Ubiquitous Spin Freezing in the Superconducting State of UTe2

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe$_2$ is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe$_2$ are unclear. By way of muon spin rotation/relaxation ($\mu$SR) measurements on independently grown UTe$_2$ single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat ($C$) and the low-temperature upturn in $C/T$ versus $T$. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe$_2$.Comment: 33 pages, 9 figure

Characterization of the Magnetic Phase in Ti-Doped Vanadium Dioxide

Here we present a characterization of the recently discovered magnetic phase in titanium-doped vanadium dioxide (VO2:Ti with 1, 3 & 5 at%) via Muon Spin Relaxation (MuSR) measurements. Specifically, variations in the magnetic phase were studied as a function of titanium dopant concentration in an effort to understand the fundamental mechanism responsible for magnetism and other transitions exhibited by the material. Muons are spin 1/2 particles with an average lifetime of 2.2 us and a gyromagnetic ratio of 135.54 MHz/T that are used to study the local magnetic environment through a technique known as Muon Spin Relaxation/Rotation/Resonance (MuSR). Implanted muons precess about the field in their local environment until they decay into a positron that is preferentially aligned with the spin direction at the time of decay (and associated neutrinos). By tracking these positrons, the time evolution of the muon spin polarization is determined and used to map the local magnetic field distribution at the muon stopping site. VO2 exhibits an ultra-fast, reversible metal−semiconductor transition (MST) at TMST near 340 K. Above TMST it is metallic, reflective and electrically conductive; below, the electrical conductivity drops by several orders of magnitude, it is transparent with a bandgap of 0.7 eV. The MST can be triggered by thermal, optical, electrical or barometric means and is accompanied by a structural transition. Dopants like tungsten and Ti reduce TMST to below room temperature while having minimal effect on the electronic or optical properties of the host, whereas dopants such as fluorine and chromium can raise it to above 400 K. While VO2 has been studied since the 1960s, its low-temperature magnetic phase was first reported in 2014 by our collaboration. This contribution is part of a large-scale project aimed at understanding the fundamental mechanism responsible for transitions in VO2 compounds, a question still highly debated within the community

Optical spectroscopy of muon/hydrogen defects in 6H-SiC

Positive muons can be implanted into silicon carbide (SiC), where they model the isolated hydrogen defect in the negative, neutral, or positive charge states and act as either an acceptor or a donor with midgap energy levels [Lichti et al., Phys. Rev. B 70, 165204 (2004); Lichti et al., Phys. Rev. Lett. 101, 136403 (2008)]. The charge states evolve after implantation depending on the temperature and material doping. We have measured optically induced effects on muons implanted in 6H-SiC using a pulsed, tunable laser [Yokoyama et al., Rev. Sci. Instrum. 87, 125111 (2016)]. In n-type 6H-SiC at 85 K and 40 K, with a laser pulse of energy below the bandgap, we observe photoionization of the doubly occupied level (Mu-) to the neutral defect Mu0 and also ionization of Mu0 to Mu+. Varying the timing of the laser pulse relative to muon arrival confirms that the laser interacts directly with the muons in a stable or metastable state. There is no evidence of any interaction when the laser pulse is timed to arrive before the muons, so either few free carriers are generated by absorption at other dopant sites or the excess carriers have a very short lifetime (\u3c0 and Mu- charge states, with the muon or hydrogen acting as a deep compensating impurity. The technique can be applied to many other semiconductors where the muon has been observed to be electrically active, modeling hydrogen

Hydrogen states in mixed-cation CuIn(1−x)GaxSe2 chalcopyrite alloys: a combined study by first-principles density-functional calculations and muon-spin spectroscopy

First-principles calculations were performed jointly with muon-spin (µSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In1−x,Gax)Se2 (CIGS) alloys and other related compounds commonly used as absorbers in solar-cell technology. The study targeted the range of Ga concentrations most relevant in typical solar cells. By means of a hybridfunctional approach the charge-transition levels of hydrogen were determined and the evolution of the defect pinning level, E(+/–), was monitored as a function of the Ga content. The use of E(+/–) as a metric of the charge-neutrality level allowed the alignment of band structures, thus providing the band offsets between the CuInSe2 compound and the CIGS alloys. The µSR measurements in both thin-film and bulk CIGS materials confirmed that the positively-charged state is the thermodynamically stable configuration of hydrogen for p-type conditions. The interpretation of the µSR data further addressed the existence of a metastable quasi-atomic neutral configuration that was resolved from the calculations and led to a formation model for muon implantation

Muon probes of temperature-dependent charge carrier kinetics in semiconductors

We have applied the photoexcited muon spin spectroscopy technique to intrinsic germanium with the goal of developing a method for characterizing excess carrier kinetics in a wide range of semiconductors. Muon spin relaxation rates can be a unique measure of excess carrier density and utilized to investigate carrier dynamics. By virtue of the localized nature of implanted muons, the obtained carrier lifetime spectrum can be modeled with a simple 1-dimensional diffusion equation to determine bulk recombination lifetime and carrier diffusivity. Temperature dependent studies of these parameters can reveal the recombination and diffusion mechanism

Ubiquitous spin freezing in the superconducting state of UTe2

UTe2 receives significant attention as it may be an example of a spin-triplet superconductor but many features of this material are still to be fully understood. Here, the authors use muon spin rotation to investigate the existence of low-temperature magnetic clusters in single crystals of UTe2 and discuss the potential relationship with the temperature dependent behaviour of the specific heat