arXiv subjects

A. McCollam

Publications and source records attributed to A. McCollam.

At least 37 records · Page 2Linked to original sources

arXiv2015

Emergence of the nematic electronic state in FeSe

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state and elastoresistance measurements. Our high resolution ARPES allows us to track the Fermi surface deformation from four-fold to two-fold symmetry across the structural transition at ~87 K which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character. The low temperature Fermi surface is that a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility as approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice effects and, in the absence of magnetic fluctuations above the structural transition, points clearly towards an electronically driven transition in FeSe stabilized by orbital-charge ordering.

cond-mat.supr-con↗

arXiv2015

Dichotomy between the hole and electrons behavior in the multiband FeSe probed by ultra high magnetic fields

Magnetoresistivity \r{ho}xx and Hall resistivity \r{ho}xy in ultra high magnetic fields up to 88T are measured down to 0.15K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel and independent approach to identify the sign of corresponding cyclotron orbit in a compensated metal from magnetotransport measurements. The observed significant differences in the relative amplitudes of the quantum oscillations between the \r{ho}xx and \r{ho}xy components, together with the positive sign of the high-field \r{ho}xy , reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron bands, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.

cond-mat.supr-con↗

arXiv2014

Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations

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 formation 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.

cond-mat.supr-con↗

arXiv2014

Cascade of field-induced magnetic transitions in a frustrated antiferromagnetic metal

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 measurements 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.

cond-mat.str-el↗

arXiv2014

Angular variation of the magnetoresistance of the superconducting ferromagnet UCoGe

We report a magnetoresistance study of the superconducting ferromagnet UCoGe. The data, taken on single-crystalline samples, show a pronounced structure at $B^* = 8.5$~T for a field applied along the ordered moment $m_0$. Angle dependent measurements reveal this field-induced phenomenon has an uniaxial anisotropy. Magnetoresistance measurements under pressure show a rapid increase of $B^*$ to 12.8~T at 1.0~GPa. We discuss $B^*$ in terms of a field induced polarization change. Upper critical field measurements corroborate the unusual S-shaped $B_{c2}(T)$-curve for a field along the $b$-axis of the orthorhombic unit cell.

cond-mat.supr-con↗

arXiv2014

k.p subband structure of the LaAlO3/SrTiO3 interface

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 variety 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.

cond-mat.mes-hall↗

arXiv2013

Thermally excited multi-band conduction in LaAlO3/SrTiO3 heterostructures exhibiting magnetic scattering

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 electron-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.

cond-mat.mtrl-sci↗

arXiv2013

Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$

We report a high magnetic field study up to 55 T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$) ((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ (x=0.078(6)) with a Tc 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22 T, when the magnetic field is applied perpendicular to the ab planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition not of long-range ordered moments, but of nematic-like antiferromagnetic fluctuations.

cond-mat.supr-con↗

arXiv2013

The Fermi volume as a probe of hidden order

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 β-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.

cond-mat.str-el↗

arXiv2012

Optically excited multi-band conduction in LaAlO3/SrTiO3 heterostructures

The low-temperature resistance of a conducting LaAlO3/SrTiO3 interface with a 10 nm thick LaAlO3 film decreases by more than 50% after illumination with light of energy higher than the SrTiO3 band-gap. We explain our observations by optical excitation of an additional high mobility electron channel, which is spatially separated from the photo-excited holes. After illumination, we measure a strongly non-linear Hall resistance which is governed by the concentration and mobility of the photo-excited carriers. This can be explained within a two-carrier model where illumination creates a high-mobility electron channel in addition to a low-mobility electron channel which exists before illumination.

cond-mat.mtrl-sci↗

arXiv2012

Quantum oscillations and subband properties of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface

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.

cond-mat.mtrl-sci↗

arXiv2012

Pressure Tuned Insulator to Metal Transition in Eu$_2$Ir$_2$O$_7$

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 continuously, 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.

cond-mat.str-el↗

arXiv2011

A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs

We report a de Haas-van Alphen (dHvA) oscillation study of the 111 iron pnictide superconductors LiFeAs with T_c ~18K and LiFeP with T_c~5K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and shows quasi-nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs respectively.

cond-mat.supr-con↗

arXiv2011

Spin splitting in graphene studied by means of tilted magnetic-field experiments

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 angle 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.

cond-mat.mes-hall↗

arXiv2011

Reply to Comment by Borisenko et al. on article `A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs'

Recently, Borisenko et al have posted a Comment (arXiv:1108.1159) where they suggest an alternative interpretation of our de Haas-van Alphen (dHvA) measurements on the superconductor LiFeAs. In our original paper (arXiv:1107.4375) we concluded that our measurements of the bulk Fermi surface were not consistent with the surface bands observed thus far by ARPES. Borisenko et al dispute this and suggest the two measurements are consistent if some of the orbits we observe are due to magnetic breakdown. We argue here that this scenario is inconsistent with the experimental data and therefore that our original conclusion stands.

cond-mat.supr-con↗

arXiv2011

Quantum critical metamagnetism of Sr3Ru2O7 under hydrostatic pressure

Using ac susceptibility, we have determined the pressure dependence of the metamagnetic critical end point temperature T* for field applied in the ab-plane in the itinerant metamagnet Sr3Ru2O7. We find that T* falls monotonically to zero as pressure increases, producing a quantum critical end point (QCEP) at Pc~13.6 kbar. New features are observed near the QCEP -- the slope of T* vs pressure changes at ~12.8 kbar, and weak subsidiary maxima appear on either side of the main susceptibility peak at pressures near Pc -- indicating that some new physics comes into play near the QCEP. Clear signatures of a nematic phase, however, that were seen in field-angle tuning of T*, are not observed. As T* is suppressed by pressure, the metamagnetic peak in the susceptibility remains sharp as a function of applied magnetic field. As a function of temperature, however, the peak becomes broad with only a very weak maximum, suggesting that, near the QCEP, the uniform magnetization density is not the order parameter for the metamagnetic transition.

cond-mat.str-el↗

arXiv2009

Evolution of the Fermi surface of BaFe_2(As_{1-x}P_x)_2 on entering the superconducting dome

Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe_2(As_{1-x}P_x)_2 as function of isoelectric substitution (As/P) for 0.41<x<1 (T_c up to 25 K). We find that the volume of electron and hole Fermi surfaces shrink linearly with decreasing x. This shrinking is accompanied by a strong increase in the quasiparticle effective mass as x is tuned toward the maximum T_c. It is likely that these trends originate from the many-body interaction which give rise to superconductivity, rather than the underlying one-electron bandstructure.

cond-mat.supr-con↗

arXiv2009

Observation of the J-sheet of the Fermi surface of YbRh2Si2

We report a previously unobserved quantum oscillation frequency in YbRh2Si2. This is the first quantum oscillation that can definitely be assigned to the larger of the two major sheets of the Fermi surface predicted by band structure calculations. Previously observed frequencies were interpreted in terms of a large Fermi surface, which includes the Yb 4f-hole in the Fermi volume, that has been so strongly spin-split that it closely resembles the small Fermi surface, which does not include the Yb 4f -hole in the Fermi volume. The new frequency can also be incorporated into this picture, however there are some indications that the a need for more advanced energy band calculations.

cond-mat.str-el↗