We report a 51 V nuclear magnetic resonance investigation of the frustrated spin-1/2 chain compound LiCuVO4, performed in pulsed magnetic fields and focused on high-field phases up to 55 T. For the crystal orientations H c and H b we find a narrow field region just below the magnetic saturation where the local magnetization remains uniform and homogeneous, while its value is field dependent. This behavior is the first microscopic signature of the spin-nematic state, breaking spin-rotation symmetry without generating any transverse dipolar order, and is consistent with theoretical predictions for the LiCuVO4 compound.PACS numbers: 75.10. Kt, 75.30.Kz, The search for new states of quantum matter is one of the most active research fields in condensed-matter physics. In this respect frustrated magnetic systems attract a lot of interest as they accommodate various unconventional quantum states, having no direct classical analogues, ordered and disordered, induced by the competing interactions [1]. One particularly interesting state is the spin-nematic phase, in which the quantum magnet behaves like a liquid crystal. Taking an external magnetic field H as the reference direction, a spin-nematic phase is defined as a state without any transverse dipolar (i.e., vector-type) order, (−1) i S + i + H.c. = 0, but possessing instead a transverse quadrupolar (tensor-type) order,The quadrupolar order parameter develops on the bonds between neighboring spins and can be described as a condensate of two-magnon pairs. It breaks the spin-rotational symmetry about the magnetic field, but only partially as π-rotations transform the order parameter into itself. The also broken translational symmetry of the order parameter is invisible in the dipolar channel. There is also an analogy between the spin-nematic phase and the superconducting state: the nematic phase can be considered as a "bosonic" superconductor formed as a result of two-magnon condensation [1,2].The concept of a spin-nematic state was developed by Andreev and Grishchuk more than 30 years ago [3], which incited intense search for a realization in real materials. However, a definite experimental proof for the existence of such a phase has not been provided yet. Several magnetic insulators have been proposed as possible candidates, including the two-dimensional magnet NiGa 2 S 4 (spin-1 system) [4-6] and thin films of 3 He [7][8][9].In the past 10 years a large number of theoretical studies have supported the formation of the spinnematic phase in frustrated zig-zag 1D (chain) systems [10][11][12][13][14]. Amongst these, orthorhombic LiCuVO 4 is one of the most promising candidates [15,16]. It consists of spin-1/2 Cu 2+ chains along the orthorhombic b axis with a dominant nearest-neighbor ferromagnetic interaction J 1 = −1.6 meV, a frustrated next-nearest-neighbor antiferromagnetic interaction J 2 = 3.8 meV, and an interchain coupling J = −0.4 meV [17,18]. At zero magnetic field an incommensurate planar spiral structure is realized below T N = 2.3 K, having the moments lying...

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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.

We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo2V2O8 as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below TN ≃ 5.5 K. In a field-induced gapless phase above Bc = 3.8 T, we identify many-body string excitations as well as low-energy fractional psinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In the vicinity of Bc, the high-energy string excitations are found to be dynamically dominant over the fractional excitations.

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.