We report the observation of the de Haas-van Alphen effect in IrTe2 measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33T. IrTe2 undergoes a major structural transition around 283 K due to the form
ation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than 1 me), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas-van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling IrTe2.
Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport mea
surements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to construct the field-temperature phase diagram. The results are discussed in the context of a frustrated easy-axis Heisenberg model for the localized moments where intermediate applied magnetic fields are predicted to stabilize a magnetic supersolid phase. Deviations in the measured phase diagram from this model predictions are attributed to the role played by the itinerant electrons.
Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostructure with 26 unit cells of LaAlO3 show Hall resistance and magnetoresistance which at low and high temperatures is described by a single channel of electr
on-like charge carriers. At intermediate temperatures, we observe non-linear Hall resistance and positive magnetoresistance, establishing the presence of at least two electron-like channels with significantly different mobilities and carrier concentrations. These channels are separated by 6 meV in energy and their temperature dependent occupation and mobilities are responsible for the observed transport properties of the interface. We observe that one of the channels has a mobility that decreases with decreasing temperature, consistent with magnetic scattering in this channel.
We demonstrate that the volume of the Fermi surface, measured very precisely using de Haas-van Alphen oscillations, can be used to probe changes in the nature and occupancy of localized electronic states. In systems with unconventional ordered states
, this allows an underlying electronic order parameter to be followed to very low temperatures. We describe this effect in the field-induced antiferroquadrupolar (AFQ) ordered phase of PrOs4Sb12, a heavy fermion intermetallic compound. We find that the phase of de Haas-van Alphen oscillations is sensitively coupled, through the Fermi volume, to the configuration of the Pr f-electron states that are responsible for AFQ order. In particular, the beta-sheet of the Fermi surface expands or shrinks as the occupancy of two competing localized Pr crystal field states changes. Our results are in good agreement with previous measurements, above 300 mK, of the AFQ order parameter by other methods. In addition, the low temperature sensitivity of our measurement technique reveals a strong and previously unrecognized influence of hyperfine coupling on the order parameter below 300 mK within the AFQ phase. Such hyperfine couplings could provide insight into the nature of hidden order states in other systems.
Heterostructures made of transition metal oxides are new tailor-made materials which are attracting much attention. We have constructed a 6-band k.p Hamiltonian and used it within the envelope function method to calculate the subband structure of a v
ariety of LaAlO3/SrTiO3 heterostructures. By use of density functional calculations, we determine the k.p parameters describing the conduction band edge of SrTiO3: the three effective mass parameters, L=0.6104 eV AA^2, M=9.73 eV AA^2, N=-1.616 eV AA^2, the spin orbit splitting Delta_SO=28.5 meV and the low temperature tetragonal distortion energy splitting Delta_T=2.1 meV. For confined systems we find strongly anisotropic non-parabolic subbands. As an application we calculate bands, density of states and magnetic energy levels and compare the results to Shubnikov-de Haas quantum oscillations observed in high magnetic fields. For typical heterostructures we find that electric field strength at the interface of F = 0.1 meV/AA for a carrier density of 7.2 10^{12} cm^-2 results in a subband structure that is similar to experimental results.
We have performed high field magnetotransport measurements to investigate the interface electron gas in LaAlO3/SrTiO3 heterostructures. Shubnikov-de Haas oscillations reveal several 2D conduction subbands with carrier effective masses between 1 and 3
m_e, quantum mobilities of order 3000 cm^2/V s, and band edges only a few millielectronvolts below the Fermi energy. Measurements in tilted magnetic fields confirm the 2D character of the electron gas, and show evidence of inter-subband scattering.
We have measured the spin splitting in single-layer and bilayer graphene by means of tilted magnetic field experiments. Applying the Lifshitz-Kosevich formula for the spin-induced decrease of the Shubnikov de Haas amplitudes with increasing tilt angl
e we directly determine the product between the carrier cyclotron mass m* and the effective g-factor g* as a function of the charge carrier concentration. Using the cyclotron mass for a single-layer and a bilayer graphene we find an enhanced g-factor g* = 2.7 pm 0.2 for both systems.
We have studied the effect of pressure on the pyrochlore iridate Eu$_2$Ir$_2$O$_7$, which at ambient pressure has a thermally driven insulator to metal transition at $T_{MI}sim120$,K. As a function of pressure the insulating gap closes, apparently co
ntinuously, near $P sim 6$,GPa. However, rather than $T_{MI}$ going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, calling into question the true nature of both ground states. The high temperature state also crosses over near 6 GPa, from an incoherent to a conventional metal, suggesting a connection between the high and the low temperature states.
Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh2Si2. We have carried out de Haas-van Alphen measurements in the magnetic-field range 8 T <= H <= 16 T, and directly observe
field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field large Fermi surface, including the Yb 4f quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at H0 ~ 10 T, without requiring 4f localization at H0.
We report transport and thermodynamic properties of stoichiometric single crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows an unusual non-metallic dependence on temperature T, rising continuously with decreasing T from ~
800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, rho(T) = rho_0 - AT^x with x<1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, gamma ~ 30 mJ/mol K^2. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T -> 0 properties suggest a new class of non-Fermi liquid.
