Combining Transport Measurements Research Articles

Interior and Edge Magnetization in Thin Exfoliated CrGeTe3 Films.

CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism down to a two-layer thick sample. Although CGT is one of the leading candidates for spintronics devices, a comprehensive analysis of CGT thickness dependent magnetization is currently lacking. In this work, we employ scanning SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with SOT images, we present the magnetic texture and hysteretic magnetism of CGT, thereby matching the global behavior of CGT to the domain structure extracted from local SOT magnetic imaging. Using this method, we provide a thickness dependent magnetization state diagram of bare CGT films. No zero-field magnetic memory was found for films thicker than 10 nm, and hard ferromagnetism was found below that critical thickness. Using scanning SOT microscopy, we identify a unique edge magnetism, contrasting the results attained in the CGT interior.

Anomalous Superconducting Proximity Effect in Bi2 Se3 /FeSe0.5 Te0.5 Thin-Film Heterojunctions.

The superconducting proximity effect (SPE) induces a superconductivity transition in otherwise non-superconducting thin films in proximity with a superconductor. The SPE usually occurs in real space and decays exponentially with film thickness. Herein, an abnormal SPE in a topological insulator (TI)/superconductor heterostructure is unveiled, which is attributed to the topologically protected surface state. Surprisingly, such abnormal SPE occurs in momentum space regardless of the TI film thickness, as long as the topological surface states are robust and form a continuous conduction loop. Combining transport measurements and scanning tunneling microscopy/spectroscopy techniques, the SPE in Bi2 Se3 /FeSe0.5 Te0.5 heterostructures is explored, where Bi2 Se3 is an ideal 3D topological insulator and FeSe0.5 Te0.5 a typical iron-based superconductor. As the thickness of the Bi2 Se3 thin film exceeds 400 nm, there still exists SPE-induced superconductivity on the surface of Bi2 Se3 thin film with a transition temperature Tc not less than 10 K. Such an extraordinary behavior is induced by the unique properties of topologically protected surface states of Bi2 Se3 . This research deepens the understanding of the important role of topologically protected surface states in the SPE.

From Low-Field Sondheimer Oscillations to High-Field Very Large and Linear Magnetoresistance in a SrTiO3-Based Two-Dimensional Electron Gas.

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, nonsaturating linear magnetoresistance (MR) has been reported in various systems, from Weyl semimetals to topological insulators. Its origin is often ascribed to unusual band structure effects, but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO3 two-dimensional electron gas and, by combining transport measurements with electron spectromicroscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semiclassical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs, and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

Polaron formation in Bi-deficient BaBiO3

$\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_{3}$ is a charged ordered Peierls-like perovskite well known for its superconducting properties upon K or Pb doping. We present a study on the transport and electronic properties of $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_{3}$ perovskite with strong Bi deficiency. We show that it is possible to synthesize $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_{3}$ thin layers with Bi vacancies above 8--10% by depositing an yttrium-stabilized zirconia capping layer. By combining transport measurements with ab initio calculations we propose a scenario where the Bi vacancies give rise to the formation of polarons and suggest that the electrical transport is dominated by the migration of these polarons trapped at ${\mathrm{Bi}}^{3+}$ sites. Our paper shows that cation vacancies engineering, hardly explored to date, appears as a promising pathway to tune the electronic and functional properties of perovskites.

Combining mesoscale thermal transport and x-ray diffraction measurements to characterize early-stage evolution of irradiation-induced defects in ceramics

In situ characterization of defects, microstructure, and properties will provide new perspectives regarding the structure-property relationship of materials in extreme environments. In this communication, we investigate the utility of laser-based thermal transport measurements in combination with X-ray diffraction as a means to characterize the early-stage evolution of irradiation-induced defects in ceramics. Uranium dioxide is used as a model system to analyze the impact of irradiation-induced defects with 2.6 MeV H and 3.9 MeV He ions up to a dose of 0.1 displacement per atom (dpa) at low temperature. For these radiation regimes, the formation of extended defects such as loops and voids is limited as compared to point defects. Lattice expansion was determined from X-ray diffraction analysis. Modulated thermoreflectance was used to measure the thermal conductivity of the ion damaged region. Both H and He irradiation leads to an expansion of the crystal lattice and a reduction in thermal conductivity. For the same dpa, the lattice expansion and conductivity reduction were notably different for H and He irradiated samples. The results were analyzed using simple models for lattice expansion and thermal conductivity reduction, informed by atomistic simulation from the literature. The modeling results suggest that the difference in the defect kinetics between two conditions can be attributed to ionization induced enhanced defect mobility and the stability of Schottky defects. These results demonstrate the utility of thermal conductivity measurements as a tool for characterization of microstructure under irradiation.

The transport properties in graphene/single-unit-cell cuprates van der Waals heterostructure

Inducing superconductivity into graphene by a proximity effect offers a chance to study exotic physics phenomena involved in Cooper pairs on a Dirac fermions system. Here we fabricate the monolayer graphene/single-unit-cell Bi2Sr2CaCu2O8+x (single-unit-cell Bi2212) van der Waals heterostructure, where the atomically flat surface from mechanically exfoliated single-unit-cell Bi2212 is the key to the pronounced proximity effect. The transport measurements in combination with direct and alternating current are performed on graphene/single-unit-cell Bi2212. The superconductivity is observed at the graphene/single-unit-cell Bi2212 heterostructure below 86 K. In the superconducting state, the nonlinear drain current–voltage relation is seen reflecting the characterisation of the Dirac–fermionic band structure of graphene. While for a lower bias, the differential conductance spectra shows a small subgap up to 60 K, implying a proximity-induced high-temperature superconductivity in graphene. Our results open a new way to study the superconductivity in two-dimensional Dirac fermion systems.

Topological surface states in thick partially relaxed HgTe films

Surface states of topological insulators (TIs) have been playing the central role in the majority of outstanding investigations in low-dimensional electron systems for more than 10 years. TIs based on high-quality strained HgTe films demonstrate a variety of subtle physical effects. The strain leads to a bulk band gap but limits a maximum HgTe strained film thickness, and therefore, the majority of experiments were performed on films with a thickness of less than 100 nm. Since a spatial separation of topological states is crucial for the study of a single-surface response, it is essential to increase the HgTe thickness further. In this work, by combining transport measurements together with capacitance spectroscopy, we perform an analysis of a 200-nm partially relaxed HgTe film. The Drude fit of the classical magnetotransport reveals the ambipolar electron-hole transport with a high electron mobility. A detailed analysis of Shubnikov-de Haas oscillations in both conductivity and capacitance allows us to distinguish three groups of electrons, identified as electrons on top and bottom surfaces and bulk electrons. The indirect bulk energy gap value is found to be close to zero. It is established that the significant gap decrease does not affect the surface states, which are found to be well resolved and spin nondegenerate. The presented techniques allow investigations of other three-dimensional TIs, regardless of the presence of bulk conductivity.

Electrodynamics of granular aluminum from superconductor to insulator: Observation of collective superconducting modes

We report on a detailed study of the optical response and $T_c-\rho$ phase diagram ($T_c$ being the superconducting critical temperature and $\rho$ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The $T_c-\rho$ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion and the superconducting gap $\Delta$. We provide a direct evidence for two different types of well resolved sub-gap absorptions, at $\omega_1\simeq\Delta$ and at $\Delta\lesssim\omega_2\lesssim2\Delta$ (decreasing with increasing resistivity).

Observation of dynamic charge stripes in Tm0.19Yb0.81B12 at the metal–insulator transition

Accurate low temperature charge transport measurements in combination with high-precision x-ray diffraction experiments have allowed detection of the symmetry lowering in the single domain Tm0.19Yb0.81B12 crystals that belong to the family of dodecaborides with metal–insulator transition. Based on the fine structure analysis we discover the formation of dynamic charge stripes within the semiconducting matrix of Tm0.19Yb0.81B12. The charge dynamics in these conducting nano-size channels is characterized by broad-band optical spectroscopy that allowed estimating the frequency (~2.4 × 1011 Hz) of quantum motion of the charge carriers. It is suggested that cooperative Jahn–Teller effect in the boron sublattice is a cause of the large-amplitude rattling modes of the Tm and Yb ions responsible for the ‘modulation’ of the conduction band along one of the directions through the variation of 5d-2p hybridization of electron states.

Anti-PbO-type CoSe film: a possible analog to FeSe superconductors

Quasi-two-dimensionality is well known as a key electronic and structural element of high temperature superconductivity. Here we prepared ultrathin films of anti-PbO-type CoSe down to the monolayer on a SrTiO3(001) substrate using molecular beam epitaxy, and investigated their electronic structures by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and transport measurements in combination with first-principles calculations. We found that the monolayer CoSe had similar band structure and non-ferromagnetism nature with monolayer FeSe on SrTiO3, except for the Fermi level upshift by ∼0.25 eV and weaker electronic correlation with a renormalization factor of 1.7, due to extra electron filling from Co 3d7 compared with Fe 3d6. The results hint that superconductivity might emerge in hole-doped tetragonal CoSe films.

Pressure-induced enhancement of non-polar to polar transition temperature in metallic LiOsO3

LiOsO3 undergoes a continuous transition from a centrosymmetric R3¯c structure to a polar R3c structure at Ts = 140 K. By combining transport measurements and first-principles calculations, we find that Ts is enhanced by applied pressures, and it reaches a value of ∼250 K at ∼6.5 GPa. The enhancement is due to the fact that the polar R3c structure of LiOsO3 has a smaller volume than the centrosymmetric R3¯c structure. Pressure generically favors the structure with the smallest volume, and therefore further stabilizes the polar R3c structure over the R3¯c structure, leading to the increase in Ts.

Evolution of the Electrode–Electrolyte Interface of LiNi0.8Co0.15Al0.05O2 Electrodes Due to Electrochemical and Thermal Stress

For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode–electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li2CO3, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species, but conflicting reports exist regarding their progression during (dis)charging. Utilizing transport measurements in combination with X-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi0.8Co0.15Al0.05O2 (NCA) particle electrodes using a LiPF6 ethylene carbonate:dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li2CO3 and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF6 breakdown at room temperature above 4.25 V, thermal aggravation is found ...

Disorder-driven topological phase transition inBi2Se3films

Topological insulators (TI) are a phase of matter that host unusual metallic states on their surfaces. Unlike the states that exist on the surface of conventional materials, these so-called topological surfaces states (TSS) are protected against disorder-related localization effects by time reversal symmetry through strong spin-orbit coupling. By combining transport measurements, angle-resolved photo-emission spectroscopy and scanning tunneling microscopy, we show that there exists a critical level of disorder beyond which the TI Bi2Se3 loses its ability to protect the metallic TSS and transitions to a fully insulating state. The absence of the metallic surface channels dictates that there is a change in topological character, implying that disorder can lead to a topological phase transition even without breaking the time reversal symmetry. This observation challenges the conventional notion of topologically-protected surface states, and will provoke new studies as to the fundamental nature of topological phase of matter in the presence of disorder.

Polar asymmetry of La(1−δ)Al(1+δ)O3/SrTiO3 heterostructures probed by optical second harmonic generation

By combining transport measurements and optical second harmonic generation, we have investigated heterostructures formed between crystalline thin films of LaAlO3, with varying stoichiometry and TiO2-terminated SrTiO3(001) substrates. Optical second harmonic generation directly probes the polarity of these heterostructures, thus complementing the transport data. The stoichiometry and the growth temperature are found to be critical parameters for controlling both the interfacial conductivity and the heterostructure polarity. In agreement with the previous work, all of the samples display an insulator-to-metal transition in the Al-reach region, with the conductivity first increasing and then saturating at the highest Al/La ratios. The second harmonic signal also increases as a function of the Al/La ratio, but, at the highest growth temperature, it does not saturate. This unusual behavior is consistent with the formation of an ordered structure of defect dipoles in the LaAlO3 film caused by the off-centering of excess Al atoms in agreement with the theory.

Characterization of aluminum oxide tunnel barriers by combining transport measurements and transmission electron microscopy imaging

We present two approaches for studying the uniformity of a tunnel barrier. The first approach is based on measuring single-electron and two-electron tunneling in a hybrid single-electron transistor. Our measurements indicate that the effective area of a conduction channel is about one order of magnitude larger than predicted by theoretical calculations. With the second method, transmission electron microscopy, we demonstrate that variations in the barrier thickness are a plausible explanation for the larger effective area and an enhancement of higher order tunneling processes.

A road to hydrogenating graphene by a reactive ion etching plasma

We report the hydrogenation of single and bilayer graphene by an argon-hydrogen plasma produced in a reactive ion etching (RIE) system. Electronic transport measurements in combination with Raman spectroscopy are used to link the electric mean free path to the optically extracted defect concentration. We emphasize the role of the self-bias of the graphene in suppressing the erosion of the flakes during plasma processing. We show that under the chosen plasma conditions the process does not introduce considerable damage to the graphene sheet and that hydrogenation occurs primarily due to the hydrogen ions from the plasma and not due to fragmentation of water adsorbates on the graphene surface by highly accelerated plasma electrons. For this reason the hydrogenation level can be precisely controlled. The hydrogenation process presented here can be easily implemented in any RIE plasma system.

High-frequency giant magnetoresistance measurement to study the dynamics of a micron-scale spin-valve sensor

We report on the study of the magnetization dynamic of a single micrometric spin-valve element by high-frequency giant magnetoresistance measurement. We have combined transport measurements in the microwave range and large band ferromagnetic resonance with microantennas. The spin valve is used as an in situ demodulator to probe the individual dynamics of each layer and the relative phase of their oscillations. We deduce the conditions for coupling the hard and free layer dynamics and show the influence of a static magnetic configuration on the sensitivity of spin valves in the microwave range. This work offers a different approach for studying the dynamics of single isolated micron-sized systems.

Ferroelectric polymer gate on AlGaN∕GaN heterostructures

There has been much investigation into the implementation of field effect transistors; however, a commercial device is still not available. An attempt is made here to develop a ferroelectric gate device using an Al0.3Ga0.7N∕GaN heterostructure with a two dimensional electron gas (2DEG). The ferroelectric copolymer poly(vinylidene fluoride∕trifluoroethylene) [P(VDF∕TrFE)] was chosen as a gate material due to advantages such as its small dielectric constant and low crystallization temperature in comparison with the perovskite ferroelectrics. A P(VDF∕TrFE) layer was successfully deposited onto an Al0.3Ga0.7N∕GaN heterostructure without degrading the transport properties of the 2DEG. The polarization reversal in the gate was proved by piezoelectric scanning force microscopy. With this structure, it was possible to demonstrate the ferroelectric gate operation when modulating the transport properties of the 2DEG due to the stable reversal of the spontaneous polarization in the gate. Transport measurements in combination with piezoelectric scanning probe microscopy enable an insight into the physical origin of the retention loss in the system.

Use of 1 H NMR to study transport processes in porous biosystems

The operation of bioreactors and the metabolism of microorganisms in biofilms or soil/sediment systems are strongly dictated by the transport processes therein. Nuclear magnetic resonance (NMR) spectroscopy or magnetic resonance imaging (MRI) allow nondestructive and noninvasive quantification and visualisation (in case of MRI) of both static and dynamic water transport phenomena. Flow, mass transfer and transport processes can be measured by mapping the (proton) displacement in a defined time interval directly in a so-called pulsed field gradient (PFG) experiment. Other methods follow the local intensity in time-controlled sequential images of water or labelled molecules, or map the effect of contrast agents. Combining transport measurements with relaxation-time information allows the discrimination of transport processes in different environments or of different fluids, even within a single picture element in an image of the porous biosystem under study. By proper choice of the applied NMR method, a time window ranging from milliseconds to weeks (or longer) can be covered. In this paper, we present an overview of the principles of NMR and MRI techniques to visualise and unravel complex, heterogeneous transport processes in porous biological systems. Applications and limitations will be discussed, based on results obtained in (model) biofilms, bioreactors, microbial mats and sediments.

Use of (1)H NMR to study transport processes in porous biosystems.

The operation of bioreactors and the metabolism of microorganisms in biofilms or soil/sediment systems are strongly dictated by the transport processes therein. Nuclear magnetic resonance (NMR) spectroscopy or magnetic resonance imaging (MRI) allow nondestructive and noninvasive quantification and visualisation (in case of MRI) of both static and dynamic water transport phenomena. Flow, mass transfer and transport processes can be measured by mapping the (proton) displacement in a defined time interval directly in a so-called pulsed field gradient (PFG) experiment. Other methods follow the local intensity in time-controlled sequential images of water or labelled molecules, or map the effect of contrast agents. Combining transport measurements with relaxation-time information allows the discrimination of transport processes in different environments or of different fluids, even within a single picture element in an image of the porous biosystem under study. By proper choice of the applied NMR method, a time window ranging from milliseconds to weeks (or longer) can be covered. In this paper, we present an overview of the principles of NMR and MRI techniques to visualise and unravel complex, heterogeneous transport processes in porous biological systems. Applications and limitations will be discussed, based on results obtained in (model) biofilms, bioreactors, microbial mats and sediments.