Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5

The quantum limit (QL) of an electron liquid, realised at strong magnetic fields, has long been proposed to host a wealth of strongly correlated states of matter. Electronic states in the QL are, for example, quasi-one dimensional (1D), which implies perfectly nested Fermi surfaces prone to instabilities. Whereas the QL typically requires unreachably strong magnetic fields, the topological semimetal ZrTe5 has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe5 at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe5 enables an efficient tuning of the 1D Landau band structure with magnetic field. This results in a Lifshitz transition to a 1D Weyl regime in which perfect charge neutrality can be achieved. Since no instability-driven phase transitions destabilise the 1D electron liquid for the investigated field strengths and temperatures, our analysis establishes ZrTe5 as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures

Unconventional Hall response in the quantum limit of HfTe5

Interacting electrons confined to their lowest Landau level in a high magnetic field can form a variety of correlated states, some of which manifest themselves in a Hall effect. Although such states have been predicted to occur in three-dimensional semimetals, a corresponding Hall response has not yet been experimentally observed. Here, we report the observation of an unconventional Hall response in the quantum limit of the bulk semimetal HfTe5, adjacent to the three-dimensional quantum Hall effect of a single electron band at low magnetic fields. The additional plateau-like feature in the Hall conductivity of the lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the longitudinal electrical resistivity and its magnitude relates as 3/5 to the height of the last plateau of the three-dimensional quantum Hall effect. Our findings are consistent with strong electron-electron interactions, stabilizing an unconventional variant of the Hall effect in a three-dimensional material in the quantum limit

Observation of a three-dimensional fractional Hall response in HfTe5

Interacting electrons in two dimensions can bind magnetic flux lines to form composite quasiparticles with fractional electric charge, manifesting themselves in the fractional quantum Hall effect (FQHE). Although the FQHE has also been predicted to occur in three dimensions, it has not yet been experimentally observed. Here, we report the observation of fractional plateaus in the Hall conductivity of the bulk semimetal HfTe5 at magnetic fields beyond the quantum limit. The plateaus are accompanied by Shubnikov-de Haas minima of the longitudinal electrical resistivity. The height of the Hall plateaus is given by twice the Fermi wave vector in the direction of the applied magnetic field and scales with integer and particular fractional multiples of the conductance quantum. Our findings are consistent with strong electron-electron interactions, stabilizing a fractionalized variant of the Hall effect in three dimensions.Comment: 35 pages with 17 figure

Magnetic order and single-ion anisotropy in Tb3_3Ga5_5O12_{12}

Terbium gallium garnet (TGG), Tb$_3$Ga$_5$O$_{12}$, is well known for its applications in laser optics but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic Tb$^{3+}$ ions forming six sublattices of magnetic moments aligned parallel and antiparallel to the ⟨100⟩ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a twofold axis of symmetry in the local orthorhombic environment of the Tb$^{3+}$ sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetization measurements are in good agreement with values based on the crystal-field model down to 20 K, where the onset of magnetic correlations is observed

Noncollinear magnetic order in epitaxial thin films of the centrosymmetric MnPtGa hard magnet

Magnetic systems exhibiting spin-canted states have garnered much attention recently for their promising rich exotic properties driven by the real-space spin textures and competing magnetic orders. In this study, we present the structural and magnetic properties of hexagonal 60 nm MnPtGa epitaxial thin films grown by magnetron sputtering on Al2O3(0001) single-crystalline substrates. The MnPtGa film crystallizes in the centrosymmetric P6(3)/mmc (No. 194) space group, showing perpendicular magnetic anisotropy along the c-axis, with a Curie temperature T-C = 263 K. In addition, the MnPtGa film undergoes a spin reorientation transition at T-sr = 160 K. We investigated the MnPtGa magnetic ground states using single-crystal neutron diffraction. A structurally forbidden (001) magnetic Bragg reflection emerges below T-sr, indicating the existence of a spin-canted state, where the magnetic moments align ferromagnetically perpendicular to the basal plane, and a non-zero in-plane component exhibits an antiferromagnetic ordering along the c-axis. At 2 K, the refined magnetic moments of Mn are mu(z) = 4.2(4) mu(B) and mu(x) = 1.5(3) mu(B), projected onto the c-axis and basal plane, respectively. Hence, we determined a 20 degrees Mn spin canting angle off from the c-axis

Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5_5

The quantum limit (QL) of an electron liquid, realised at strong magnetic fields, has long been proposed to host a wealth of strongly correlated states of matter. Electronic states in the QL are, for example, quasi-one dimensional (1D), which implies perfectly nested Fermi surfaces prone to instabilities. Whereas the QL typically requires unreachably strong magnetic fields, the topological semimetal ZrTe$_5$ has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe$_5$ at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe$_5$ enables an efficient tuning of the 1D Landau band structure with magnetic field. This results in a Lifshitz transition to a 1D Weyl regime in which perfect charge neutrality can be achieved. Since no instability-driven phase transitions destabilise the 1D electron liquid for the investigated field strengths and temperatures, our analysis establishes ZrTe$_5$ as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures