arXiv subjects

Mazhar N. Ali

Publications and source records attributed to Mazhar N. Ali.

At least 37 records · Page 2Linked to original sources

arXiv2020

Hard Magnetic Topological Semimetals in XPt3: Harmony of Berry Curvature

Topological magnetic semimetals, like Co3Sn2S2 and Co2MnGa, are known to display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt3 compounds display anti-crossing gapped nodal lines, which are a driving mechanism in the intrinsic Berry curvature Hall effects. Uniquely, these compounds contain two sets of gapped nodal lines that harmoniously dominate the Berry curvature in this complex multiband system. We calculate a maximum AHE of 1965 S/cm in the CrPt3 by a state-of-the-art first principle electronic structure. We have grown high-quality thin films by magnetron sputtering and measured a robust AHE of 1750 S/cm for different sputtering growth conditions. Additionally, the cubic films display a hard magnetic axis along [111] direction. The facile and scalable fabrication of these materials is prime candidates for integration into topological devices.

cond-mat.mtrl-sci↗

arXiv2020

Fermi Surface Geometry

Motivated by the famous and pioneering mathematical works by Perelman, Hamilton, and Thurston, we introduce the concept of using modern geometrical mathematical classifications of multi-dimensional manifolds to characterize electronic structures and predict non-trivial electron transport phenomena. Here we develop the Fermi Surface Geometry Effect (FSGE), using the concepts of tangent bundles and Gaussian curvature as an invariant. We develop an index, $\mathbb{H}_F$, for describing the the "hyperbolicity" of the Fermi Surface (FS) and show a universal correlation (R$^2$ = 0.97) with the experimentally measured intrinsic anomalous Hall effect of 16 different compounds spanning a wide variety of crystal, chemical, and electronic structure families, including where current methods have struggled. This work lays the foundation for developing a complete theory of geometrical understanding of electronic (and by extension magnonic and phononic) structure manifolds, beginning with Fermi surfaces. In analogy to the broad impact of topological physics, the concepts begun here will have far reaching consequences and lead to a paradigm shift in the understanding of electron transport, moving it to include geometrical properties of the E vs k manifold as well as topological properties.

cond-mat.str-el↗

arXiv2020

Doping-induced spin Hall ratio enhancement in A15-phase, Ta-doped Beta-W thin films

As spintronic devices become more and more prevalent, the desire to find Pt free materials with large spin Hall effects is increasing. Previously it was shown that Beta W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that Beta W(Ta) alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase Beta W films doped with Ta (W(4-x)Tax where x is between 0.28 and 0.4, deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was approx. 9 percent larger than pure Beta W films. We also found that the SHRs in devices with Co2Fe6B2 were nearly twice as large as the SHRs in devices with Co4Fe4B2. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W3Ta alloy should be feasible, opening the door to commercially viable, Pt free, spintronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous, anomalous Hall effect in the layered, Kagome, Dirac semimetal KV$_3$Sb$_5$

The electronic anomalous Hall effect (AHE), where charge carriers acquire a velocity component orthogonal to an applied electric field, is one of the most fundamental and widely studied phenomena in physics. There are several different AHE mechanisms known, and material examples are highly sought after, however in the highly conductive (skew scattering) regime the focus has centered around ferromagnetic metals. Here we report the observation of a giant extrinsic AHE in KV$_3$Sb$_5$, an exfoliable, Dirac semimetal with a Kagome layer of Vanadium atoms. Although there has been no reports of magnetic ordering down to 0.25 K, the anomalous Hall conductivity (AHC) reaches $\approx$ 15,507 $Ω^{-1}$cm$^{-1}$ with an anomalous Hall ratio (AHR) of $\approx$ 1.8$ \%$; an order of magnitude larger than Fe. Defying expectations from skew scattering theory, KV$_3$Sb$_5$ shows an enhanced skew scattering effect that scales quadratically, not linearly, with the longitudinal conductivity ($σ_{xx}$), opening the possibility of reaching an anomalous Hall angle (AHA) of 90$^{\circ}$ in metals; an effect thought reserved for quantum anomalous Hall insulators. This observation raises fundamental questions about the AHE and opens a new frontier for AHE (and correspondingly SHE) exploration, stimulating investigation in a new direction of materials, including metallic geometrically frustrated magnets, spin-liquid candidates, and cluster magnets.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous thickness-dependent electrical conductivity in van der Waals layered transition metal halide, Nb_3Cl_8

Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 μm to 5 nm, at 300 K. At low temperatures and below ~50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed.

cond-mat.mtrl-sci↗

arXiv2019

Evidence of Higher Order Topology in Multilayer WTe$_2$ from Josephson Coupling through Anisotropic Hinge States

The noncentrosymmetric Td-WTe$_2$, previously known as a type-II Weyl semimetal, is expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when their scarcely separated Weyl points get annihilated. However, the detection of these hinge states is difficult in the presence of the semimetallic behaviour of the bulk. Here, we spatially resolved the hinge states by analysing the magnetic field interference of supercurrent in Nb-WTe$_2$-Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe$_2$ crystal, but not along the b-axis, showed sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D conduction channel. Our experimental observations provide evidence of the higher order topological phase in WTe$_2$ and its corresponding anisotropic topological hinge states, in good agreement with theoretical calculations. Our work paves the way for hinge transport studies on topological semimetals in superconducting heterostructures, including their topological superconductivity.

cond-mat.mes-hall↗

arXiv2019

A Proposal to Detect Dark Matter Using Axionic Topological Antiferromagnets

Antiferromagnetically doped topological insulators (A-TI) are among the candidates to host dynamical axion fields and axion-polaritons; weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle antiferromagnetic resonance in A-TI's in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with mass 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals. The benefits of this method at high frequency are the tunability of the resonance with applied magnetic field, and the use of A-TI samples with volumes much larger than 1 mm$^3$.

hep-ph↗

arXiv2017

Symmetry Demanded Topological Nodal-line Materials

The realization of Dirac and Weyl physics in solids has made topological materials one of the main focuses of condensed matter physics. Recently, the topic of topological nodal line semimetals, materials in which Dirac or Weyl-like crossings along special lines in momentum space create either a closed ring or line of degeneracies, rather than discrete points, has become a hot topic in topological quantum matter. Here we review the experimentally confirmed and theoretically predicted topological nodal line semimetals, focusing in particular on the symmetry protection mechanisms of the nodal lines in various materials. Three different mechanisms: a combination of inversion and time-reversal symmetry, mirror reflection symmetry, and non-symmorphic symmetry, and their robustness under the effect of spin orbit coupling are discussed. We also present a new Weyl nodal line material, the Te-square net compound KCu$_2$EuTe$_4$, which has several Weyl nodal lines including one extremely close to the Fermi level ($<$30 meV below E$_F$). Finally, we discuss potential experimental signatures for observing exotic properties of nodal line physics.

cond-mat.mtrl-sci↗

arXiv2016

Butterfly Magnetoresistance, Quasi-2D Dirac Fermi Surfaces, and a Topological Phase Transition in ZrSiS

Magnetoresistance (MR), the change of a material's electrical resistance in response to an applied magnetic field, is a technologically important property that has been the topic of intense study for more than a quarter century. Here we report the observation of an unusual "butterfly" shaped titanic angular magnetoresistance (AMR) in the non-magnetic, Dirac material, ZrSiS. The MR is large and positive, reaching nearly 1.8 x 10^5 percent at 9 T and 2 K at an angle of 45o between the applied current (along the a-axis) and the applied field (90o is H parallel to the c-axis). Approaching 90o, a "dip" is seen in the AMR which can be traced to an angle dependent deviation from the H^2 law. By analyzing the SdH oscillations at different angles, we find that ZrSiS has a combination of 2D and 3D Dirac pockets comprising its Fermi surface and that the anomalous transport behavior coincides with a topological phase transition whose robust signature is evident despite transport contributions from other parts of the Fermi surface. We also find that as a function of angle, the temperature dependent resistivity in high field displays a broad peak-like behavior, unlike any known Dirac/Weyl material. The combination of very high mobility carriers and multiple Fermi surfaces in ZrSiS allow for large bulk property changes to occur as a function of angle between applied fields makes it a promising platform to study the physics stemming from the coexistence of 2D and 3D Dirac electrons.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in Weyl Semimetal Candidate MoTe2

In recent years, layered transition-metal dichalcogenides (TMDs) have attracted considerable attention because of their rich physics; for example, these materials exhibit superconductivity, charge density waves, and the valley Hall effect. As a result, TMDs have promising potential applications in electronics, catalysis, and spintronics. Despite the fact that the majority of related research focuses on semiconducting TMDs (e.g., MoS2), the characteristics of WTe2 are provoking strong interest in semimetallic TMDs with extremely large magnetoresistance, pressure-driven superconductivity, and the predicted Weyl semimetal (WSM) state. In this work, we investigate the sister compound of WTe2, MoTe2, which is also predicted to be a WSM and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that MoTe2 exhibits superconductivity with a resistive transition temperature Tc of 0.1 K. The application of a small pressure (such as 0.4 GPa) is shown to dramatically enhance the Tc, with a maximum value of 8.2 K being obtained at 11.7 GPa (a more than 80-fold increase in Tc). This yields a dome-shaped superconducting phase diagram. Further explorations into the nature of the superconductivity in this system may provide insights into the interplay between strong correlations and topological physics.

cond-mat.mtrl-sci↗

arXiv2015

Prediction of the Weyl semimetal in the orthorhombic MoTe2

We investigate the orthorhombic phase (Td) of layered transition-metal-dichalcogenide MoTe$_2$ as a Weyl semimetal candidate, which was discovered to be a superconductor in our recent experiment. MoTe$_2$ exhibits four pairs of Weyl points lying slightly ($\sim$ 6 meV) above the Fermi energy in the bulk band structure. Unlike its cousin WTe$_2$, which was predicted to be a type-II Weyl semimetal recently, the spacing between each pair of Weyl points is found to be as large as 4 percent of the reciprocal lattice in MoTe$_2$ (six times larger than that of WTe$_2$). When projected to the surface, Weyl points are connected by Fermi arcs, which can be easily verified by ARPES due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive MoTe$_2$ from the type-II to type-I Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2015

Dirac Cone Protected by Non-Symmorphic Symmetry and 3D Dirac Line Node in ZrSiS

Materials harboring exotic quasiparticles, such as Dirac and Weyl fermions\cite{xu2015discovery,borisenko2015time,weng2015weyl,xu2015observation}, have garnered much attention from the physics and material science communities. These fermions are massless and, in some materials, have shown exceptional physical properties such as ultrahigh mobility and extremely large magnetoresistances \cite{liang2015ultrahigh,ali2014large,du2015unsaturated,shekhar2015large}. Recently, new materials have been predicted to exist which exhibit line nodes of Dirac cones \cite{PhysRevLett.115.036806,xie2015new,burkov2011topological,rhim2015landau}. Here, we show with angle resolved photoemission studies supported by \textit{ab initio} calculations that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones which form a Fermi surface with a diamond-shaped line of Dirac nodes. We also experimentally show, for the first time, that the square Si lattice in ZrSiS is an excellent template for realizing the new types of 2D Dirac cones recently predicted by Young and Kane \cite{young2015dirac} and image an unforseen surface state that arises close to the 2D Dirac cone. Finally, we find that the energy range of the linearly dispersed bands is as high as 2\,eV above and below the Fermi level; much larger than of any known Dirac material so far. This makes ZrSiS a very promising candidate to study the exotic behavior of Dirac electrons, or Weyl fermions if a magnetic field is applied, as well as the properties of lines of Dirac nodes

cond-mat.mtrl-sci↗

arXiv2015

Breakdown of Three-dimensional Dirac Semimetal State in pressurized Cd3As2

We report the first observation of a pressure-induced breakdown of the 3D-DSM state in Cd3As2, evidenced by a series of in-situ high-pressure synchrotron X-ray diffraction (XRD) and single crystal transport measurements. We find that Cd3As2 undergoes a structural phase transition from a metallic tetragonal (T) phase in space group I41/acd to a semiconducting monoclinic (M) phase in space group P21/c at critical pressure 2.57 GPa, above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3D-DSM state in pressurized Cd3As2.

cond-mat.supr-con↗

arXiv2015

Correlation of Crystal Quality and Extreme Magnetoresistance of WTe$_2$

High quality single crystals of WTe$_2$ were grown using a Te flux followed by a cleaning step involving self-vapor transport. The method is reproducible and yields consistently higher quality single crystals than are typically obtained via halide assisted vapor transport methods. Magnetoresistance (MR)values at 9 Tesla and 2 Kelvin as high as 1.75 million \%, nearly an order of magnitude higher than previously reported for this material, were obtained on crystals with residual resistivity ratio (RRR) of approximately 1250. The MR follows a near B$^2$ law (B = 1.95(1)) and, assuming a semiclassical model, the average carrier mobility for the highest quality crystal was found to be ~167,000 cm$^2$/Vs at 2 K. A correlation of RRR, MR ratio and average carrier mobility ($μ_{avg}$) is found with the cooling rate during the flux growth.

cond-mat.mtrl-sci↗

arXiv2014

Z$_2$ topology and superconductivity from symmetry lowering of a 3D Dirac Metal Au$_2$Pb

3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties It has been suggested that structurally distorting a DSM can create a Topological Insulator (TI), but this has not yet been experimentally verified. Furthermore, quasiparticle excitations known as Majorana Fermions have been theoretically proposed to exist in materials that exhibit superconductivity and topological surface states. Here we show that the cubic Laves phase Au$_2$Pb has a bulk Dirac cone above 100 K that gaps out upon cooling at a structural phase transition to create a topologically non trivial phase that superconducts below 1.2 K. The nontrivial Z$_2$ = -1 invariant in the low temperature phase indicates that Au$_2$Pb in its superconducting state must have topological surface states. These characteristics make Au$_2$Pb a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states.

cond-mat.supr-con↗

arXiv2014

Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures

The electronic properties of two-dimensional materials such as graphene are extremely sensitive to their environment, especially the underlying substrate. Planar van der Waals bonded substrates such as hexagonal boron nitride (hBN) have been shown to greatly improve the electrical performance of graphene devices by reducing topographic variations and charge fluctuations compared to amorphous insulating substrates}. Semiconducting transition metal dichalchogenides (TMDs) are another family of van der Waals bonded materials that have recently received interest as alternative substrates to hBN for graphene as well as for components in novel graphene-based device heterostructures. Additionally, their semiconducting nature permits dynamic gate voltage control over the interaction strength with graphene. Through local scanning probe measurements we find that crystalline defects intrinsic to TMDs induce scattering in graphene which results in significant degradation of the heterostructure quality, particularly compared to similar graphene on hBN devices.

cond-mat.mtrl-sci↗

arXiv2014

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd$_3$As$_2$

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na$_3$Bi and Cd$_3$As$_2$ were predicted to be Dirac semimetals [4,5], and recently confirmed to be so by photoemission [6-8]. Several novel transport properties in a magnetic field $\bf H$ have been proposed for Dirac semimetals [2,9-11]. Here we report an interesting property in Cd$_3$As$_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero $\bf H$. In single crystals, the protection results in a very high mobility that exceeds $>10^7$ cm$^2$/Vs below 4 K. Suppression of backscattering results in a transport lifetime 10$^4\times$ longer than the quantum lifetime. The lifting of this protection by $\bf H$ leads to an unusual giant $\bf H$-linear magnetoresistance that violates Kohler's rule. We discuss how this may relate to changes to the Fermi surface induced by $\bf H$.

cond-mat.str-el↗

arXiv2014

Titanic Magnetoresistance in WTe2

Magnetoresistance is the change of a material's electrical resistance in response to an applied magnetic field. In addition to its intrinsic scientific interest, it is a technologically important property, placing it in "Pasteur's quadrant" of research value: materials with large magnetorsistance have found use as magnetic sensors 1, in magnetic memory 2, hard drives 3, transistors 4, and are the subject of frequent study in the field of spintronics 5, 6. Here we report the observation of an extremely large one-dimensional positive magnetoresistance (XMR) in the layered transition metal dichalcogenide (TMD) WTe2; 452,700 percent at 4.5 Kelvin in a magnetic field of 14.7 Tesla, and 2.5 million percent at 0.4 Kelvin in 45 Tesla, with no saturation. The XMR is highly anisotropic, maximized in the crystallographic direction where small pockets of holes and electrons are found in the electronic structure. The determination of the origin of this effect and the fabrication of nanostructures and devices based on the XMR of WTe2 will represent a significant new direction in the study and uses of magnetoresistivity. *The published version of the paper includes co-authors Tian Liang and Max Hirschberger. **This paper has been published with new MR data to 60T where the MR of WTe2 reaches 13 million percent (at 0.5K) and still shows no signs of saturation. We also have new electron diffraction patterns to lower temperature (10K). We discuss the possible origin of the MR as coming from an electron-hole 'resonance' condition established by a perfect n/p ratio of 1 (more details in a new "extended data" section). This makes WTe2, possibly, the first realization of a perfectly balanced semimetal. ***The paper is published as "Large non-saturating magnetoresistance in WTe2" in Nature (2014), DOI:10.1038/nature13763

cond-mat.mtrl-sci↗