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Ryan A. Nelson

Publications and source records attributed to Ryan A. Nelson.

3 recordsLinked to original sources

Revealing the EuCd_{2}As_{2} Semiconducting Band Gap via n-type La-Doping

EuCd_{2}As_{2} has attracted considerable interest as one of the few magnetic Weyl semimetal candidate materials, although recently there have been emerging reports that claim it to have a semiconducting electronic structure. To resolve this debate, we established the growth of n-type EuCd_{2}As_{2} crystals, to directly visualize the nature of the conduction band using angle resolve photoemission spectroscopy (ARPES). We show that La-doping leads to n-type transport signatures in both the thermopower and Hall effect measurements, in crystals with doping levels at 2 - 6 x 10^{17} e^{-} cm^{-3}. Both p-type and n-type doped samples exhibit antiferromagnetic ordering at 9 K. ARPES experiments at 6 K clearly show the presence of the conduction band minimum at 0.8 eV above the valence band maximum, which is further corroborated by the observation of a 0.71 - 0.72 eV band gap in room temperature diffuse reflectance absorbance measurements. Together these findings unambiguously show that EuCd_{2}As_{2} is indeed a semiconductor with a substantial band gap and not a topological semimetal.

cond-mat.mtrl-sci

Quantum Octets in Air Stable High Mobility Two-Dimensional PdSe2

Two-dimensional (2D) materials have drawn immense interest in scientific and technological communities, owing to their extraordinary properties that are profoundly altered from their bulk counterparts and their enriched tunability by gating, proximity, strain, and external fields. For digital applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large-scale synthesis. Here we demonstrate air-stable field-effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with record high saturation current >350μA/μm, and field effect mobilities 700 and 10,000 cm2/Vs at 300K and 2K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.

cond-mat.mtrl-sci

Magnetic field-controlled lattice thermal conductivity in MnBi2Te4

Phonon properties and the lattice thermal conductivity are not generally understood to be sensitive to magnetic fields. Using an applied field to change MnBi2Te4 between antiferromagnetic (AFM), canted (CAFM) and ferromagnetic (FM) phases we discovered a new way to control the lattice thermal conductivity, generating both a positive and a negative magnetic field dependence. We report the field dependence of the thermal conductivity, k, in the in-plane direction under an applied magnetic field along the cross-plane direction in MnBi2Te4 from 2K to 30K. k decreases with field in the AFM phase, saturates in the CAFM phase, and increases with field in the FM phase. We explain this in terms of the field-induced changes of the magnon gap which modifies which magnon-phonon scattering processes are allowed by energy conservation. We also report magneto-Seebeck coefficient, Nernst coefficient and thermal Hall data measured in the same configuration. This finding may open a way to design magnetically controlled heat switches.

cond-mat.mtrl-sci