Transport Properties Of Single Crystals Research Articles

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.

Correlation between thermopower and carrier mobility in the thermoelectric semimetal Ta2PdSe6

We have investigated the transport properties of single crystals of a thermoelectric semimetal Ta2PdSe6 and its niobium-substituted (Ta1−xNbx)2PdSe6 by means of resistivity, thermopower, and Hall resistivity measurements. The residual resistivity ratio systematically decreases by the Nb-substitution, indicating enhanced impurity scattering. The slope and peak-top value of thermopower also systematically decrease upon increasing x. We have analyzed the set of transport data by using a two-carrier model, then revealed strong correlation between the thermopower and carrier mobility in the titled compound. We propose that controlling carrier mobility is a possible route to achieve a high-performance thermoelectric semimetal.

Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator Cr2Ge2Te6

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic two-dimensional (2D) material ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ under high pressure. In contrast to previous studies, our high-pressure single-crystal x-ray diffraction under hydrostatic conditions shows prominent anisotropic compressibility in the layered structure of the crystalline $R\overline{3}$ phase of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. The loss of crystallinity starts at 20 GPa; however, the crystalline phase and amorphous state coexist even at the maximum pressure of 31.2 GPa. High-pressure powder x-ray diffraction data and electrical resistivity measurements of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ using NaCl as the pressure-transmitting medium reveal an insulator-to-metal transition in the absence of a phase transition at \ensuremath{\sim}3.9 GPa; at a considerably lower pressure than the previously reported (7--14 GPa). Density functional theory calculations demonstrate the density of states around the Fermi level are primarily dominated by Cr $3d$ and Te $5p$ states. Hence the large reduction of Cr-Te bond lengths within the ${\mathrm{CrTe}}_{6}$ octahedra under compression is most likely responsible for the band-gap closure. This study clarifies that the phase stability and onset of metallization pressure in the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.

Direction-Dependent Thermoelectric Properties of a Layered Compound In2Te5 Single Crystal

We examine the anisotropic electrical and thermal transport properties in single crystals of In2Te5 belonging to the monoclinic space group C12/c1 with the temperature gradient applied parallel (||) and perpendicular (\(\perp \) ) to the crystallographic c-axis of the crystals. A systematic investigation of structural, electrical, and thermal properties suggests the role of layered structure in this material in guiding its thermoelectric behavior. The thermal conductivity along the c-axis (κ||c) was found to be smaller by a factor of 2 compared to the thermal conductivity along the direction perpendicular to the c-axis (κ\(_{\perp{c}}) \) over the entire temperature range. In contrast, the Seebeck coefficient along the c-axis (S||c) was found to be higher than its value along the direction perpendicular to the c-axis \((S_{\perp{c}}) \). At room temperature, the figure of merit zT||c is found to be four times larger as compared to the figure of merit \(zT_{{\perp}c} \). Our results provide insights into how the resistivity, thermal conductivity, and thermopower depends on the crystalline anisotropy and its impact on the overall zT.

Topological surface conduction in Kondo insulator YbB12

Kondo insulators have recently aroused great interest because they are promising materials that host a topological insulator state caused by the strong electron interactions. Moreover, recent observations of the quantum oscillations in the insulating state of Kondo insulators have come as a great surprise. Here, to investigate the surface electronic state of a prototype Kondo insulator YbB12, we measured the transport properties of single crystals and microstructures. In all samples, the temperature dependence of the electrical resistivity is insulating at high temperatures and the resistivity exhibits a plateau at low temperatures. The magnitude of the plateau value decreases with reducing sample thickness, which is quantitatively consistent with the surface electronic conduction in the bulk insulating YbB12. Moreover, the magnetoresistance of the microstructures exhibits a weak-antilocalization effect at low field. These results are consistent with the presence of a topologically protected surface state, suggesting that YbB12 is a candidate material for a topological Kondo insulator. The high field resistivity measurements up to µ 0 H = 50 T of the microstructures provide supporting evidence that the quantum oscillations of the resistivity in YbB12 occurs in the insulating bulk.

Topological Hall effect and magnetic states in the Nowotny chimney ladder compound Cr11Ge19

We have investigated the magnetic and charge transport properties of single crystals of Nowotney Chimney Ladder compound Cr$_{11}$Ge$_{19}$ and mapped out a comprehensive phase diagram reflecting the complicated interplay between the Dzyaloshinskii-Moriya (DM) interaction, the dipolar interaction, and the magnetic anisotropy. We have identified a set of interesting magnetic phases and attributed a finite topological Hall effect to the recently discovered bi-skyrmion phase. These data also suggest the existence of an anti-skyrmion state at finite fields for temperatures just below the magnetic ordering temperature, $T_c$, as indicated by a distinct change in sign of the topological Hall effect. Above $T_c$, we discovered a region of enhanced magnetic response corresponding to a disordered phase likely existing near the ferromagnetic critical point under small magnetic fields. Strong spin chirality fluctuations are demonstrated by the large value of the topological Hall resistivity persisting up to 1 T which is most likely due to the existence of the DM interaction. We argue that changes to the topological Hall effect correspond to different topological spin textures that are controlled by magnetic dipolar and DM interactions that vary in importance with temperature.

Crystal Growth by the Floating Zone Method of Ce-Substituted Crystals of the Topological Kondo Insulator SmB6

SmB6 is a mixed valence topological Kondo insulator. To investigate the effect of substituting Sm with magnetic Ce ions on the physical properties of samarium hexaboride, Ce-substituted SmB6 crystals were grown by the floating zone method for the first time as large, good quality single crystal boules. The crystal growth conditions are reported. Structural, magnetic and transport properties of single crystals of Sm1−xCexB6 (x=0.05, 0.10 and 0.20) were investigated using X-ray diffraction techniques, electrical resistivity and magnetisation measurements. Phase composition analysis of the powder X-ray diffraction data collected on the as-grown boules revealed that the main phase was that of the parent compound, SmB6. Substitution of Sm ions with magnetic Ce ions does not lead to long-range magnetic ordering in the Sm1−xCexB6 crystals. The substitution with 5% Ce and above suppresses the cross-over from bulk conductivity at high temperatures to surface-only conductivity at low temperatures.

Going beyond polaronic theories in describing charge transport in rubrene single crystals

We show that the charge transport properties of single crystals of rubrene can be well described without using polaron theories along the two high-mobility axes. The charge carriers can be considered as holes with coupling to the lattice but not to the degree, which requires the use of polaron theories. It is possible to use the Boltzmann Transport Equation (BTE) in the relaxation time approximation (RTA) to evaluate mobilities due to various scattering mechanisms after introducing a transport reduction factor (PTRF). PTRF takes into account the fraction of charge carriers, which have path lengths that are larger than the lattice constant, and permits the use of the BTE in the RTA even when the magnitude of the overall mobility is lower than the value typically required for the use of the BTE. We are then able to calculate mobilities due to various scattering mechanisms. We calculate the effective electron–phonon coupling constant from the published values for various phonon modes. The values of the effective mass from calculations and measurements reported in the literature vary slightly; we assume an intermediate value for the effective mass. With no fitting parameters needed for calculating temperature-dependent mobilities for trap-free crystals, we are able to get excellent agreement with the measured values along the two high-mobility crystallographic directions. In samples, with some trapping, a small density of exponentially distributed trap states is assumed and gives a very good fit to the measured data. Our work provides strong evidence that it is not necessary to invoke polaronic effects to understand charge transport in rubrene crystals.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multipoint contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

Síntesis e investigación del mono cristal BiSBTeSe dopado con Zr obtenido mediante el método de Bridgman

El lingote del mono cristal de BiSbTeSe dopado con Zr se sintetizó utilizando el método de Bridgman. La composición química se determinó mediante análisis con espectroscopía de dispersión de energía (EDS). Mediante difracción de rayos X (DRX) se demostró que el lingote de cristal obtenido es un mono cristal y confirmó que se trata de un compuesto del tipo Bi2Te3, con orientación del mono cristal (001). El punto de fusión se determinó por medidas dilatométricas. La movilidad, concentración, resistividad/conductividad, y el coeficiente de Hall del BiSbTeSe dopado con Zr, se determinaron utilizando un sistema de medición de efecto Hall basado en el método de Van der Pauw. El efecto Hall se midió a temperatura ambiente con una intensidad de campo magnético aplicada de 0,37 T a diferentes intensidades de corriente. 
 Las muestras de lingotes medidos se cortaron y se rompieron de diferentes regiones. Las muestras de lingotes utilizadas en las medidas fueron obtenbidas mediante corte y escisión en distintas zonas. Los resultados obtenidos utilizando un sistema de medición de efecto Hall (Ecopia, HMS-3000) en las distintas muestras, se compararon entre sí. Los resultados confirmaron que las propiedades eléctricas y de transporte del mono cristal dependen de la dirección del crecimiento del cristal. La movilidad se mejoró significativamente en comparación no solo con el valor teórico de Bi2Te3 sino también con los datos existentes en la literatura.

Magnetotransport studies of superconductingPr4Fe2As2Te1−xO4

We report a detailed study of the electrical transport properties of single crystals of Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$, a recently discovered iron-based superconductor. Resistivity, Hall effect and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic c-axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$. The model can successfully explain the strongly temperature dependent negative Hall coefficient and the high magnetic anisotropy assuming that the mobility of electrons is higher than that of holes.

Spin dynamics, electronic, and thermal transport properties of two-dimensional CrPS4 single crystal

2-Dimensional (2D) CrPS4 single crystals have been grown by the chemical vapor transport method. The crystallographic, magnetic, electronic, and thermal transport properties of the single crystals were investigated by the room-temperature X-ray diffraction, electrical resistivity ρ(T), specific heat CP(T), and the electronic spin response (ESR) measurements. CrPS4 crystals crystallize into a monoclinic structure. The electrical resistivity ρ(T) shows a semiconducting behavior with an energy gap Ea = 0.166 eV. The antiferromagnetic transition temperature is about TN = 36 K. The spin flipping induced by the applied magnetic field is observed along the c axis. The magnetic phase diagram of CrPS4 single crystal has been discussed. The extracted magnetic entropy at TN is about 10.8 J/mol K, which is consistent with the theoretical value R ln(2S + 1) for S = 3/2 of the Cr3+ ion. Based on the mean-field theory, the magnetic exchange constants J1 and Jc corresponding to the interactions of the intralayer and between layers are about 0.143 meV and −0.955 meV are obtained based on the fitting of the susceptibility above TN, which agree with the results obtained from the ESR measurements. With the help of the strain for tuning the magnetic properties, monolayer CrPS4 may be a promising candidate to explore 2D magnetic semiconductors.

Multiband character of β-FeSe: Angular dependence of the magnetoresistance and upper critical field

We studied ab-plane transport properties in single crystals of the superconductor β-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (Tc=8.87(5) K) the upper critical field anisotropy Hc2||ab/Hc2||c changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.

Магнитные и транспортные свойства кристаллов железосодержащих сверхпроводников семейства 122

Магнитные и транспортные свойства кристаллов железосодержащих сверхпроводников семейства 122, Ельцев Ю.Ф., Перваков К.С., Власенко В.А., Гаврилкин С.Ю., Хлыбов Е.П., Пудалов В.М.

Observation of a phase transition at 55 K in single-crystal CaCu1.7As2

We present the structural, magnetic, thermal and $ab$-plane electronic transport properties of single crystals of ${\mathrm{CaCu}}_{1.7}{\mathrm{As}}_{2}$ grown by the self-flux technique that were investigated by powder x-ray diffraction, magnetic susceptibility $\ensuremath{\chi}$, isothermal magnetization $M$, specific heat ${C}_{\mathrm{p}}$, and electrical resistivity $\ensuremath{\rho}$ measurements as a function of temperature $T$ and magnetic field $H$. X-ray diffraction analysis of crushed crystals at room temperature confirm the collapsed tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure with $\ensuremath{\sim}15%$ vacancies on the Cu sites as previously reported, corresponding to the composition ${\mathrm{CaCu}}_{1.7}{\mathrm{As}}_{2}$. The $\ensuremath{\chi}(T)$ data are diamagnetic, anisotropic, and nearly independent of $T$. The $\ensuremath{\chi}$ is larger in the $ab$ plane than along the $c$ axis, as also observed previously for ${\mathrm{SrCu}}_{2}{\mathrm{As}}_{2}$ and for pure and doped BaFe${}_{2}$As${}_{2}$. The ${C}_{\mathrm{p}}(T)$ and $\ensuremath{\rho}(T)$ data indicate metallic $sp$-band character. In contrast to the $\ensuremath{\chi}(T)$ and ${C}_{\mathrm{p}}(T)$ data that do not show any evidence for phase transitions below 300 K, the $\ensuremath{\rho}(T)$ data exhibit a sharp decrease on cooling below a temperature ${T}_{\mathrm{t}}=54$--56 K, depending on the crystal. The $\ensuremath{\rho}(T)$ data show no hysteresis on warming and cooling through ${T}_{\mathrm{t}}$ and the transition thus appears to be second order. The phase transition may arise from spatial ordering of the vacancies on the Cu sublattice. The ${T}_{\mathrm{t}}$ is found to be independent of $H$ for $H\ensuremath{\le}8$ T. A positive magnetoresistance is observed below ${T}_{\mathrm{t}}$ that increases with decreasing $T$ and attains a value in $H=8.0$ T of 8.7$%$ at $T=1.8$ K.

Large magnetothermopower effect in Dirac materials (Sr/Ca)MnBi2

We report temperature and magnetic field dependence of the thermal transport properties in single crystals of (Sr/Ca)MnBi$_2$ with linear energy dispersion. In SrMnBi$_2$ thermopower is positive, indicating hole-type carriers and the magnetic field enhances the thermopower significantly. The maximum change of thermopower is about 1600% in 9 T field and at 10 K. A negative thermopower is observed in CaMnBi$_2$ with dominant electron-type carriers and, in contrast, the magnetic field suppresses the absolute value of thermopower. First-principle band structure shows that the chemical potential is close to the Dirac-cone-like points in linear bands. The magnetic field suppresses the apparent Hall carrier density of CaMnBi$_2$ below 50 K. The large magnetothermopower effect in (Sr/Ca)MnBi$_2$ is attributed to the magnetic field shift of chemical potential

Anomalous Hall effect in the high-temperature ferrimagnetic semiconductorsBaFe2±xRu4∓xO11

Field and temperature dependences of the magnetic and transport properties of single crystals of ${\text{BaFe}}_{2+x}{\text{Ru}}_{4\ensuremath{-}x}{\text{O}}_{11}$ $(x=0.5,0.9,1.4)$ R-type ferrites are reported. A wide homogeneity range of Ru/Fe compositions allows to tune the strength of the anomalous Hall effect (AHE), the width of the semiconducting gap $(\ensuremath{\Delta})$, and the temperature of magnetic ordering $({T}_{C})$ within one structural family. These parameters are enhanced for higher Fe content. A large Hall resistivity ${\ensuremath{\rho}}_{xy}=77\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{cm}$ observed in single-crystal ${\text{BaFe}}_{3.4}{\text{Ru}}_{2.6}{\text{O}}_{11}$ at 300 K is several orders of magnitude large than that observed for bulk magnetic materials, and comparable to the Hall resistivity of thin films of magnetite. The anomalous Hall coefficient ${R}_{s}$ changes sign at a characteristic temperature ${T}^{\ensuremath{\ast}}$ that depends on the Fe/Ru ratio, which we attribute to variations in the electronic band structure. The ${R}_{s}$ vs longitudinal resistivity $({\ensuremath{\rho}}_{xx})$ dependence of ${\text{BaFe}}_{3.4}{\text{Ru}}_{2.6}{\text{O}}_{11}$ can be described in terms of negative skew and positive side-jump scattering contributions that differ above and below ${T}^{\ensuremath{\ast}}$. The very small side jump $\ensuremath{\Delta}y$ estimated from the contribution to the AHE that is quadratic in ${\ensuremath{\rho}}_{xx}$ suggests that the AHE is caused by a Karplus-Luttinger (Berry phase) mechanism.

Hole and electron contributions to the transport properties ofBa(Fe1−xRux)2As2single crystals

We report a systematic study of structural and transport properties in single crystals of Ba(Fe_(1-x)Ru_x)_2As_2 for x ranging from 0 to 0.5. The isovalent substitution of Fe by Ru leads to an increase of the a parameter and a decrease of the c parameter, resulting in a strong increase of the AsFeAs angle and a decrease of the As height above the Fe planes. Upon Ru substitution, the magnetic order is progressively suppressed and superconductivity emerges for x > 0.15, with an optimal Tc ~ 20K at x = 0.35 and coexistence of magnetism and superconductivity between these two Ru contents. Moreover, the Hall coefficient RH which is always negative and decreases with temperature in BaFe2As2, is found to increase here with decreasing T and even change sign for x > 0.15. For x_Ru = 0.35, photo-emission studies have shown that the number of holes and electrons are similar with n_e = n_h ~ 0.11, that is twice larger than found in BaFe2As2 [1]. Using this estimate, we find that the transport properties of Ba(Fe_0.65Ru_0.35)_2As_2 can be accounted for by the conventional multiband description for a compensated semi-metal. In particular, our results show that the mobility of holes is strongly enhanced upon Ru addition and overcomes that of electrons at low temperature when x_Ru > 0.15.

Thermoelectric Properties of Electron-Doped KTaO3

To explore the possibility of large thermoelectric responses on the basis of d-electron orbital degeneracy, we investigated electrical and thermal transport properties of single crystals of electron-doped KTaO3. The electron-type carrier density (n) can be increased up to 1.4×1020 cm-3 (x = 0.009) by partially substituting K with Ba in the form of K1-xBaxTaO3. The power factor and dimensionless figure of merit at room temperature steeply increase with n, up to 14 µW cm-1 K-2 and 0.03 for x = 0.009, respectively. This suggests that K1-xBaxTaO3 is a potential thermoelectric material, provided that n can be further increased.

Uniquef-level resonant state within the gap inCe3Au3Sb4single crystals: Magnetic, thermal, and transport properties

The magnetic, thermal, and transport properties of single crystals of the narrow gap semiconductor Ce3Au3Sb4 have been studied. Transport data are consistent with an exponential activation and variable range hopping at low temperatures, which is characteristic of weak disorder and localization. The specific heat and the magnetic susceptibility data suggest the existence of a large density of states of localized states in the gap. The physics is then different from standard Kondo insulators. From its unique physical properties, we propose a three band model valence, conduction, and f bands with weak disorder that explains most of the experimental findings.