highly anisotropic. In the presence of a parallel electric and magnetic field, we observe a strong negative longitudinal magneto-resistance that has been consid- ered as a smoking-gun for the presence of chiral fermions in a certain class of semi-metals due to the so-called axial anomaly. Its observation in a three-dimensional topological insulator implies that the axial anomaly may be in fact a far more generic phenomenon than originally thought.

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