Low-temperature Transport Properties Research Articles

Low-temperature magnetoresistance hysteresis in vanadium-doped Bi2−xTe2.4Se0.6 bulk topological insulators

Bi2−xTe2.4Se0.6 single crystals show gapless topological surface states, and doping (x) with vanadium allows to shift the chemical potential in the bulk bandgap. Accordingly, the resistivity, carrier density, and mobility are constant below 10 K, and the magnetoresistance shows weak antilocalization as expected for low-temperature transport properties dominated by gapless surface states of so-called three-dimensional topological insulators. However, the magnetoresistance also shows a hysteresis depending on the sweep rate and the magnetic field direction. Here, we provide evidence that such magnetoresistance hysteresis is enhanced if both three-dimensional bulk states and quasi-two-dimensional topological states contribute to the transport (x = 0 and 0.03), and it is mostly suppressed if the topological states govern transport (x = 0.015). It is proposed that the hysteresis in the magnetoresistance results from different spin-dependent scattering rates of the topological surface and bulk states. Generally, this observation is of relevance to the studies of topologically insulating materials in which both topological surface and bulk states exist.

Understanding the impact of Ca doping on structural, morphological and low temperature transport properties of Nd2NiMnO6

Double perovskites of the form R2NiMnO6 (R=Rare-earth) are recognized for their multifunctional nature. Here in this study we have considered one of the members of this class of double perovskites i.e., Nd2NiMnO6 and modified its various physical properties through Ca doping at Nd site. X-ray diffraction patterns and their analysis show the monoclinic structure that can be indexed with P21/n space group for all the three compounds of Nd2-xCaxNiMnO6 (x = 0.0, 0.2 and 0.4). Scanning electron microscopy shows that the grains are irregular in shape and size for the three samples and the average grain size decreases with increase in Ca concentration. The variation of dielectric constant as well as dielectric loss with frequency depicts a decrease in their value with increase in frequency from 20 Hz to 2 MHz. On the other hand the variation of dielectric constant with temperature shows a rise in the value of dielectric constant as the temperature is increased from 100 K to 300 K, while the temperature variation of dielectric loss is associated with relaxation behavior. The peak in the tanδ curve corresponds to the temperature at which the dielectric constant starts increasing and this peak in the dielectric loss gets shifted towards the side of high temperature with the rise in frequency. A rise in the Ca concentration increases the dielectric constant as well as dielectric loss. The ac conductivity for all the three samples of Nd2-xCaxNiMnO6 (x = 0.0, 0.2 and 0.4) rises with rise in temperature indicating the existence of negative temperature coefficient of resistance. The ac conductivity also increases with increase in Ca concentration. A further analysis of conductivity behavior has been done by determining the activation energy through the involvement of the Arrhenius equation. The activation energy decreases with increase in Ca concentration. The resistivity measurements again indicate the existence of negative temperature coefficient of resistance in all the three samples of Nd2-xCaxNiMnO6 (x = 0.0, 0.2 and 0.4) and the value of resistivity reduces as the Ca concentration is increased. The various results obtained in this study have been interrelated.

Low-temperature electron transport of rutile-type GexSn1−xO2

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K.

Magnetic Field-Induced Resistivity Upturn and Non-Topological Origin in the Quasi-One-Dimensional Semimetals

As a layered topological nodal line semimetals hosting a quasi-one-dimensional (quasi-1D) crystalline structure, TaNiTe5 has attracted intensive attention. In this research, we analyze the low temperature (low-T) transport properties in single crystals of TaNiTe5. The high anisotropic transport behaviors confirm the anisotropic electronic structure in quasi-1D TaNiTe5. The resistivity shows a magnetic field-induced resistivity upturn followed by a plateau at low temperatures when current is parallel to the c axis and magnetic field is parallel to the b axis. An extremely large magnetoresistance of 1000% has been observed at 2 K and 13 T. Such a magnetic field-induced phenomenon can be generally explained using the topological theory, but we find that the behaviors are well accounted with the classical Kohler’s rule. The analysis of the Hall resistivity points to carrier compensation in TaNiTe5, fully justifying Kohler’s rule. Our findings imply that analogous magnetic field-induced low-T properties in nodal line semimetals TaNiTe5 can be understood in the framework of classical magnetoresistance theories that do not require to invoke the topological surface states.

Low-temperature transport properties of isovalent-substituted La0.9Sr0.1YbO3–δ ceramic materials

Low-temperature transport properties of isovalent-substituted La0.9Sr0.1YbO3–δ ceramic materials

Quantum phase transition in NbN superconducting thin films

We systematically investigated the low-temperature transport properties of a series of NbN epitaxial films with thickness $t$ ranging from $\sim$2.0 to $\sim$4.0 nm. The films undergo a superconductor-insulator transition (SIT) with decreasing film thickness, and the critical sheet resistance for the SIT is close to the quantum resistance of Cooper pairs $h/4e^2$ (6.45 k$\Omega$). Besides the Berezinski-Koterlitz-Thouless transition, a magnetic-field-driven SIT is observed in those two-dimensional (2D) superconducting films (2.6 nm $\lesssim t \lesssim 4.0$ nm). Interestingly, it is found that the low-temperature magnetoresistance isotherms do not cross at a single fixed point but at a well-distinguished region for these superconducting films. The dynamical critical exponent obtained by analyzing these magnetoresistance isotherms is divergent as the quantum critical point is being approached. The behavior of the dynamical critical exponent, originating from quenched disorder at ultralow temperatures, provides direct evidence for the occurrence of quantum Griffiths singularity in the quantum phase transition process of the films. The field-driven anomalous metal (quantum metal) state does not appear in these films. Our results suggest that the quantum Griffiths singularity not only occurs in the highly crystalline 2D superconductors with superconductor-metal transition but also in those with SIT.

Impurity metallic conduction below the critical concentration of metal-insulator transition in FeCo x Si

Analysis on very detailed measurements of resistivity (ρ) and thermoelectric power (S) of magnetic impurity (Co) substituted iron silicide (FeSi) has been presented in this report. The impurity valence electrons of Co dominate the whole physical properties at low temperatures below 35 K, below the critical concentration x c (≈0.02). The negative thermopower and the positive slope in the resistivity at low temperatures are exciting and show that the system is not entirely insulator below the critical concentration of metal–insulator transition (x c ). So, due to the external impurity electrons, the system’s magnetic ground state could change considerably compared to the parent compound FeSi. This report may help unveil the interesting low-temperature transport properties between x = 0 and x = 0.04 (FeCo x Si). Two band model and variable range hopping model were employed to explain the low-temperature electrical and thermal transport properties.

Low-temperature electrical transport and tunable optical properties of Mo-doped V2O3 thin films

This work reports the structural and low-temperature electrical transport properties of Mo doped V2O3 single-layer thin films (i.e., with compositions between Mo and V2O3 [V2−xMoxO3−δ (x = 0, 0.05–0.1)]) deposited on alumina (Al2O3) substrates by sol–gel dip-coating technique. The crystallinity of the V2O3 films is modified with the high concentration of Mo doping and exhibits a relatively defective structure compared to that of pure V2O3 films. The low temperature (i.e., over a temperature range of 253.15 to 273.15 K) electrical hysteresis curves reveal that the triggering of metal–insulator transition at low temperatures. The metal–insulator transition (MIT) temperature (Tc) values are found to be 258.49 and 261.11 K for 8 and 10 mol% Mo doped V2O3 films, respectively, which are higher than that of undoped V2O3 (Tc = 253.09 K). The experimental results contribute a promising method to develop a novel material for high-performance low-temperature engineering applications.

Anomalous resistivity upturn in the van der Waals ferromagnet Fe5GeTe2

FenGeTe2 (n = 3, 4, and 5) has recently attracted increasing attention due to its two-dimensional van der Waals characteristic and high temperature ferromagnetism, which makes promises for spintronic devices. A Fe(1) split site is an important structural characteristic of Fe5GeTe2, which makes it very different from other FenGeTe2 (n = 3 and 4) systems. The local atomic disorder and short-range order can be induced by the split site. In this work, high-quality van der Waals ferromagnet Fe5GeTe2 single crystals were grown to study low-temperature transport properties. We found a resistivity upturn below 10 K. The temperature and magnetic field dependence of the resistivity are in good agreement with a combination of the theory of disorder-enhanced three-dimensional electron–electron and single-channel Kondo effect. The Kondo effect exists only at low magnetic fields B<3 T, while electron–electron interaction dominates the appearance for the low-temperature resistivity upturn. We believe that the enhanced three-dimensional electron–electron interaction in this system is induced by the local atomic structural disorder due to the split site of Fe(1). Our results indicate that the split site of Fe plays an important role for the exceptional transport properties.

Low-temperature transport properties of the intermetallic compound HoAgGe with a kagome spin-ice state

We study the magnetic susceptibility, magnetization, resistivity, and thermal conductivity of intermetallic HoAgGe single crystals at low temperatures and in magnetic fields along the $a$ and $c$ axis, while the electric and heat currents are along the $c$ axis. The magnetization curves show a series of metamagnetic transitions and small hysteresis at low field for $B\ensuremath{\parallel}a$, and a weak metamagnetic transition for $B\ensuremath{\parallel}c$, respectively. Both the magnetic susceptibility and $\ensuremath{\rho}(T)$ curve show anomalies at the antiferromagnetic transition (${T}_{\mathrm{N}}\ensuremath{\sim}11.3$ K) and spin reorientation transition ($\ensuremath{\sim}7$ K). In zero field and at very low temperatures, the electrons are found to be the main heat carriers. For $B\ensuremath{\parallel}a$, the $\ensuremath{\rho}(B)$ curves display large and positive transverse magnetoresistance (MR) with extraordinary field dependence between ${B}^{2}$ and $B$ linear, accompanied with anomalies at the metamagnetic transitions and low-field hysteresis; meanwhile, the $\ensuremath{\kappa}(B)$ mainly decrease with increasing field and display some anomalies at the metamagnetic transitions. For $B\ensuremath{\parallel}c$, there is weak and negative longitudinal MR while the $\ensuremath{\kappa}(B)$ show rather strong field dependence, indicating the role of phonon heat transport.

Low-temperature nanoscale heat transport in a gadolinium iron garnet heterostructure probed by ultrafast x-ray diffraction.

Time-resolved x-ray diffraction has been used to measure the low-temperature thermal transport properties of a Pt/Gd3Fe5O12//Gd3Ga5O12 metal/oxide heterostructure relevant to applications in spin caloritronics. A pulsed femtosecond optical signal produces a rapid temperature rise in the Pt layer, followed by heat transport into the Gd3Fe5O12 (GdIG) thin film and the Gd3Ga5O12 (GGG) substrate. The time dependence of x-ray diffraction from the GdIG layer was tracked using an accelerator-based femtosecond x-ray source. The ultrafast diffraction measurements probed the intensity of the GdIG (1 −1 2) x-ray reflection in a grazing-incidence x-ray diffraction geometry. The comparison of the variation of the diffracted x-ray intensity with a model including heat transport and the temperature dependence of the GdIG lattice parameter allows the thermal conductance of the Pt/GdIG and GdIG//GGG interfaces to be determined. Complementary synchrotron x-ray diffraction studies of the low-temperature thermal expansion properties of the GdIG layer provide a precise calibration of the temperature dependence of the GdIG lattice parameter. The interfacial thermal conductance of the Pt/GdIG and GdIG//GGG interfaces determined from the time-resolved diffraction study is of the same order of magnitude as previous reports for metal/oxide and epitaxial dielectric interfaces. The thermal parameters of the Pt/GdIG//GGG heterostructure will aid in the design and implementation of thermal transport devices and nanostructures.

Systematic study on thermal conductivity of organic triangular lattice systems β′-X[Pd(dmit)2]2

Systematic variation of the low-temperature heat transport properties of the quasi-two-dimensional quantum spin liquid (QSL) candidate ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{EtMe}}_{3}\mathrm{Sb}[\mathrm{Pd}({\mathrm{dmit})}_{2}{]}_{2}$ and its analog compounds showing antiferromagnetic (AFM) ordering is discussed using thermal conductivity $\ensuremath{\kappa}$ measurements. All compounds with the QSL ground state show a monotonic decrease in $\ensuremath{\kappa}$ with decreasing temperature, which is a typical feature of glass or amorphous materials, caused by their low-energy phonon structures. In contrast, the AFM compounds exhibit a peak structure of $\ensuremath{\kappa}$ around 10 K, which is the characteristic temperature dependence of normal crystals with coherent phonons. Our results indicate that the anomalous glassy behavior in ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{EtMe}}_{3}\mathrm{Sb}[\mathrm{Pd}({\mathrm{dmit})}_{2}{]}_{2}$ is an intrinsic and universal feature of the organic QSL phase and is not caused by extrinsic factors, such as the quality of the samples and the microcracks occurring during the cooling process. We propose a scenario for the origin of the glassy behavior of QSL compounds in which the charge fluctuation within the Pd(${\mathrm{dmit})}_{2}$ dimers changes the phonon properties from crystalline to glasslike.

Unconventional Landau level transitions in Weyl semimetal NbP

Weyl semimetals were experimentally discovered as a new quantum phase of matter that exhibits topologically protected states characterized by separated Weyl points. Similar to other topological materials, the research of Landau level transitions can provide abundant information on Fermi surfaces. Extensive experimental efforts on low-temperature transport and optical properties have been dedicated to investigating the nontrivial topology structure. However, there are some theoretically predicted unconventional Landau level transitions in Weyl semimetals that have not been observed experimentally. And, their contribution to the electromagnetic response of Weyl semimetal remains elusive. Here, we report the magneto-optical study of Landau quantization and selection rules in Weyl semimetal NbP under Voigt geometry. By changing the direction of the electric vector of the incoming light under Voigt and Faraday geometry, abundant Landau level transition modes which contain $\mathrm{\ensuremath{\Delta}}=0$, \ifmmode\pm\else\textpm\fi{}1, \ifmmode\pm\else\textpm\fi{}2 selection rules are obtained. The richness of the optical spectra, particularly the ones from the Voigt geometry, allows us to determine the location of Fermi energy and its evolution with magnetic fields. We further extract the transitions between Landau levels and Fermi energy and deduce the Landau index. Our results reveal the track of Fermi energy under varying magnetic fields based on the Landau level transitions under Voigt geometry and suggest that a combination of the magneto-optical spectra under Voigt and Faraday geometry can effectively determine the key properties of topological materials that are otherwise inaccessible.

Energy-gap driven low-temperature magnetic and transport properties inCr1/3MS2(M= Nb, Ta)

The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations that demonstrate the materials are half-metals. There is also a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta), with the gap-like feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($\mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $\mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase.

Majorana-Kondo competition in a cross-shaped double quantum dot-topological superconductor system

We examine the transport properties of a double quantum dot system coupled to a topological superconducting nanowire hosting Majorana quasiparticles at its ends, with the central quantum dot attached to the left and right leads. We focus on the behavior of the local density of states and the linear conductance, calculated with the aid of the numerical renormalization group method, to describe the influence of the Majorana coupling on the low-temperature transport properties induced by the Kondo correlations. In particular, we show that the presence of Majorana quasiparticles in the system affects both the spin-up and spin-down transport channels, affecting the energy scales associated with the first-stage and second-stage Kondo temperatures, respectively, and modifying the low-energy behavior of the system.

Low-Temperature Electrical Transport and Tunable Optical Properties of Mo-Doped V2o3 Thin Films

Low-Temperature Electrical Transport and Tunable Optical Properties of Mo-Doped V2o3 Thin Films

Band convergence and thermoelectric performance enhancement of InSb via Bi doping

The thermoelectric performance of InSb has been significantly improved via defect engineering or nanostructuring to reduce thermal conductivity in the past decade; however, less attention has been paid to improve its electrical properties via other key factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here, we show that Bi doping on the Sb site not only significantly suppresses the high-temperature bipolar effect, but also leads to band convergence which enhances the Seebeck coefficient. As a result, a maximum zT of ∼0.43 @ 675 K is achieved in InSb0.97Bi0.03, which is increased by ∼43% as compared to that of pure InSb. In addition, the comparison of Bi and Ga doping (J. Mater. Chem., 2011, 21, 12398–12401) on the low-temperature electrical transport properties reveals that Bi doping is more beneficial for achieving high power factor due to the mixed acoustic phonon and ionized impurity scattering. Finally, it is suggested that Bi should be an effective dopant for electrical optimization of InSb and a further experiment on co-doping of Ga and Bi will be a considerable alternative for thermoelectric performance enhancement of InSb-based materials.

Structural, Magnetic, and Low-Temperature Electrical Transport Properties of YIG Thin Films with Heavily Reduced Oxygen Contents

Structural, Magnetic, and Low-Temperature Electrical Transport Properties of YIG Thin Films with Heavily Reduced Oxygen Contents

Low-temperature transport properties of doped Ba0.57K0.43Fe2As2 superconductors in high magnetic field

For high-critical-temperature (high-${T}_{\text{c}}$) superconductors, the normal state transport properties at low temperatures are of great importance to understand the Fermi surface structure and carrier profile. In this work we studied the doped ultrathin ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ system, a member of the optimally doped 122-type multigapped iron-based superconductors with complex superconducting gap structure. To explore the normal state resistivity below the zero-field superconducting ${T}_{\text{c}}$, both Co and Zn dopants were substituted into the superconducting ${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ layer to suppress the superconductivity. The temperature dependence of in-plane resistivity below ${T}_{\text{c}}$ reveals a typical metallic behavior with the magnetic fields up to 45 T along $c$ axis. A linear dependence of Hall resistivity $({\ensuremath{\rho}}_{xy})$ is observed for ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{({\mathrm{Fe}}_{0.96}{\mathrm{Zn}}_{0.04})}_{2}{\mathrm{As}}_{2}$ at high magnetic field, indicating a conventional metal profile. Nevertheless, the Co dopants lead to a decreasing $d{\ensuremath{\rho}}_{xy}/dH$ when the field is above 20 T. Such transformation of carrier transport properties may be contributed by band shift within a two-carrier model.

High mobility field-effect transistors based on MoS2 crystals grown by the flux method

Two-dimensional (2D) molybdenum disulphide (MoS2) transition metal dichalcogenides (TMDs) have great potential for use in optical and electronic device applications; however, the performance of MoS2 is limited by its crystal quality, which serves as a measure of the defects and grain boundaries in the grown material. Therefore, the high-quality growth of MoS2 crystals continues to be a critical issue. In this context, we propose the formation of high-quality MoS2 crystals via the flux method. The resulting electrical properties demonstrate the significant impact of crystal morphology on the performance of MoS2 field-effect transistors. MoS2 made with a relatively higher concentration of sulphur (a molar ratio of 2.2) and at a cooling rate of 2.5 °C h−1 yielded good quality and optimally sized crystals. The room-temperature and low-temperature (77 K) electrical transport properties of MoS2 field-effect transistors (FETs) were studied in detail, with and without the use of a hexagonal boron nitride (h-BN) dielectric to address the mobility degradation issue due to scattering at the SiO2/2D material interface. A maximum field-effect mobility of 113 cm2 V−1 s−1 was achieved at 77 K for the MoS2/h-BN FET following high-quality crystal formation by the flux method. Our results confirm the achievement of large-scale high-quality crystal growth with reduced defect density using the flux method and are key to achieving higher mobility in MoS2 FET devices in parallel with commercially accessible MoS2 crystals.