Whereas the discovery of Dirac-and Weyl-type excitations in electronic systems is a major breakthrough in recent condensed matter physics, finding appropriate materials for fundamental physics and technological applications is an experimental challenge. In all of the reported materials, linear dispersion survives only up to a few hundred millielectronvolts from the Dirac or Weyl nodes. On the other hand, real materials are subject to uncontrolled doping during preparation and thermal effect near room temperature can hinder the rich physics. In ZrSiS, angle-resolved photoemission spectroscopy measurements have shown an unusually robust linear dispersion (up to ∼2 eV) with multiple nondegenerate Dirac nodes. In this context, we present the magnetotransport study on ZrSiS crystal, which represents a large family of materials (WHM with W = Zr, Hf; H = Si, Ge, Sn; M = O, S, Se, Te) with identical band topology. Along with extremely large and nonsaturating magnetoresistance (MR), ∼1.4 × 10 5 % at 2 K and 9 T, it shows strong anisotropy, depending on the direction of the magnetic field. Quantum oscillation and Hall effect measurements have revealed large hole and small electron Fermi pockets. A nontrivial π Berry phase confirms the Dirac fermionic nature for both types of charge carriers. The long-sought relativistic phenomenon of massless Dirac fermions, known as the Adler-Bell-Jackiw chiral anomaly, has also been observed.Dirac semimetal | extreme magnetoresistance | chiral anomaly | quantum oscillation | Fermi surface T he discovery of topological insulators (1) and 3D Dirac and Weyl semimetals (2, 3) has emerged as one of the major breakthroughs in condensed matter physics in recent time. Materials with topologically nontrivial band structure serve as a template to explore the quantum dynamics of relativistic particles in low-energy condensed matter systems. In addition to rich physics, these systems offer the possibility of practical applications in magnetic memory, magnetic sensor, or switch and spintronics, due to the novel transport phenomena such as extreme magnetoresistance and ultrahigh mobility (4-6). In Dirac semimetals, bulk valence and conduction bands undergo linear band crossings at fourfold degenerate Dirac points protected by timereversal symmetry (TRS), inversion symmetry (IS), and crystal symmetry (CS) (7,8). By breaking either TRS or IS, each Dirac point can be broken into a pair of doubly degenerate Weyl points, accompanied by the surface Fermi arc (7,8). Theoretical prediction (7, 8) followed by angle-resolved photoemission spectroscopy (ARPES) and transport measurements have verified the existence of bulk Dirac points in Cd3As2 and Na3Bi (2, 9-11) and Weyl nodes in the IS-breaking TX (T = Ta, Nb; X = As, P) family of materials (3,(12)(13)(14) and TRS-breaking YbMnBi2 (15). Apart from these compounds, recently, topological nodal line semimetals (TNLSM) have emerged, where the bands cross along one-dimensional closed lines in k space instead of discrete points. Although proposed in few materials ...
We have systematically investigated the magnetic properties and magnetocaloric effect (MCE) in RMnO3 (R=Dy, Tb, Ho and Yb) single crystals. Above a critical value of applied field (Hc), RMnO3 undergo a first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition below the ordering temperature (T R N ) of R 3+ moment and a second-order FM to paramagnetic (PM) transition above T R N . Both H and T dependence of M shows that the system is highly anisotropic in the FM as well as PM states and, as a result, the magnetic entropy change (∆SM ) is extremely sensitive to the direction of applied field and can be negative (normal MCE) or positive (inverse MCE). For hexagonal HoMnO3 and YbMnO3 systems, a very small inverse MCE is observed only for H parallel to c axis and it decreases with increasing H and crosses over to normal one above Hc. On the other hand, for orthorhombic DyMnO3 and TbMnO3, though the inverse MCE disappears above Hc along easy-axis of magnetization, it increases rapidly with H along hard-axis of magnetization for T T R N . Except for YbMnO3, the values of ∆SM , relative cooling power and adiabatic temperature change along easy-axis of magnetization are quite large in the field-induced FM state for a moderate field strength. The large values of these parameters, together with negligible hysteresis, suggest that the multiferroic manganites could be potential materials for magnetic refrigeration in the low-temperature region.
In Ce-doped RMnO 3 the rare earth (R) is partially replaced with cerium ions instead of divalent alkalineearth metals. These compounds are very sensitive to annealing. The resistivity and thermoelectric power ͑TEP, S) of R 0.7 Ce 0.3 MnO 3 ͑RϭLa, Pr, and Nd͒ have been studied as a function of annealing conditions. They show a ferromagnetic to paramagnetic transition, and a resistive peak near T c , marking a metal-insulator ͑MI͒ transition. A pronounced giant magnetoresistance effect is also observed in these compounds, and T c can be changed by annealing in various atmospheres. The (T) curve of oxygen overdoped samples shows double peaks along with a hysteresis loop ͑in the heating and cooling cycles͒, which bears the signature of the first-order nature of the transition. The thermopower of as-prepared samples show a peak near the MI transition, and changes sign from positive to negative at temperatures below the ferromagnetic transition. The magnitude of S at low temperatures is enhanced to an abnormally large value. With increasing of oxygen content, several characteristic features develop in the temperature dependence of the thermopower which is consistent with the resistive behavior. At high temperatures, the resistivity and thermopower follow the predictions of the Emin-Holstein theory of adiabatic polaron hopping.
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