Angular-dependent Magnetoresistance Research Articles

Magnetotransport phenomena in Bi2Se3 thin film topological insulators grown by hybrid physical chemical vapor deposition

Intrinsic defects in Bi2Se3 topological insulators tend to produce a high carrier concentration and current leakage through the bulk material. Bi2Se3 thin films were grown by hybrid physical chemical vapor deposition on (0001) Al2O3 substrates with high Se vapor pressure to reduce the occurrence of Se vacancies as the main type of defect. Consequently, the carrier concentration was reduced to ∼5.75 × 1018 cm−3 comparable to reported carrier concentration in Bi2Se3 thin films. Magnetotransport measurements were performed on the films and the data were analyzed for weak anti-localization using the Hikami-Larkin-Nagaoka model. The estimated α and lϕ values showed good agreement with the symplectic case of 2-D transport of topological surface states in the quantum diffusion regime. The temperature and angular dependence of magnetoresistance indicate a large contribution of the 2-D surface carriers to overall transport properties of Bi2Se3 thin film.

Dimensional crossover in ultrathin buried conductingSrVO3layers

The structure and resistive properties of buried ${\mathrm{SrVO}}_{3}$ layers between two insulating ${\mathrm{LaVO}}_{3}$ layers are investigated by varying the thickness of the ${\mathrm{SrVO}}_{3}$ layers between 3 and 35 monolayers. The thickest ${\mathrm{SrVO}}_{3}$ layer shows a bulklike metallic behavior, while in the thinnest ${\mathrm{SrVO}}_{3}$ layer, a weak localization regime is observed below 100 K manifesting a logarithmic temperature dependence. Angular-dependent magnetoresistance measurements indicate a cylindric shape of the Fermi surface, and therefore a two-dimensional transport in the thinnest buried ${\mathrm{SrVO}}_{3}$ layer. The modification of the charge carrier properties by the reduced thickness of the ${\mathrm{SrVO}}_{3}$ layer are furthermore underlined by the appearance of a relatively strong positive magnetoresistance under a magnetic field perpendicular to the sample surface. The present study therefore highlights a way to synthesize oxide electrodes with reduced dimension for future oxide electronics application.

Influence of the Fe Concentration on the Superconducting Properties of Fe $$_{1-y}$$ 1 - y Se

We present a comparative study of electrical transport properties in the normal state and in the dissipative superconducting state between pure \(\beta \)-FeSe phase and Fe deficient Fe\(_{1-y}\)Se crystals. We discuss the influence of the intergrowth of the magnetic hexagonal phase (Fe\(_7\)Se\(_8\)) in Fe deficient samples when compared to pure \(\beta \)-FeSe samples. In the superconducting state, we measured the \(ab\)-plane electrical resistivity with magnetic field up to 16 T and the electrical resistivity as a function of the angle between the \(c\) axis and the applied field. The angular dependence at fixed temperature below the superconducting critical temperature, \(T_c(H=0)\), is very different for both sets of crystals. The Fe deficient samples display a vortex pinning-related feature at \(\sim \)57\(^\circ \) off the plane while the pure \(\beta \)-FeSe phase samples show the persistence of a strong angular-dependent magnetoresistance characteristic of the normal state electronic structure.

Robust surface state transport in thin bismuth nanoribbons.

While a two-dimensional (2D) metallic surface state in bismuth has been proposed, experimental 2D evidence of quantum transport, e.g., angular dependent Shubnikov-de Haas (SdH) oscillations is still lacking. Here, we report the angular-dependent magnetoresistance measurements in single-crystal Bi nanoribbons, and found that both the low-field weak antilocalization behavior and the high-field angle-dependent SdH oscillations follow exactly the 2D character, indicative of the 2D metallic surface states which dominate the transport properties of thin Bi nanoribbons. Moreover, by controllable exposing the ribbons to ambient environment (1 atm and room temperature), the metallic surface states were found to be robust to the oxidation although the carrier density in the surface states are modified after the exposures. These results suggest that the metallic surface states in Bi nanoribbons should be topologically protected which can provide key information in understanding the surface properties of Bi in nanometer scale.

Angular dependence of magnetoresistance in strongly anisotropic quasi-two-dimensional metals: Influence of Landau-level shape

We present the quantum-mechanical calculations of the angular dependence of interlayer conductivity $\sigma _{zz}(\theta )$ in a tilted magnetic field in quasi-2D layered metals. Our calculation shows that the LL shape is important for this angular dependence. In particular, the amplitude of angular magnetoresistance oscillations (AMRO) is much stronger for the Gaussian LL shape than for the Lorentzian. The ratio $\sigma_{zz}(\theta =0)/ \sigma_{zz}(\theta ->\pm 90^{\circ})$ is also several times larger for the Gaussian LL shape. AMRO and Zeeman energy splitting lead to a spin current. For typical organic metals and for a medium magnetic field 10T this spin current is only a few percent of the charge current, but its value may almost reach the charge current for special tilt angles of magnetic field. The spin current has strong angular oscillations, which are phase-shifted as compared to the usual AMRO.

Evidence of topological two-dimensional metallic surface states in thin bismuth nanoribbons.

Understanding the exotic quantum phenomena in bulk bismuth beyond its ultraquantum limit remains controversial and gives rise to renewed interest. The focus of the issues is whether these quantum properties have a conventional bulk nature or just the surface effect due to the significant spin-orbital interaction and in relation to the Bi-based topological insulators. Here, we present angular-dependent magnetoresistance (AMR) measurements on single-crystal bismuth nanoribbons of different thicknesses with magnetic fields up to 31 T. In thin nanoribbons with thickness of ∼40 nm, a two-fold rational symmetry of the low field AMR spectra and two sets of 1/2-shifted (i.e., γ = 1/2) Shubnikov-de Haas (SdH) quantum oscillations with exact two- dimensional (2D) character were obtained. However, when the thickness of the ribbon increases, a 3D bulk-like SdH oscillations with γ = 0 and a four-fold rotational symmetry of the AMR spectra appear. These results provided unambiguous transport evidence of the topological 2D metallic surface states in thinner nanoribbons with an insulating bulk. Our observations provide a promising pathway to understand the quantum phenomena in Bi arising from the surface states.

Low anisotropy of the upper critical field in a strongly anisotropic layered cuprateBi2.15Sr1.9CuO6+δ:Evidence for a paramagnetically limited superconductivity

We study angular-dependent magnetoresistance in a low-${T}_{c}$ layered cuprate ${\mathrm{Bi}}_{2.15}{\mathrm{Sr}}_{1.9}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$. The low ${T}_{c}\ensuremath{\sim}4$ K allows complete suppression of superconductivity by modest magnetic fields and facilitates accurate analysis of the upper critical field ${H}_{c2}$. We observe a universal exponential decay of fluctuation conductivity in a broad range of temperatures above ${T}_{c}$ and propose a method for extraction of ${H}_{c2}(T)$ from the scaling analysis of the fluctuation conductivity at $T>{T}_{c}$. Our main result is observation of a surprisingly low ${H}_{c2}$ anisotropy $\ensuremath{\sim}2$, which is much smaller than the effective mass anisotropy of the material $\ensuremath{\sim}300$. We show that the anisotropy is decreasing with increasing field and saturates at a small value when the field reaches the paramagnetic limit. We argue that the dramatic discrepancy of high-field and low-field anisotropies is clear evidence for paramagnetically limited superconductivity.

Coexistence of spin density waves and superconductivity in (TMTSF)2PF6.

We present simultaneous measurements of angular-dependent magnetoresistance and thermopower along all three crystal axes in (TMTSF)2PF6 for pressures to 7.4kbar and magnetic fields to 35T. (TMTSF)2PF6 under pressure shows the coexistence of spin density wave and metal-superconducting orders. We suggest that this coexistence results neither in microscopic coexistence nor in a new soliton wall phase, contrary to previous suggestions, but in phase separation into domains of the high-pressure metal and the low-pressure spin density wave phases. Simultaneous measurement of transport along all crystal axes allows us to unambiguously describe the domain structure, whereas the superconducting transition temperature and four independent Fermi surface-sensitive magnetoresistance signatures allow us to unambiguously characterize the coexisting metallic domains.

Magnetic-field-induced phase transitions in the quasi-one-dimensional organic conductor HMTSF–TCNQ

Motivated by an interest to see if the field-induced (FI) phase in the charge-density wave (CDW) system is similar to the field-induced-SDW (FISDW) in (TMTSF)2X, (TMTSF: tetramethyltetraselenafulvalene), we examined the magnetic-field-induced phases in a quasi-one-dimensional (Q1D) organic conductor HMTSF–TCNQ (hexamethylene- tetraselenafulvalene- tetracyanoquinodimethane) under a pressure of 1.1 GPa, where the CDW occurring at 30 K is suppressed. The work was carried out by measurements of angular-dependent magnetoresistance oscillations and exploratory work on the Hall effect. It turned out that the FI-phase, most likely a FICDW for B > 0.1 T, accompany a quantum Hall effect, and the FI-phase transitions are controlled by the field component along the least conducting axis. Above 10 T, the lowest Landau level of the small 2D Fermi pocket (due to incomplete nesting of Fermi surface) exceeds the Fermi level, reaching the quantum limit. Although there are many differences between the CDW (HMTSF–TCNQ) and SDW ((TMTSF)2X) systems, a similar scenario for field-induced phases seems to hold.

Localization of charge carriers in the normal state of underdoped Bi2+xSr2−xCuO6+δ

We report the transport results of underdoped Bi${}_{2+x}$Sr${}_{2\ensuremath{-}x}$CuO${}_{6+\ensuremath{\delta}}$ ($0.05\ensuremath{\le}x\ensuremath{\le}0.40$) single crystals. A close relationship between the upturn of in-plane resistivity [${\ensuremath{\rho}}_{ab}(T)$] and the sign of magnetoresistance in the normal state upon Bi substitutions is found. Combining the results of the field and angular dependence of magnetoresistance, the model fitting of ${\ensuremath{\rho}}_{ab}(T)$, as well as the Hall coefficient results, we suggest a crossover from weak to strong localization of charge carriers in the underdoped Bi${}_{2+x}$Sr${}_{2\ensuremath{-}x}$CuO${}_{6+\ensuremath{\delta}}$, which may be responsible for the rather narrow superconducting dome in this system.

Recovery of oscillatory magneto-resistance in phase separated La0.3Pr0.4Ca0.3MnO3 epitaxial thin films

In-plane angular dependent magneto-resistance has been studied in La0.3Pr0.4Ca0.3MnO3 (LPCMO) manganite thin films deposited on the (100) oriented NdGaO3, and (001) oriented SrTiO3 and LaAlO3 substrates. At temperatures where the electronic phase separation is the strongest, a metastable irreversible state exists in the films whose resistivity ρ attains a large time dependent value. The ρ decreases sharply with an increasing angle θ between the magnetic field and the current, and does not display an expected oscillatory cos2θ/sin2θ dependence for all films. The regular oscillations are recovered during repetitive sweeping of θ between 0° and 180°. We discuss possible factors that could produce these unusual changes in the resistivity.

Interlayer Charge Disproportionation in the Layered Organic SuperconductorκH−(DMEDO−TSeF)2[Au(CN)4](THF)with Polar Dielectric Insulating Layers

We report the molecular dipole effect on conduction electrons in the title superconductor. The angular-dependent magnetoresistance has a peak for fields nearly parallel to the conducting layer, and the peak width scales as the field component perpendicular to the layer, indicating incoherent interlayer transport. However, two closed Fermi surfaces are observed in quantum oscillation. Accordingly, crystallographically independent layers have different charge densities in a bulk single crystal. The electric dipole of tetrahydrofuran gives rise to interlayer charge disproportionation.

Magnetocrystalline anisotropy-dependent six-fold symmetric anisotropic magnetoresistance in epitaxial CoxFe3−xO4 films

The angular-dependent anisotropic magnetoresistance (AMR) of epitaxial CoxFe3−xO4 films has been investigated systematically. The six-fold symmetric AMR was observed in the (111)-oriented epitaxial CoxFe3−xO4 films in contrast to the four-fold symmetric AMR of the (100)- and (110)-oriented CoxFe3−xO4 films. The six-fold term appears at temperatures below 200 K, and becomes more pronounced with increasing x, while no six-fold term was found in epitaxial NixFe3−xO4 films. The six-fold characteristics observed in the epitaxial CoxFe3−xO4 films are considered to be induced by the large magnetocrystalline anisotropy caused by Co addition, which directly proves that the AMR of magnetite is intimately related to magnetocrystalline anisotropy.

Exchange field-induced topological phase transition in ultrathin films of three-dimension topological insulators

We investigate the topological phase transition in ultrathin films of three-dimension topological insulators, induced by a rotation of the exchange field. It is found that, as the exchange field rotates from the vertical to the horizontal orientation, there is a topological phase transition from a quantum anomalous Hall phase to the trivial insulating one at a critical angle, yielding an angular-dependent quantum magnetoresistance.

Misfit dislocations of anisotropic magnetoresistant Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (1 1 0) substrates

Nd0.45Sr0.55MnO3 is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd0.45Sr0.55MnO3 thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (110) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd0.45Sr0.55MnO3 thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both〈111〉and [001]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a〈110〉-type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO3 (110) and (111), which are attracting attention for their potentially new functions.

Anomalous in-plane magnetoresistance in a EuFe2As2single crystal: Evidence of strong spin-charge-lattice coupling

In-plane magnetotransport properties of a EuFe${}_{2}$As${}_{2}$ single crystal are investigated by angular-dependent magnetoresistance (MR) measurements. Strong anisotropy in magnetotransport properties is observed below the magnetic ordering temperature and it exhibits intimate correlations with the ordering states of both Eu${}^{2+}$ and Fe${}^{2+}$ spins as well as the twinned structure in EuFe${}_{2}$As${}_{2}$ crystal. Such correlations lead to giant MR effect around the magnetic phase boundary, e.g., negative MR of $\ensuremath{\sim}$26$%$ is observed at 5 K and 1 T upon the reorientation of Eu${}^{2+}$ spins from antiferromagnetic to ferromagnetic configuration. Compared with Eu${}^{2+}$ moments, Fe${}^{2+}$ moments with antiferromagnetic arrangement generated relatively weak effect on MR due to its small ordered moment. All obtained MR results can be well understood based on the scenario of superzone boundary effect and the electron scattering by fluctuation of spins and twinning boundaries. It also suggests that the magnetic order state plays an important role for the determination of transport properties through the strong coupling between itinerant electrons and ordered spins.

Additional Evidence for the Surface Origin of the Peculiar Angular-Dependent Magnetoresistance Oscillations Discovered in a Topological Insulator Bi1−xSbx

We present detailed data on the unusual angular-dependent magnetoresistance oscillation phenomenon recentiy discovered in a topological insulator Bi0.91Sb0.09. Direct comparison of the data taken before and after etching the sample surface gives compelling evidence that this phenomenon is essentially originating from a surface state. The symmetry of the oscillations suggests that it probably comes from the (111) plane, and obviously a new mechanism, such as a coupling between the surface and the bulk states, is responsible for this intriguing phenomenon in topological insulators.

Correlation between the ferromagnetic metal percolation and the sign evolution of angular dependent magnetoresistance in Pr0.7Ca0.3MnO3 film

Angular dependent magnetoresistance (AMR) phenomena in Pr0.7Ca0.3MnO3 film have been investigated. A transition between cos2θ dependent AMR in an insulating state at low fields and sin2θ dependent AMR in a metal state at high fields is observed at intermediate fields, depending on the temperature and/or strength of an applied magnetic field. Although the AMR sign evolution process from cos2θ dependence to sin2θ dependence at low temperature is different from that at high temperature due to existence of ferromagnetic insulator besides charge ordering antiferromagnetic insulator, we believe that such AMR sign evolutions are closely related with magnetic-field-induced ferromagnetic metal percolation behavior.

Quantum transport of two-dimensional Dirac fermions in SrMnBi2

We report two-dimensional quantum transport in SrMnBi${}_{2}$ single crystals. The linear energy dispersion leads to unusual nonsaturated linear magnetoresistance since all Dirac fermions occupy the lowest Landau level in the quantum limit. The transverse magnetoresistance exhibits a crossover at a critical field ${B}^{*}$ from semiclassical weak-field ${B}^{2}$ dependence to the high-field linear-field dependence. With an increase in temperature, the critical field ${B}^{*}$ increases and the temperature dependence of ${B}^{*}$ satisfies the quadratic behavior which is attributed to the Landau-level splitting of the linear energy dispersion. The effective magnetoresistant mobility ${\ensuremath{\mu}}_{\text{MR}}\ensuremath{\sim}3400$ cm${}^{2}$/V s is derived. Angular-dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces. Our results illustrate the dominant 2D Dirac fermion states in SrMnBi${}_{2}$ and imply that bulk crystals with Bi square nets can be used to study low-dimensional electronic transport commonly found in 2D materials such as graphene.

Fermi surface and interlayer transport in the two-dimensional magnetic organic conductor (Me-3,5-DIP)[Ni(dmit)2]2

Resistance and magnetic torque measurements at low temperatures under high magnetic fields have been performed for a magnetic organic conductor (Me-3,5-DIP)[Ni(dmit)${}_{2}$]${}_{2}$ to investigate the electronic state. This conductor contains two types of Ni(dmit)${}_{2}$]${}_{2}$ anion layers, layers I and II. Shubnikov-de Haas and angular-dependent magnetoresistance oscillations clearly show that there exists a two-dimensional Fermi surface in layer II, whose spins are strongly coupled with the localized spins in layer I. When the magnetic field is applied parallel to the layers, the interlayer resistance shows a sharp minimum at $~8$ T and then slow oscillation at higher fields. The minimum is explained by the combined effect of the field-dependent magnetic potential and momentum shift in the interlayer tunneling. The mechanism of the slow oscillation is not clarified yet.