Reflectance confocal microscopy of optical phantoms

A reflectance confocal scanning laser microscope (rCSLM) operating at 488-nm wavelength imaged three types of optical phantoms: (1) 100-nm-dia. polystyrene microspheres in gel at 2% volume fraction, (2) solid polyurethane phantoms (INO BiomimicTM), and (3) common reflectance standards (SpectralonTM). The noninvasive method measured the exponential decay of reflected signal as the focus (zf) moved deeper into the material. The two experimental values, the attenuation coefficient μ and the pre-exponential factor ρ, were mapped into the material optical scattering properties, the scattering coefficient μs and the anisotropy of scattering g. Results show that μs varies as 58, 8–24, and 130–200 cm-1 for phantom types (1), (2) and (3), respectively. The g varies as 0.112, 0.53–0.67, and 0.003–0.26, respectively

Disorder-induced critical exponents near a ferromagnetic quantum critical point in Mn1−xCrxSi

We report the observation of critical behavior in Mn1−xCrxSi (0≤x≤1) close to a T = 0 K quantum critical point, consistent with the Belitz-Kirkpatrick-Vojta (BKV) theory of disordered metallic ferromagnets. The critical exponents are in good agreement with the theoretical predictions of the BKV theory in the preasymptotic limit. A non-Fermi liquid-like behavior is seen down to 200 mK in the transport and thermodynamic properties around the critical concentration xC = 0.2. Quantum criticality and self-consistency of the exponents is further confirmed using a scaling analysis of the magnetization and heat capacity data. A recovery to Fermi liquid-like behavior is displayed on moving away from the critical composition, as well as with the application of a magnetic field

Anomalous thermal expansion of Sb2_2Te3_3 topological insulator

We have investigated the temperature dependence of the linear thermal expansion along the hexagonal c axis ($\Delta L$), in-plane resistivity ($\rho$), and specific heat ($C_p$) of the topological insulator Sb$_2$Te$_3$ single crystal. $\Delta L$ exhibits a clear anomaly in the temperature region 204-236 K. The coefficient of linear thermal expansion $\alpha$ decreases rapidly above 204 K, passes through a deep minimum at around 225 K and then increases abruptly in the region 225-236 K. $\alpha$ is negative in the interval 221-228 K. The temperature dependence of both $\alpha$ and $C_p$ can be described well by the Debye model from 2 to 290 K, excluding the region around the anomaly in $\alpha$

Spin-valve nature and giant coercivity of a ferrimagnetic spin semimetal Mn2_2IrGa

Spin semimetals are amongst the most recently discovered new class of spintronic materials, which exhibit a band gap in one spin channel and semimetallic feature in the other, thus facilitating tunable spin transport. Here, we report Mn$_2$IrGa to be a candidate material for spin semimetal along with giant coercivity and spin-valve characteristics using a combined experimental and theoretical study. The alloy crystallizes in an inverse Heusler structure (without any martensitic transition) with a para- to ferri-magnetic transition at $T_\mathrm{C} \sim$ 243 K. It shows a giant coercive field of about 8.5 kOe (at 2 K). The negative temperature coefficient, relatively low magnitude and weak temperture dependance of electrical resistivity suggest the semimetallic character of the alloy. This is further supported by our specific heat measurement. Magnetoresistance (MR) confirms an irreversible nature (with its magnitude $\sim$1\%) along with a change of sign across the magnetic transition indicating the potentiality of Mn$_2$IrGa in magnetic switching applications. In addition, asymmetric nature of MR in the positive and negative field cycles is indicative of spin-valve characteristics. Our ab-initio calculations confirm the inverse Heusler structure with ferrimagnetic ordering to be the lowest energy state, with a saturation magnetization of 2 $\mu_\mathrm{B}$. $$ is found to be the easy magnetic axis with considerable magneto-crystalline anisotropy energy. A large positive Berry flux at/around $\Gamma$ point gives rise to an appreciable anomalous Hall conductivity ($\sim$-180 S/cm).Comment: Mn2IrGa, Inverse Heusler alloys, Giant Coercivity, Ferrimagnets, Spin Semimetal, Spin valve, Fleur, FLAPW, Spintronic

Quantum Griffiths phase in disordered Mn1-xFexSi

We show the presence of magnetic rare regions consistent with the quantum Griffiths phase in Fe-doped MnSi using detailed heat capacity, magnetization, and muon spin relaxation (μSR) measurements down to millikelvin temperatures. The slow dynamics of these rare regions at low temperatures leads to the non-Fermi-liquid behavior in heat capacity and magnetization. The μSR and magnetization results further indicate that the dynamics freezes into a cluster-glass state below Tf ∼ 1.25 K. The results are in agreement with theoretical models proposed in the literature for metallic systems with Heisenberg symmetry that exhibit the quantum Griffiths phase in the presence of strong disorder