Unsaturated Magnetoresistance Research Articles

Large magnetoresistance and nontrivial Fermi surface topology in quasi-one-dimensional Ta2PdSe6

We report the experimental investigation of quasi-one-dimensional (quasi-1D) Ta2PdSe6 single crystals by magnetotransport measurements under high magnetic fields up to 31 T. The crystals show large unsaturated magnetoresistance up to (1.8 × 105%) under 31 T at 1.6 K, accompanied by remarkable resistivity oscillations. The analysis of quantum oscillations reveals the existence of five Fermi pockets, consistent with theoretical calculations. Furthermore, our investigation suggested that band structure is modified by the electron correlations in Ta2PdSe6, which may provide a platform for exploring the interplay between electron effects and topological properties in quasi-1D materials.

Anomalous Hall effect and Fermi surface reconstruction in topological antiferromagnet candidate GdAuPb

The combination of topology and magnetism has demonstrated great potential in the search of emergent quantum matters. In this study, we report the synthesis, magnetic, and electrical properties of GdAuPb, a rare-earth-based half-Heusler topological antiferromagnet candidate. Magnetization and specific heat studies reveal highly frustrated antiferromagnetic ordering of Gd at TN = 11.5 K. Interestingly, this material possesses a large, unsaturated magnetoresistance up to 800% below TN and anomalous Hall conductivity as large as 750 Ω−1cm−1 in the antiferromagnetic state. Moreover, the electric transports including the SdH quantum oscillation patterns all exhibit a sudden change around 9 T, indicating a field-driven quantum transition related to the Fermi surface reconstruction. These results suggest that GdAuPb is close to a topological critical point, which can be easily tuned via external parameters. Our study demonstrates a strong interplay between band topology and magnetism in GdAuPb and offers clues for material designs for topological antiferromagnets.

The vertical longitudinal magnetoresistance in a van der Waals thin film of WTe2

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.

Large unsaturated magnetoresistance and electronic structure studies of single-crystal GdBi

Single crystal of the binary equiatomic compound GdBi crystallizing in the rock-salt-type cubic crystal structure with the space group $Fm\overline{3}m$ has been grown by flux method. The electrical and magnetic measurements have been performed on well-oriented single crystals. The antiferromagnetic ordering of the Gd moments is confirmed at ${T}_{\mathrm{N}}=27.5$ K. The magnetization measurement performed at 2 K along the principal crystallographic direction [100] attained a value of $1.3\phantom{\rule{0.16em}{0ex}}{\textmu{}}_{B}$/Gd in a field of 7 T, which is about 5 times smaller than its saturation value of $7\phantom{\rule{0.16em}{0ex}}{\textmu{}}_{B}$. Zero-field electrical resistivity reveals a sudden drop at 27.5 K, suggesting a reduction in the spin disorder scattering due to the antiferromagnetic alignment of the Gd moments. The residual resistivity at 2 K is 390 $\mathrm{n}\mathrm{\ensuremath{\Omega}}\mathrm{cm}$, suggesting a good quality of the grown crystal. The magnetoresistance attains a value of ${10}^{4}%$ with no signature of saturation, in a field of 14 T at $T=2$ K. Shubnikov--de Hass oscillations have been observed in the high field range of the magnetoresistance with three fundamental frequencies corresponding to the extremal areas of the Fermi surface. Our results suggest that the extremely large unsaturated magnetoresistance in this compound is due to near compensation of the charge carriers. We also performed angle-resolved photoelectron spectroscopy measurements with different photon energies and compared our results with the calculated band structure of GdBi.

Synthesis and electromagnetic transport of large-area 2D WTe2 thin film

Tungsten telluride thin films were successfully prepared on monocrystal sapphire substrates by using atomic layer deposition and chemical vapor deposition technology, and the effects of different tellurization temperatures on the properties of tungsten telluride films were investigated. The growth rate, crystal structure and composition of the film samples were characterized and analyzed by using scanning electron microscope, Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that tungsten telluride thin films with good crystal orientation in (001) were obtained at telluride temperature of 550 °C. When the telluride temperature reached 570 °C, the tungsten telluride began to decompose and unsaturated magnetoresistance was found.

A ferromagnetic topological half-metal candidate: Carbon-doped Bi2Se3

The high quality CxBi2Se3-x single crystals have been prepared by melt-grown reaction. The transport characteristicsmeasurement showed that the doping leads to the presence of ferromagnetism and the high external magnetic field could induce the metal-insulation transition. The metal-insulation transition temperature (Tm) is 169 K and 253 K for 6 T and 9 T, respectively. Analyze the curves of ρxy-B, we used the two-band model to fit MR and Hall coefficient of samples. The results indicated that electron-hole compensation may not be the only reason for the existence of unsaturated magnetoresistance in the material. To further, first principles are used to calculate the band structure of CxBi2Se3-x single crystals, and the feature of the topological half-metals has been suggested. According to the above results, we infer that CxBi2Se3-x single crystals may be candidate materials for topological half-metals.

Large unsaturated magnetoresistance of 2D magnetic semiconductor Fe-SnS2 homojunction

A magnetic semiconductor whose electronic charge and spin can be regulated together will be an important component of future spintronic devices. Here, we construct a two-dimensional (2D) Fe doped SnS2 (Fe-SnS2) homogeneous junction and investigate its electromagnetic transport feature. The Fe-SnS2 homojunction device showed large positive and unsaturated magnetoresistance (MR) of 1800% in the parallel magnetic field and 600% in the vertical magnetic field, indicating an obvious anisotropic MR feature. In contrast, The MR of Fe-SnS2 homojunction is much larger than the pure diamagnetic SnS2 and most 2D materials. The application of a gate voltage can regulate the MR effect of Fe-SnS2 homojunction devices. Moreover, the stability of Fe-SnS2 in air has great application potential. Our Fe-SnS2 homojunction has a significant potential in future magnetic memory applications.

Magnetoresistance retraction behaviour of Ag/p‐Ge:Ga/Ag device under pulsed high magnetic field

In non-magnetic semiconductor materials, unsaturated magnetoresistance (MR) effect has attracted lots of attention due to its physical interests and potential applications in electronic devices. Under the extremely high magnetic field, the stability and reliability of MR effects based on the non-ohmic transport has been rarely researched. In this paper, the transport properties of non-magnetic Ag/p-Ge:Ga/Ag devices under 45 T pulsed high magnetic field at 300 K are investigated. It is found that in ohmic conduction region ( I <5 mA) where the single dominant carrier is hole, the MR values increase with increasing the applied magnetic field, presenting a conventional unsaturated behavior. In the two non-ohmic regions (5 mA≤ I ≤ 100 mA) where the transport is dominated by bipolar (electrons and holes), a MR retraction has been obviously observed under pulsed high magnetic field. Combining the Hall measurement results and calculation of Hall effect with bipolar-driven transport model, the mechanism of the MR retraction is analysed, in which the MR retraction may be related to the strong regulation of electron-to-hole density ratio by pulsed high magnetic field. This work provides a reference for evaluating the stability and reliability of the properties of non-magnetic semiconductor based MR devices under the interference of strong magnetic pulses.

Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe7S8 with Robust Environmental Stability.

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe7S8 with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe7S8 nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe7S8, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 105 S m-1 at room temperature and ∼1.7 × 106 S m-1 at 2 K). More interestingly, the CVD-synthesized 2D Fe7S8 nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Excitation polarization-independent photo-induced restoration of inversion symmetry in Td-WTe2

Td-WTe2 is a topologically nontrivial material and exhibits a variety of physical properties, such as giant unsaturated magnetoresistance and the unconventional thermoelectric effect, due to its topological nature. It is also known to exhibit ultrafast topological phase transitions that restore its inversion symmetry by intense terahertz and mid-infrared pulses, and these properties demonstrate the possibility of ultrafast control of devices based on topological properties. Recently, a novel photo-induced topological phase transition by using polarization-controlled infrared excitation has been proposed, which is expected to control the material topology by rearranging the atomic orbitals near the Weyl point. To examine this topological phase transition, we experimentally studied the excitation-polarization dependence of the infrared-induced phase dynamics in a thin-layer of Td-WTe2. Time-resolved second harmonic generation (SHG) measurements showed that SHG intensity decreases after the infrared pump regardless of the polarization. Polarization-resolved infrared pump–probe measurements indicated that the polarization-selected excited state relaxes quite rapidly (i.e., within 10–40 fs). Considering these experimental results, we conclude that it is difficult to control the photo-induced phase transition through orbital-selective excitation owing to the rapid loss of carrier distribution created by polarization-selective excitation in thin-layer Td-WTe2 under our experimental condition. These results indicate that the suppression of the electron scattering process is crucial for experimentally realizing the photo-induced phase transition based on the polarization selection rule of the materials.

Large magnetoresistance and de Haas–van Alphen oscillations in the topological semimetal candidates BaX4 (X=Ga, In)

We report the magnetoresistance (MR), de Haas--van Alphen (dHvA) oscillations, and first-principles calculation studies on topological semimetal candidates ${\mathrm{Ba}X}_{4}$ $(X=\mathrm{Ga}, \mathrm{In})$. Large unsaturated MR that reaches $3\ifmmode\times\else\texttimes\fi{}{10}^{3}%$ in ${\mathrm{BaGa}}_{4}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{4}%$ in ${\mathrm{BaIn}}_{4}$ are observed, which is due to the carrier compensations and high mobility. Evident dHvA oscillations are detected with $B//[001]$ configuration, from which the quantum mobilities are extracted. The light cyclotron effective masses are extracted from the fitting of the thermal damping term in the LK formula. According to the first-principles calculations, ${\mathrm{Ba}X}_{4}$ possesses several Dirac points in the absence of spin-orbit coupling (SOC), some of which open negligible gaps when SOC is considered. The ${Z}_{2}$ invariants of ${\mathrm{Ba}X}_{4}$ are equal to 1, indicating the nontrivial topological properties.

Linear unsaturated magnetoresistance in YSi single crystal

Linear magnetoresistance is a phenomenon observed in a few topological compounds originating from both classical and quantum phenomena. Here, we performed electrical transport measurements, in zero and applied magnetic fields, on the YSi single crystal along all three principal crystallographic directions of the orthorhombic crystal structure. For the J∥[001]; B∥[100] direction above ≈10 T, mobility fluctuations driven linear magnetoresistance is observed without any sign of saturation up to the 14 T magnetic field. Anisotropy in the Fermi surface is imminent from the angular dependence of the magnetoresistance. Kohler rule violation is observed in this system, and Hall data signify multiple charge carriers in YSi.

High-resolution angle-resolved photoemission study of large magnetoresistance topological semimetal CaAl4 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774421, 12074425, 11874422, 11574391, and 11774424), the National Key Research and Development Program of China (Grant No. 2019YFA0308602), and the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China

Extremely large magnetoresistance (XMR) has been explored in many nonmagnetic topologically nontrivial/trivial semimetals, while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures. In this paper, the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated, exhibiting unsaturated magnetoresistance of ∼ 3000% at 2.5 K and 14 T as the fingerprints of XMR materials. By the combination of ARPES and the first-principles calculations, we elaborate multiband features and anisotropic Fermi surfaces, which can explain the mismatch of isotropic two-band model. Although the structural phase transition from I4/mmm to C2/m has been recognized, the subtle impact on electronic structure is revealed by our ARPES measurements. Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4, our findings report CaAl4 as a new reference material for exploring the XMR phenomena.

Extremely large magnetoresistance from electron-hole compensation in the nodal-loop semimetalZrP2

Several early transition metal dipnictides have been found to host topological semimetal states and exhibit large magnetoresistance. In this study, we use angle-resolved photoemission spectroscopy (ARPES) and magneto-transport to study the electronic properties of a new transition metal dipnictide ZrP$_2$. We find that ZrP$_2$ exhibits an extremely large and unsaturated magnetoresistance of up to 40,000 % at 2 K, which originates from an almost perfect electron-hole compensation. Our band structure calculations further show that ZrP$_2$ hosts a topological nodal loop in proximity to the Fermi level. Based on the ARPES measurements, we confirm the results of our calculations and determine the surface band structure. Our study establishes ZrP$_2$ as a new platform to investigate near-perfect electron-hole compensation and its interplay with topological band structures.

Quantum Oscillations and Electronic Structure in the Large-Chern-Number Topological Chiral Semimetal PtGa*Supported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0308602, 2018YFA0305700 and 2016YFA0300600), the National Natural Science Foundation of China (Grant Nos. 11874422 and 11574391), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (Grant Nos. 19XNLG18

We report the magnetoresistance (MR), de Haas-van Alphen (dHvA) oscillations and the electronic structures of single-crystal PtGa. The large unsaturated MR is observed with the magnetic field B ∥ [111]. Evident dHvA oscillations with the B ∥ [001] configuration are observed, from which twelve fundamental frequencies are extracted and the spin-orbit coupling (SOC) induced band splitting is revealed. The light cyclotron effective masses are extracted from the fitting by the thermal damping term of the Lifshitz–Kosevich formula. Combining with the calculated frequencies from the first-principles calculations, the dHvA frequencies F1/F3 and F11/F12 are confirmed to originate from the electron pockets at Γ and R, respectively. The first-principles calculations also reveal the existence of spin-3/2 Rarita–Schwinger–Weyl fermions and time-reversal doubling of the spin-1 excitation at Γ and R with large Chern numbers of ± 4 when SOC is included.

Large magnetoresistance in topological insulator candidate TaSe3

Large unsaturated magnetoresistance (XMR) with magnitude ∼103% is observed in topological insulator candidate TaSe3 from our high field (up to 38 T) measurements. Two oscillation modes, associated with one hole pocket and two electron pockets in the bulk, respectively, are detected from our Shubnikov-de Hass measurements, consistent with our first-principles calculations. With the detailed Hall measurements performed, our two-band model analysis exhibits an imperfect density ratio nh/ne ≈ 0.9 at T &amp;lt; 20 K, which suggests that the carrier compensations account for the XMR in TaSe3.

Effect of annealing on magnetic and magnetoresistance characteristics of Gd/CoFeTaB multilayer film

By using the magnetron sputtering, a Gd/CoFeTaB multilayer structure film was fabricated, and investigated the effect of annealing temperature on magnetic and magnetoresistance characteristics of the multilayer film. A magnetic compensation phenomenon was observed, which means there is an antiferromagnetic coupling in the Gd/CoFeTaB system. It was found that the annealing enhances the antiferromagnetic coupling, lead to increase the Gilbert damping coefficient. Furthermore, an unsaturated magnetoresistance effect was observed, which is explained by the electron-electron interaction in the system. It is expected that the artificial antiferromagnetic multilayer system with weak macro-magnetism and internal magnetic order will has a good application prospect for the development of spintronics devices.

Signature of Dirac semimetal states in gray arsenic studied by de Haas–van Alphen and Shubnikov–de Haas quantum oscillations

We report systematically studies of de Haas--van Alphen (dHvA) and Shubnikov--de Haas (SdH) quantum oscillations under high magnetic fields on gray arsenic, which has been suggested to be a topological insulator by first-principles calculations and angle-resolved photoemission spectroscopy. We observed large unsaturated magnetoresistance and pronounced quantum oscillations in two methods. By analysis of these quantum oscillation spectra, we obtained two major frequency branches around 3.8--18 and 153--162 T, and the light effective quasiparticle mass and nontrivial Berry phase. These results suggest that bulk crystals of gray arsenic may belong to the category of topological Dirac semimetals rather than topological insulators.

Negative differential resistance and unsaturated magnetoresistance effects based on avalanche breakdown

The negative differential resistance (NDR) effect and magnetoresistance (MR) effect attract a lot of attention since they have been widely applied in fields such as circuit amplifiers and information storage, respectively. In general, the NDR and MR effects derive from different physical mechanisms, which makes it difficult to obtain both an NDR effect and a large unsaturated MR effect in a device based on the same physical mechanism. In this paper, the NDR and unsaturated MR effects were observed simultaneously in In/SiO2/p-Si/SiO2/In hetero junction devices, and their physical mechanisms were investigated. It is found that under zero magnetic field conditions, the NDR effect can be observed up to 25 K which is much higher than the temperature of NDR obtaining in intrinsic silicon. The NDR effect is significantly enhanced by applying a small magnetic field of 0.1 T. Meanwhile, the unsaturated positive MR effect is obtained when an external magnetic field is applied. Our analysis shows that the NDR effect and the unsaturated MR effect can be explained on the basis of the same physical mechanism of the avalanche breakdown in the In/SiO2/p-Si hetero junction. The NDR effect mainly results from complex effects containing the carrier injection after avalanche breakdown in the hetero junction, the reduction of the Schottky barrier height and the Joule heating effect. The large unsaturated MR effect derives from the suppression of the plasma due to avalanche breakdown by the applied magnetic field. The largest value of MR based on avalanche breakdown can reach about 530%@1 T at 10 K, and MR sensitivity can be as high as about 29.4 T−1 under 0.05 T.

Quantum oscillations and electronic structure in the large–Chern number semimetal RhSn

We report the magnetoresistance, Hall effect, de Haas-van Alphen (dHvA) oscillations and the electronic structures of single crystal RhSn, which is a typical material of CoSi family holding a large Chern number. The large unsaturated magnetoresistance is observed with B//[001]. The Hall resistivity curve indicates that RhSn is a multi-band system with high mobility. Evident quantum oscillations have been observed, from which the light effective masses are extracted. Ten fundamental frequencies are extracted after the fast Fourier transform analysis of the dHvA oscillations with B//[001] configuration. The two low frequencies F$_1$ and F$_2$ do not change obviously and the two high frequencies F$_9$ and F$_{10}$ evolve into four when B rotates from B//[001] to B//[110], which is consistent with the band structure in the first-principles calculations with spin-orbit coupling (SOC). The extracted Berry phases of the relative pockets show a good agreement with the Chern number $\pm4$ (with SOC) in the first-principles calculations. Above all, our studies indicate that RhSn is an ideal platform to study the unconventional chiral fermions and the surface states.