Dmitry G. Eshchenko

Formation and dynamics of muonium centres in semiconductors—a new approach

Various processes of muonium atom formation in semiconductors via electron capture by a positive muon have been studied using μSR techniques, including those with applied electric field. Experiments in GaAs, GaP and CdS suggest that the electron is initially captured into a highly excited state, from which the cascade down to the muonium ground state goes through an intermediate weakly bound state determined by the electron effective mass and the dielectric constant of the host. The electronic structure of this weakly bound state is shown to be hydrogenic. The nature of the final (on the μSR timescale) muonium state depends on the energy releasing mechanisms in the cascade process. We suggest that muonium dynamics in semiconductors (including the effects of electric and magnetic fields and temperature) reflect the electron dynamics in weakly bound muonium state(s) in which the electron is delocalized over distances of about 100 A.

Spin gap in heavy fermion compound UBe13

Heavy fermion (HF) compounds are well known for their unique properties, such as narrow bandwidths, loss of coherence in a metal, non-Fermi-liquid behaviour, unconventional superconductivity, huge magnetoresistance etc. While these materials have been known since the 1970s, there is still considerable uncertainty regarding the fundamental mechanisms responsible for some of these features. Here we report transverse-field muon spin rotation (μ +SR) experiments on the canonical HF compound UBe13 in the temperature range from 0.025 to 300 K and in magnetic fields up to 7 T. The μ +SR spectra exhibit a sharp anomaly at 180 K. We present a simple explanation of the experimental findings identifying this anomaly with a gap in the spin excitation spectrum of f-electrons opening near 180 K. It is consistent with anomalies discovered in heat capacity, NMR and optical conductivity measurements of UBe13, as well as with the new resistivity data presented here. The proposed physical picture may explain several long-standing mysteries of UBe13 (as well as other HF systems).

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe2

It has long been believed that coexistence among ferromagnetic ordering, superconductivity or heavy-fermion behaviour is impossible, as the former supports parallel spin alignment while the latter two phenomena assume a spin-singlet configuration. This understanding has recently been challenged by a number of observations in uranium intermetallic systems where superconductivity (SC) is found within a ferromagnetic state and both ordering phenomena are facilitated by the same set of comparatively heavy quasiparticles which bind into spin-triplet pairs in the SC state. Within the heavy-fermion scenario, this mechanism necessarily assumes that the magnetism has a band character. This band is expected to be responsible for all three phenomena – heavy-fermion behaviour, ferromagnetism and superconductivity – although its nature and the nature of the heavy quasiparticles have so far remained unclear. Our high-field muon spin rotation measurements are indicative of spin polarons of subnanometer size in UGe2. These spin polarons behave as heavy quasiparticles made of 5f electrons. Once coherence is established, they may form a narrow spin-polaron band which thus may provide a natural reconciliation of itinerant ferromagnetism with spin-triplet superconductivity and heavy-fermion behaviour.

Observation of magnetic polarons in the magnetoresistive pyrochlore Lu2V2O7

Materials that exhibit colossal magnetoresistance (CMR) have attracted much attention due to their potential technological applications. One particularly interesting model for the magnetoresistance of low-carrier-density ferromagnets involves mediation by magnetic polarons (MP)-electrons localized in nanoscale ferromagnetic droplets by their exchange interaction. However, MP have not previously been directly detected and their size has been difficult to determine from macroscopic measurements. In order to provide this crucial information, we have carried out muon spin rotation measurements on the magnetoresistive semiconductor Lu(2)V(2)O(7) in the temperature range from 2 to 300 K and in magnetic fields up to 7 T. Magnetic polarons with characteristic radius R ≈ 0.4 nm are detected below about 100 K, where Lu(2)V(2)O(7) exhibits CMR; at higher temperature, where the magnetoresistance vanishes, these MP also disappear. This observation confirms the MP-mediated model of CMR and reveals the microscopic size of the MP in magnetoresistive pyrochlores.

Radio-frequency μSR experiments in an applied electric field

Abstract A new technique is introduced which combines the application of an electric field (EF) with the signal detection capabilities of radio-frequency (RF)-μSR. The power of the combined technique resides in the natural complementarity of the underlying methods. By using an EF in a semiconductor or insulator, one can modify the concentration of electrons and holes around the muon and thereby change the initial muon/muonium fractions. Simultaneous use of RF-μSR then allows for the selective detection of the resulting changes in the final states. We report the first experimental test of the EF+RF-μSR idea in semi-insulating GaAs.

Coupling of magnetic orders in La 2 CuO 4 + x

High transverse magnetic field and zero field muon spin rotation and relaxation measurements have been carried out in a lightly oxygen-doped high-

Intra-unit-cell magnetic order in stoichiometric La 2 CuO 4

{T}_{c}

Localization of muonium atoms in KCl at low temperature

parent compound

Spin-singlet state of electrons in filled skutterudites

{mathrm{La}}_{2}{mathrm{CuO}}_{4}

Avoided level crossing measurements of electric field enhanced diamagnetic states in gallium arsenide

in a temperature range from 2 K to 300 K. As in the stoichiometric compound, muon spin rotation spectra reveal, along with the antiferromagnetic local field, the presence of an additional source of magnetic field at the muon. The results indicate that this second magnetic order is driven by the antiferromagnetism at low temperature but the two magnetic orders decouple at higher temperature. The ability of