Nonmonotonic Magnetoresistance Research Articles

Anisotropic magnetotransport properties coupled with spiral spin modulation in a magnetic semimetal EuZnGe

We investigate the thermodynamic, magnetic, and electrical transport properties of a triangular-lattice antiferromagnet EuZnGe using single crystals grown from Eu-Zn flux in sealed tantalum tubes. Magnetic properties are found to be isotropic in the paramagnetic state while we observe an enhancement of in-plane magnetic susceptibility at the temperature near T* =11.3 K, suggesting an easy-plane anisotropy at low temperatures. Magnetic transition temperature is lower than T* as specific heat shows a peak at TN =7.6 K. We reveal the magnetic modulation along the c axis by resonant x-ray scattering at Eu L2 edge, which suggests competing magnetic interaction among Eu triangular-lattice layers. We observe a double-peak structure in the intensity profile along (0, 0, L) below TN, which is mainly composed of a dominant helical modulation with q ~ (0, 0, 0.4) coexisting with a secondary contribution from q ~ (0, 0, 0.5). We reproduce the intensity profile with a random mixture of five- and four-sublattice helices with spin rotation skipping due to hexagonal in-plane anisotropy. The metallic conductivity is highly anisotropic with the ratio rho_zz/rho_xx exceeding 10 over the entire temperature range and additionally exhibits a sharp enhancement of rho_zz at TN giving rise to rho_zz/rho_xx ~ 50, suggesting a coupling between out-of-plane electron conduction and the spiral magnetic modulations. In-plane magnetic field induces a spin-flop like transition, where the q = 0.4 peak disappears and an incommensurate peak of approximately qICM ~ 0.47 emerges, while the q = 0.5 modulation retains a finite intensity. This transition correlates with non-monotonic magnetoresistance and Hall resistivity, suggesting a significant interplay between electrons and spin structures through Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction.

Symmetry breaking and skyrmionic transport in twisted bilayer graphene

Motivated by recent low-temperature magnetoresistance measurements in twisted bilayer graphene aligned with hexagonal Boron Nitride substrate, we perform a systematic study of possible symmetry breaking orders in this device at a filling of two electrons per Moir\'e unit cell. We find that the surprising non-monotonic dependence of the resistance on an out-of-plane magnetic field is difficult to reconcile with particle-hole charge carriers from the low-energy bands in symmetry broken phases. We invoke the non-zero Chern numbers of the twisted bilayer graphene flat bands to argue that skyrmion textures provide an alternative for the dominant charge carriers. Via an effective field-theory for the spin degrees of freedom, we show that the effect of spin Zeeman splitting on the skyrmion excitations provides a possible explanation for the non-monotonic magnetoresistance. We suggest several experimental tests, including the functional dependence of the activation gap on the magnetic field, for our proposed correlated insulating states at different integer fillings. We also discuss possible exotic phases and quantum phase transitions that can arise via skyrmion-pairing on doping such an insulator.

Quantum Oscillations and Chiral Anomaly in a Bi0.96Sb0.04 Single Crystal

Bi1-xSbx alloys are of special significance in topological insulator research. Here we focus on the Bi0.96Sb0.04 alloy in which the conduction band edge just touches the valence band edge. Transport measurements show quantum oscillations in the longitudinal (Shubnikov–de Haas effect) and transverse magnetoresistance originating from a spheroidal Fermi surface pocket. Further investigation of the longitudinal magnetoresistance for the magnetic field parallel to the electrical current shows a small nonmonotonic magnetoresistance that is attributed to a competition of weak-antilocalization effects and a topological term related to the chiral anomaly.

Temperature and current dependence of the magnetoresistive behavior of poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)

We investigate the magnetic field effects in thin-film diodes made of the conducting polymer poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) as a function of temperature and electrical current. Magnetoresistance of these devices can be measured to high precision on two distinct magnetic field scales: <3 mT, where a pronounced nonmonotonic magnetoresistance response can be resolved, owing to weak hyperfine coupling, and at intermediate magnetic fields, ranging between 3 and 10 mT, where strong monotonic magnetoresistance is seen. The detailed examination of the magnetoresistance effects in both regimes allows one to scrutinize the accuracy of the underlying models for the behavior of these kinds of materials.

Nonmonotonic magnetoresistance of a two-dimensional viscous electron-hole fluid in a confined geometry

Ultra-pure conductors may exhibit hydrodynamic transport where the collective motion of charge carriers resembles the flow of a viscous fluid. In a confined geometry (e.g., in ultra-high quality nanostructures) the electronic fluid assumes a Poiseuille-like flow. Applying an external magnetic field tends to diminish viscous effects leading to large negative magnetoresistance. In two-component systems near charge neutrality the hydrodynamic flow of charge carriers is strongly affected by the mutual friction between the two constituents. At low fields, the magnetoresistance is negative, however at high fields the interplay between electron-hole scattering, recombination, and viscosity results in a dramatic change of the flow profile: the magnetoresistance changes its sign and eventually becomes linear in very high fields. This novel non-monotonic magnetoresistance can be used as a fingerprint to detect viscous flow in two-component conducting systems.

Vortex crossing, trapping, and pinning in superconducting nanowires of aNbSe2two-dimensional crystal

Nanowires of two-dimensional (2D) crystals of type-II superconductor ${\mathrm{NbSe}}_{2}$ prepared by electron-beam lithography were studied, focusing on the effect of the motion of Abrikosov vortices. We present magnetoresistance measurements on these nanowires and show features related to vortex crossing, trapping, and pinning. The vortex crossing rate was found to vary nonmonotonically with the applied field, which results in nonmonotonic magnetoresistance variations in agreement with theoretical calculations in the London approximation. Above the lower critical field ${H}_{c1}$ the crossing rate is also influenced by vortices trapped by sample boundaries or pinning centers, leading to sample-specific magnetoresistance patterns. We show that the local pinning potential can be modified by intentionally introducing surface adsorbates, making the magnetoresistance pattern a ``magnetofingerprint'' of the sample-specific configuration of vortex pinning centers in a 2D crystal superconducting nanowire.

Non-monotonic magnetoresistance in an AlGaN/GaN high-electron-mobility transistor structure in the ballistic region

In this report, we will discuss the nonmonotonic magnetoresistance (MR) in an AlGaN/GaN high-electron-mobility transistor (HEMT) in a perpendicular magnetic field B in the ballistic region (kBTτ/ħ < 1) and in the weakly-disordered limit (kFl = 159 ≫ 1), where kB, T, τ, ħ, k F, and l represent the Boltzmann constant, temperature, elastic scattering time, reduced Planck constant, Fermi wave vector and mean free path, respectively. The MR shows a local maximum between the weak localization (WL) and the Shubnikov-de Haas regions. In the low magnetic field regime, the quantum correction to the conductivity is proportional to T −3/2, which is consistent with a recent theory [T. A. Sedrakyan, and M. E. Raikh, Phys. Rev. Lett. 100, 106806 (2008)]. According to our results, as the temperature is increased, the position of the MR maximum in B increases. These results cannot be explained by present theories. Moreover, in the high-magnetic-field regime, neither the magnetic and nor the temperature dependences of the observed MR is consistent with present theories. We, therefore, suggest that while some features of the observed nonmonotonic MR can be successfully explained, further experimental and theoretical studies are necessary to obtain a thorough understanding of the MR effects.

Strain effect on magnetoresistance of SiGe solid solution whiskers at low temperatures

The aim of this paper is to study the strain-induced low-temperature behavior of Si 1− x Ge x whiskers' magnetoresistance and to estimate the prospects for the creation of physical values and sensing elements on the basis operating in strong magnetic fields. We have investigated the magnetoresistance and piezomagnetoresistance of low germanium fraction Si 1− x Ge x solid solution whiskers under the uniaxial strain (−4.3×10 −3 to +4.7×10 −4 rel. un.) at 4.2 K in a wide range of magnetic fields up to 14 T. Whiskers have been doped to the impurity concentration corresponding to both the insulator and the metal side of metal–insulator transition (MIT). It has been shown that magnetoresistance substantially depends on the doping level, the type and magnitude of samples' strain. The hopping conductivity with Δ Е 2 and Δ Е 3 activation energies at strain effect has been observed. The exponential character of magnetoresistance field dependencies has been obtained for heavily doped “metallic type” Si 1− x Ge x whiskers, while for weakly doped samples at the insulator side of MIT a square-law field dependencies of magnetoresistance have been observed at 4.2 K. A non-monotonic magnetoresistance change depending on the doping level of Si and Si 1− x Ge x whiskers in the vicinity of MIT.

Graphene magnetoresistance in a parallel magnetic field: Spin polarization effect

We develop a theory for graphene magnetotransport in the presence of carrier spin polarization as induced, for example, by the application of an in-plane magnetic field $(B)$ parallel to the two-dimensional graphene layer. We predict a negative magnetoresistance $\ensuremath{\sigma}\ensuremath{\propto}{B}^{2}$ for intrinsic graphene, but for extrinsic graphene we find a nonmonotonic magnetoresistance which is positive at lower magnetic fields (below the full spin polarization) and negative at very high fields (above the full spin polarization). The conductivity of the minority spin band $(\ensuremath{-})$ electrons does not vanish as the minority carrier density $({n}_{\ensuremath{-}})$ goes to zero. The residual conductivity of $(\ensuremath{-})$ electrons at ${n}_{\ensuremath{-}}=0$ is unique to graphene. We discuss experimental implications of our theory.

Nonmonotonic magnetoresistance of two-dimensional electron systems in the ballistic regime

We report experimental observations of a novel magnetoresistance (MR) behavior of two-dimensional electron systems in perpendicular magnetic field in the ballistic regime, for k_BT\tau/\hbar>1. The MR grows with field and exhibits a maximum at fields B>1/\mu, where \mu is the electron mobility. As temperature increases the magnitude of the maximum grows and its position moves to higher fields. This effect is universal: it is observed in various Si- and GaAs- based two-dimensional electron systems. We compared our data with recent theory based on the Kohn anomaly modification in magnetic field, and found qualitative similarities and discrepancies.

Island formation in disordered superconducting thin films at finite magnetic fields

It has been predicted theoretically and observed experimentally that disorder leads to spatial fluctuations in the superconducting (SC) gap. Areas where SC correlations are finite, coined SC islands, were shown experimentally to persist into the insulating side of the superconductor-insulator transition. The existence of such (possibly weakly coupled) SC islands in amorphous thin films of superconducting material accounts for numerous experimental findings related to superconductor-insulator transition and nonmonotonic magnetoresistance behavior in the insulating region. In this work, a detailed analysis pertaining to the occurrence of SC islands in disordered two-dimensional superconductors is presented. Using a locally self-consistent numerical solution of the Bogoliubov--de Gennes equations, the formation of SC islands is demonstrated, and their evolution with an applied perpendicular magnetic field is studied in some detail, along with the disorder-induced vortex pinning. While mean-field theory cannot, in principle, explore phase correlations between different islands, it is demonstrated that, by inspecting the effect of a parallel magnetic field, one can show that the islands are indeed uncorrelated SC domains. Experimental predictions based on this analysis are presented.

Superconducting islands, phase fluctuations and the superconductor–insulator transition

Properties of disordered thin films are discussed based on the viewpoint that superconducting islands are formed in the system. These lead to superconducting correlations confined in space, which are known to form spontaneously in thin films. Application of a perpendicular magnetic field can drive the system from the superconducting state (characterized by phase-rigidity between the sample edges) to an insulating state in which there are no phase-correlations between the edges of the system. On the insulating side the existence of superconducting islands leads to a non-monotonic magnetoresistance. Several other features seen in experiment are explained.

Low-temperature magnetoresistance of dirty thin films and quantum wires near a parallel-field-tuned superconducting quantum phase transition

We study the low-temperature magnetoresistance of dirty thin films and quantum wires close to a quantum phase transition from a superconducting to normal state, induced by applying a parallel magnetic field. We find that the different corrections (Aslamazov–Larkin, density of states and Maki–Thompson) to the normal state conductivity, coming from the superconducting pair fluctuations, are of the same order at zero temperature. There are three regimes at finite temperatures. In the “quantum” regime, which essentially shows a zero-temperature-like behavior, we find a negative magnetoresistance. Since in the “classical” regime the correction is positive, we predict a nonmonotonic magnetoresistance at higher temperatures.

Unexpected negative nonmonotonic magnetoresistance of the two-dimensional electrons in Si in a parallel magnetic field

We report observation of the unexpected negative and nonmonotonic magnetoresistance of 2D electrons in Si-MOSFET subjected to a varying in-plane magnetic field superimposed on a constant perpendicular field component. We show that this nonmonotonic magnetoresistance is irrelevant to the energy spectrum of mobile 2D electrons. We also observed variations of the density of mobile electrons with the in-plane field. We argue that both variations of the negative magnetoresistance and of the density of mobile electrons originate from the band of localized states. The latter coexist and interact with mobile electrons even at relatively high density, a factor of 1.5 higher than the critical density of the apparent metal-insulator transition.

Large positive quasi-classical magnetoresistance in high mobility 2D electron gas: interplay of short- and long-range disorder

We observed a giant positive quasi-classical magnetoresistance (MR) of high mobility 2D electron gas in AlGaAs/GaAs heterostructure. The MR is non-saturating and increases with the magnetic field as ρ xx ∼ B α ( α=0.9–1.2). In antidot lattice a non-monotonic behavior is observed. We show that the MR in both cases is well described by the recently advanced theory by Polyakov et al. (Phys. Rev. B 64 (2001) 205306) where non-saturating positive MR and non-monotonic MR in antidot lattice is predicted as the consequence of concurrent existence of short- and long-range scattering.

Superconductivity effect on magnetoresistance in ferromagnetic tunnel junctions

We studied the magnetoresistance of Ni/Al–Al 2O 3/Co tunnel junctions at T>1.4 K. Below the superconducting transition temperature of deposited Al films, a nonmonotonic magnetoresistance was observed. The nonmonotonic behaviour of the magnetoresistance can be explained in terms of the dependencies of the superconducting energy gap and the sheet conductivity of the Al layer on the magnetic field. We also observed an enhancement of TMR (tunneling magnetoresistance) peaks below T c in some samples. This is probably caused by the change of the distribution of the current flow at the superconducting transition.

Thermometry in high magnetic fields at low temperatures

We have investigated the behavior of carbon resistor and capacitive thermometers from 4.2 to below 0.1 K and to 19 T. We observe, in agreement with previous work at higher temperatures, a nonmonotonic magnetoresistance in a standard carbon resistor for thermometry below 1.0 K. We provide the reader with an expression for the magnetoresistance which may be used, with proper precalibration for a particular sensor, to determine the temperature from a reading of the resistance and the magnetic field. In addition, we discuss epoxy mixing chambers for dilution refrigerators and discuss results which indicate that certain materials can be magnetized and demagnetized at low temperatures. Finally, we report measurements on two types of capacitive temperature sensors which also exhibit magnetoeffects.