M. Morita

Magnetoresistance and vortex states below the 2D superconductor–insulator transition near T=0

Abstract To study the vortex states below the critical field B c of the superconductor–insulator transition in two dimensions (2D), we measure the magnetoresistance at low perpendicular fields B ( B c ) for a thin (4 nm) a -Mo x Si 1− x film. We find that the very small temperature-independent resistance remains at T →0 in finite B below B c . Such resistance is not observed for a thick (100 nm) film as well as for the thin film in parallel fields. These results suggest quantum motion of a small number of vortices (dislocations) in the 2D vortex-glass phase in the presence of the current.

Electronic Transport in the Low‐Temperature Liquid Phase of a Thin Amorphous Film without the Edge Effects

We study the possible edge‐pinning effects on the transport properties in the low‐temperature (T) liquid phase of a thin amorphous MoxSi1−x film with the Corbino‐Disk (CD) and strip‐like contact geometries. In CD, in the presence of a radial current, field‐induced vortices rotate around the center of the sample without crossing the sample edges and, hence, the edge effects are negligible. In both geometries we observe the two‐dimensional field‐driven superconductor‐insulator transition at T → 0. This result indicates that the edge effects are not important and the absence of an intermediate metallic vortex phase at T= 0 is intrinsic in our thin amorphous films.We study the possible edge‐pinning effects on the transport properties in the low‐temperature (T) liquid phase of a thin amorphous MoxSi1−x film with the Corbino‐Disk (CD) and strip‐like contact geometries. In CD, in the presence of a radial current, field‐induced vortices rotate around the center of the sample without crossing the sample edges and, hence, the edge effects are negligible. In both geometries we observe the two‐dimensional field‐driven superconductor‐insulator transition at T → 0. This result indicates that the edge effects are not important and the absence of an intermediate metallic vortex phase at T= 0 is intrinsic in our thin amorphous films.

Effects of disorder and dimensionality on the vortex phase diagram at low temperature in amorphous films

Abstract We present measurements of dc and ac complex resistivities for amorphous Mo x Si 1− x films with different disorder and dimensionality (film thickness t ). For thicker films with t =100 and 30 nm we determine the vortex-glass-transition (VGT) line B g ( T ) which persists down to low enough temperatures T up to high fields B near B c2 (0), where B c2 (0) is an upper critical field at T =0. The finite quantum-vortex-liquid (QVL) phase at T =0, B g (0) B B c2 (0), is observed for these films. We find a trend for the QVL phase to increase as the film becomes more resistive and/or thinner. This result is consistent with a view that the QVL phase is driven by strong quantum fluctuations, which are enhanced with increasing disorder and with decreasing dimensionality. For the thinnest film with t =6 nm, both the dc resistivity and vortex relaxation time follow the activated T dependence, suggestive of two-dimensional VGT.

Anomalous magnetoresistance below the 2D superconductor–insulator transition

Abstract We measure the magnetoresistance R LF ( B ) of two thin films of amorphous Mo x Si 1− x with nearly identical x in low fields B below the critical field B c of the superconductor–insulator transition. We observe small but finite R LF ( B ) for both films, which is resulting from vortex motion in the liquid phase. We observe an anomalous peak for one film and a monotonic increase in R LF ( B ) for the other, indicating that R LF ( B ) is sensitive to microscopic morphology of films responsible for pinning. We do not find the R =0 state at any nonzero B and T within our experimental resolutions, which is consistent with the picture of the vortex–glass transition in two dimensions.

Quantum Fluctuations and Vortex Phase Diagram in Uniformly Disordered Amorphous Films

We have studied effects of microscopic disorder (normal-state resistivity ρn) and dimensionality (film thickness t) on the vortex phase diagram at low temperature T on the basis of measurements of DC and AC complex resistivities for amorphous (a)-MoxSi1-x films with different ρn and t. For thick films (t=100 nm) we have commonly observed the vortex-glass transition (VGT) down to low T~0.05Tc0 and high fields up to B~0.9Bc2(0), where Tc0 and Bc2(0) are the mean-field transition temperature and upper critical field at T=0, respectively. In the limit T=0, the VGT line Bg(T) is independent of T and extrapolates to a field below Bc2(0), indicative of the presence of a quantum-vortex-liquid (QVL) phase at T=0 in the regime Bg(0)<B<Bc2(0). We find that the width of the T=0 QVL phase, [Bc2(0)-Bg(0)]/Bc2(0), grows as the film becomes more resistive. This result is consistent with the view that the QVL phase is driven by strong quantum fluctuations, which are enhanced with increasing disorder. In the mixed state of the 6-nm-thick film, both the DC resistivity and vortex relaxation time follow the activated T dependence, suggestive of two-dimensional VGT.

Anisotropic magnetoresistance on the insulating side of the zero-field superconductor–insulator transition

Abstract We present measurements of the magnetoresistance (MR) for insulating amorphous Mo x Si 1− x films. The field B is applied both perpendicular R ⊥ ( B ) and parallel R ∥ ( B ) to the plane of the film. For the most resistive film the difference between R ⊥ and R ∥ , R a (i.e., anisotropy component of MR), is positive and shows a monotonic increase with increasing B . On the other hand, for the less resistive film which lies closer to the zero-field superconductor–insulator transition, R a ( B ) also shows an increase at low fields but it then takes a broad peak at higher B . The behavior is not explained only by the orbital effect for fermions, suggesting the contribution of superconductivity.

Possible vortex phase diagram at zero temperature in disordered 2D superconductors

Abstract We study transport properties of highly disordered thin films of amorphous Mo x Si 1− x at low temperatures. For superconducting films we observe an anomalous peak in the magnetoresistance suggesting the presence of localized Cooper pairs on the insulating side ( B > B C ) of the field-driven superconductor–insulator transition. In contrast, for thick films, or for thin films in parallel fields the MR is always positive. These results may suggest the presence of insulating quantum-vortex-liquid (QVL) phase in the region B > B C . In B B C the metallic QVL phase is not evident, most likely absent, which is in contrast to the results reported by other groups.

Observation of the vortex-glass transition in the low-temperature high-field regime in thick a-MoxSi1−x films

Abstract We present measurements of dc and ac complex resistivities for thick amorphous (a-)MoxSi1−x films at low temperatures (T>0.04 K) in various fields B. The critical behavior associated with the vortex-glass transition (VGT) has been observed for both dc and ac resistivities. The VGT persists down to low enough temperatures T

Evidence for the quantum vortex liquid phase in thick a-MoxSi1-x films

Abstract We perform detailed measurements of the ac complex resistivity of thick amorphous (a-)MoxSi1−x films at low temperatures T to study the vortex states near T=0 K. We obtain evidence for the vortex-glass transition (VGT) down to low enough temperatures T∼0.1 K and up to high fields B near the upper critical field Bc2(0) at T=0 K. In the limit T→0 K, the VGT line Bg(T) becomes independent of T and extrapolates to a field Bg(0) lower than Bc2(0). From this result, together with the finding that Bg(0) coincides with the T=0 K superconductor–metal transition, we conclude that the metallic quantum vortex liquid phase is present in the field region Bg(0)

Vortex states at T=0 in disordered thin and thick films of a-MoxSi1−x

Abstract We measure the transport properties of highly disordered thin and thick films of amorphous (a-)Mo x Si 1− x at low temperatures T . For thin films we have observed an anomalous peak in the magnetoresistance in fields B higher than the critical field B C of the field-driven superconductor–insulator transition (SIT). This suggests the presence of localized Cooper pairs on the insulating side of the SIT. We present data supporting the view that this insulating regime may correspond to the quantum-vortex-liquid (QVL) phase. The metallic QVL phase in B B C is not evident, most likely absent. On the other hand, for thick films the existence of the vortex-glass transition and the metallic QVL phase at T =0 has been demonstrated. Based on these data, we construct the T =0 phase diagram of disordered two-dimensional and three-dimensional superconductors.