No abstract

The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe5 samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe5 electronic structure and its Dirac-type semi-metallic character.

Magnetochiral effect (MChE) of phonons, a nonreciprocal acoustic property arising due to the symmetry principles, is demonstrated in a chiral-lattice ferrimagnet Cu2OSeO3. Our high-resolution ultrasound experiments reveal that the sound velocity differs for parallel and antiparallel propagation with respect to the external magnetic field. The sign of the nonreciprocity depends on the chirality of the crystal in accordance with the selection rule of the MChE. The nonreciprocity is enhanced below the magnetic ordering temperature and at higher ultrasound frequencies, which is quantitatively explained by a proposed magnon-phonon hybridization mechanism.

No abstract

The magnetic-field and temperature dependencies of ultrasound propagation and magnetization of single-crystalline CoCr2O4 have been studied in static and pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies with significant changes in the sound velocity and attenuation are found in this spinel compound at the onset of long-range incommensurate spiral-spin order at Ts = 27 K and at the transition from the incommensurate to the commensurate state at T l = 14 K, evidencing strong spin-lattice coupling. While the magnetization evolves gradually with field, steplike increments in the ultrasound clearly signal a transition into a new magneto-structural state between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a high-symmetry phase with only the longitudinal component of the magnetization being ordered, while the transverse helical component remains disordered. This phase is metastable in an extended H-T phase space. Multiferroic materials, which exhibit concomitant magnetic and ferroelectric order, are of great current interest, both from a fundamental as well as applicationoriented view. They challenge our understanding of ordering phenomena, but in addition provide new functionalities in spintronics, since these dielectric and magnetic polarizations can be tuned either by external magnetic or electric fields [1][2][3][4]. Among multiferroics magnetic AB 2 X 4 compounds with spinel structure attracted considerable interest revealing colossal magnetocapacitance and spontaneous dielectric polarization in the magnetically ordered state [5][6][7][8][9]. The appearance of dielectric polarization is associated either with a non-collinear arrangement of spins, with charge order, or with magnetic ions moving off-center from their symmetric site positions in the lattice due to strong magneto-elastic effects.Significant spin-lattice coupling and magnetic frustration are important features of spinels, specifically, of chromium oxides and chalcogenides. Despite quenched orbital moments these compounds reveal structural instabilities which are governed explicitly by ordering of spins, e.g., giant magnetostriction, negative thermal expansion [10], and spin Jahn-Teller instabilities [11][12][13][14][15]. In Cr spinels with only one magnetic sublattice, where the Cr 3+ ions are located solely on the pyrochlore lattice of the B sites, it is well known that the oxides with dominating antiferromagnetic (AFM) exchange reveal strong geometrical frustration, while in the sulfides and selenides ferromagnetic (FM) exchange becomes important. ZnCr 2 S 4 and ZnCr 2 Se 4 are strongly frustrated due to competing AFM and FM interactions, with the ground state still being antiferromagnetic [16]. At low temperatures, the geometrically frustrated oxide spinels exhibit magnetization plateaus as function of an external magnetic field, with a 3-up 1-down spin configuration [17][18][19]. Theoretically, it has been suggested that these plateaus are stabilized by lattice distortions [20]. Indeed, in high-fiel...

We report on magnetization, sound-velocity, and magnetocaloric-effect measurements of the Ising-like spin-1/2 antiferromagnetic chain system BaCo_{2}V_{2}O_{8} as a function of temperature down to 1.3 K and an applied transverse magnetic field up to 60 T. While across the Néel temperature of T_{N}∼5  K anomalies in magnetization and sound velocity confirm the antiferromagnetic ordering transition, at the lowest temperature the field-dependent measurements reveal a sharp softening of sound velocity v(B) and a clear minimum of temperature T(B) at B_{⊥}^{c,3D}=21.4  T, indicating the suppression of the antiferromagnetic order. At higher fields, the T(B) curve shows a broad minimum at B_{⊥}^{c}=40  T, accompanied by a broad minimum in the sound velocity and a saturationlike magnetization. These features signal a quantum phase transition, which is further characterized by the divergent behavior of the Grüneisen parameter Γ_{B}∝(B-B_{⊥}^{c})^{-1}. By contrast, around the critical field, the Grüneisen parameter converges as temperature decreases, pointing to a quantum critical point of the one-dimensional transverse-field Ising model.

High magnetic field studies in MnCr2S4 reveal an extended magnetization plateau and the realization of supersolid phase.

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.