SmB6 has been predicted and verified as a prototype of topological Kondo insulators (TKIs). Here we report longitudinal magnetoresistance and Hall coefficient measurements on co-sputtered nanocrystalline SmB6 films and try to find possible signatures
of their topological properties. The magnetoresistance (MR) at 2 K is positive and linear (LPMR) at low field and becomes negative and quadratic at higher field. While the negative part is known from the reduction of the hybridization gap due to Zeeman splitting, the positive dependence is similar to what has been observed in other topological insulators (TI). We conclude that the LPMR is a characteristic feature of TI and is related to the linear dispersion near the Dirac cone. The Hall resistance shows a sign change around 50 K. It peaks and becomes nonlinear at around 10 K then decreases below 10 K. This indicates that carriers with opposite signs emerge below 50 K. Two films with different geometries (thickness and lateral dimension) show contrasting behavior below and above 50K, which proves the surface origin of the low temperature carriers in these films. The temperature dependence of magnetoresistance and the Hall data indicates that the surface states are likely non-trivial.
Topological insulators are a class of materials with insulating bulk but protected conducting surfaces due to the combination of spin-orbit interactions and time-reversal symmetry. The surface states are topologically non-trivial and robust against n
on-magnetic backscattering, leading to interesting physics and potential quantum computing applications1, 2. Recently there has been a fast growing interest in samarium hexboride (SmB6), a Kondo insulator predicted to be the first example of a correlated topological insulator3, 4. Here we fabricated smooth thin films of nanocrystalline SmB6 films. Their transport behavior indeed shows that SmB6 is a bulk insulator with topological surface states. Upon decreasing the temperature, the resistivity r{ho} of Sm0.14B0.86 (SmB6) films display significant increase below 50 K due to hybridization gap formation, and it shows a saturation behavior below 10 K. The saturated resistance of our textured films is similar to that of the single crystals, suggesting that this conduction is from the surface and robust against grain boundary scatterings. Point contact spectroscopy (PCS) of the film using a superconducting tip displays both a Kondo Fano resonance and Andreev reflection, suggesting the existence of both an insulating Kondo lattice and metallic surface states.
Evidence of two-dimensional (2D) quantum critical fluctuations is observed in the superfluid density ns(T) propto $lambda$ -2(T) of deeply underdoped Bi2Sr2CaCu2O8+x (Bi-2212) films, indicating that quantum fluctuations play a dominant role in underd
oped cuprates in general. 2D fluctuations are expressed by the linear scaling, Tc propto ns(0). 2D scaling in Bi-2212 contrasts with 3D scaling seen in the much less anisotropic YBa2Cu3O7-x. Quantum critical fluctuations could also account for the absence of thermal critical behavior in lambda^{-2}(T) of strongly underdoped Bi-2212 samples, Tc < 48 K.
Due to their proximity to an antiferromagnetic phase and to the mysterious pseudogap, underdoped cuprates have attracted great interest in the high Tc community for many years. A central issue concerns the role of quantum and thermal fluctuations of
the phase of the superconducting order parameter. The evolution of superfluid density ns with temperature and doping is a powerful probe of this physics. Here, we report superfluid density measurements on underdoped Bi2Sr2CaCu2O8+x (Bi-2212) films at much lower dopings than have been achieved previously, and with excellent control on doping level - Tc ranges from Tc,min ~ 6K to Tc,max ~ 80K in steps of about 5K. Most famous studies on Bi-2212 like angle-resolved photoemission and scanning probe microscopy are surface-sensitive while superfluid density measurements are bulk-sensitive. We find that strong two-dimensional quantum fluctuations are evident in the observed linear scaling of Tc with ns(0) when Tc is below about 45 K, which contrasts with three-dimensional quantum fluctuations evident in the square root scaling, Tc $propto sqrt$ns(0), seen in the much less anisotropic cuprate, YBa2Cu3O7 (YBCO). On the other hand, consistent with YBCO, ns(T) in severely underdoped Bi-2212 loses its strong downward curvature near Tc, becoming quasi-linear without any obvious critical behavior near Tc. We argue that the quasi-linear T dependence arises from thermal phase fluctuations, although the current theory needs modification in order to understand some features.
