Roland Mathieu

Bio: Roland Mathieu is an academic researcher from Uppsala University. The author has contributed to research in topics: Magnetization & Antiferromagnetism. The author has an hindex of 32, co-authored 220 publications receiving 4514 citations. Previous affiliations of Roland Mathieu include Royal Institute of Technology & National Institute of Advanced Industrial Science and Technology.

The anomalous Hall effect and magnetic monopoles in momentum space.

Abstract: Efforts to find the magnetic monopole in real space have been made in cosmic rays and in particle accelerators, but there has not yet been any firm evidence for its existence because of its very heavy mass, ∼10 16 giga–electron volts We show that the magnetic monopole can appear in the crystal momentum space of solids in the accessible low-energy region (∼01 to 1 electron volts) in the context of the anomalous Hall effect We report experimental results together with first-principles calculations on the ferromagnetic crystal SrRuO 3 that provide evidence for the magnetic monopole in the crystal momentum space

Near-room-temperature colossal magnetodielectricity and multiglass properties in partially disordered La2NiMnO6

Abstract: We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.

Memory and superposition in a spin glass

Abstract: Nonequilibrium dynamics in a Ag(Mn) spin glass are investigated by measurements of the temperature dependence of the remanent magnetization. Using specific cooling protocols before recording the thermo- or isothermal remanent magnetizations on reheating, it is found that the measured curves effectively disclose nonequilibrium spin glass characteristics such as aging and memory phenomena as well as an extended validity of the superposition principle for the relaxation. The usefulness of this ``simple'' dc method is discussed, as well as its applicability to other disordered magnetic systems.

Scaling of the anomalous Hall effect in Sr1-xCaxRuO3

Abstract: The anomalous Hall effect (AHE) of ferromagnetic thin films of Sr1-xCaxRuO3 (0less than or equal toxless than or equal to0.4) is studied as a function of x and temperature T. As x increases, both the transition temperature T-c and the magnetization M are reduced and vanish near xsimilar to 0.7. For all compositions, the transverse resistivity rho(H) varies nonmonotonously with T, and even changes sign, thus violating the conventional expression rho(H)=RoB+4piR(s)M(T) (B is the magnetic induction, while R-o and R-s are the ordinary and anomalous Hall coefficients). From the rather complicated data of rho(H), we find a scaling behavior of the transverse conductivity sigma(xy) with M(T), which is well reproduced by the first-principles band calculation assuming the intrinsic origin of the AHE.

Colossal magnetoresistance without phase separation: disorder-induced spin glass state and nanometer scale orbital-charge correlation in half doped manganites.

Abstract: The magnetic and electrical properties of high-quality single crystals of $A$-site disordered (solid solution) ${\mathrm{L}\mathrm{n}}_{0.5}{\mathrm{B}\mathrm{a}}_{0.5}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ are investigated near the phase boundary between the spin-glass insulator and colossal-magnetoresistive ferromagnetic metal, locating near $\mathrm{L}\mathrm{n}=\mathrm{S}\mathrm{m}$. The temperature dependence of the ac susceptibility and the x-ray diffuse scattering of ${\mathrm{E}\mathrm{u}}_{0.5}{\mathrm{B}\mathrm{a}}_{0.5}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ are analyzed in detail. The uniformity of the random potential perturbation in ${\mathrm{L}\mathrm{n}}_{0.5}{\mathrm{B}\mathrm{a}}_{0.5}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ crystals with a small bandwidth yields, rather than the phase separation, an homogeneous short ranged charge or orbital order which gives rise to a nearly atomic spin-glass state. Remarkably, this microscopically disordered ``charge-exchange-glass'' state alone is able to bring forth the colossal magnetoresistance.

Experimental observation of the quantum Hall effect and Berry's phase in graphene

Abstract: When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron–hole degeneracy and vanishing carrier mass near the point of charge neutrality1,2. Indeed, a distinctive half-integer quantum Hall effect has been predicted3,4,5 theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction—a consequence of the exceptional topology of the graphene band structure6,7. Recent advances in micromechanical extraction and fabrication techniques for graphite structures8,9,10,11,12 now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.

Experimental Observation of Quantum Hall Effect and Berry's Phase in Graphene

Abstract: When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron–hole degeneracy and vanishing carrier mass near the point of charge neutrality. Indeed, a distinctive half-integer quantum Hall effect has been predicted theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction—a consequence of the exceptional topology of the graphene band structure. Recent advances in micromechanical extraction and fabrication techniques for graphite structures now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.

Weyl and Dirac semimetals in three-dimensional solids

Abstract: Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.