arXiv subjects

Ghulam Mohmad

Publications and source records attributed to Ghulam Mohmad.

5 recordsLinked to original sources

arXiv2026

Nucleation and Enhancement of Superconductivity under Tip-Induced Strain Fields

A metallic point contact formed on a non-superconducting or weakly superconducting material often nucleates or enhances superconductivity confined under the contact. However, no unified theoretical description of the phenomenon exists. We show that the spatially inhomogeneous, predominantly uniaxial nature of the stress field under a point contact is fundamental for such tip-induced and tip-enhanced superconductivity (TISC/TESC). We also show that the coupling of such a stress field to the electronic structure can be estimated through an experimentally measurable uniaxial coupling scale $C^{\mathrm{exp}}$. Combining Hertzian contact mechanics with a Ginzburg-Landau variational analysis, we derive a criterion for the nucleation of TISC/TESC and determine $C^{\mathrm{exp}}$ for twenty-one materials. For topological semimetals with ungapped band crossings, the framework explains observed critical temperatures with no free parameters and for all others, $C^{\mathrm{exp}}$ provides a direct experimental determination of the uniaxial strain sensitivity and a target scale for microscopic theories.The work predicts TISC in elemental Sb and Y with $T_c \approx 2.8$\,K and $T_c \approx 12$\,K respectively.

cond-mat.supr-con↗

arXiv2026

A Top-Loading Point-Contact Spectroscopy Probe with In-Situ Sample Exchange for Dilution Refrigerators

We report the design and implementation of a point-contact spectroscopy (PCS) system integrated with a dilution refrigerator, enabling measurements down to 30 mK. The setup employs a needle-anvil geometry with a cryogenic piezo-driven nanopositioner for in-situ formation of mesoscopic point contacts. We discuss the thermal anchoring strategies that enable efficient cooling of the probe to ultra-low temperatures and reliable measurements. We also address positioner-related challenges and the solutions implemented to ensure stable operation at millikelvin temperatures. The performance of the probe is demonstrated through point contact spectroscopy on Ta-doped TiSe$_2$ (Ta$_x$Ti$_{1-x}$Se$_2$, $x = 0.2$), a superconductor with $T_c \approx 2.3$ K. The spectra exhibit well-defined superconducting features that systematically diminish with increasing temperature and magnetic field. The platform provides a robust and versatile tool for spectroscopic investigations of superconductors and other quantum materials at millikelvin temperatures and high magnetic fields.

cond-mat.supr-con↗

arXiv2026

Magnetic field decouples nodeless surface and nodal bulk orders in PdTe

Selective spectroscopic disentanglement of surface and bulk quantum orders remains an outstanding challenge in condensed matter physics. The candidate topological superconductor PdTe has recently been proposed to host a nodeless surface gap on top of a nodal bulk state, but their direct identification and mutual coupling remained experimentally elusive. Here, we employ magnetic-field-dependent Andreev reflection spectroscopy to spectroscopically disentangle these components. At zero magnetic field, the spectra exhibit a BCS-like gap structure, consistent with dominant transport through a fully gapped surface superconducting state. Strikingly, even a weak magnetic field leads to an abrupt suppression of the Andreev-enhanced conductance (AEC), while a residual AEC, attributable to the nodal bulk state, persists to much higher magnetic fields. The transition is accompanied by pronounced magnetic hysteresis pointing to the existence of vortex dynamics at low fields. Our findings suggest that the nodal bulk gap facilitates early vortex entry, which in turn disrupts the fragile surface superconductivity. These results establish a field-tunable decoupling of surface and bulk superconductivity and illustrate how distinct gap topologies can shape the global superconducting order in multichannel systems.

cond-mat.supr-con↗

arXiv2025

Spectroscopic signature of anisotropic order parameter in Kagome lattice superconductor LaRh$_3$B$_2$

The physics of the Kagome metal LaRh$_3$B$_2$ along with its superconductivity below 2.6 K, unlike other popular Kagome metals, is not known to be significantly influenced by the electron correlations. While the indirect techniques to probe the bulk superconducting properties of LaRh$_3$B$_2$ indicate a conventional isotropic order parameter, we show that the direct spectroscopic determination of the superconducting energy gap reveals an anomalous suppression of Andreev reflection between LaRh$_3$B$_2$ and a normal metal. This observation hints to the presence of incomplete superconducting gap formation, at least along certain momentum directions, and consequent low-lying quasiparticle states. An analysis of multiple Andreev reflection spectra captured at different points on the surface of LaRh$_3$B$_2$ reveals a distribution of the superconducting energy gap which is consistent with an anisotropic superconducting order parameter.

cond-mat.supr-con↗

arXiv2024

Topological surface states host superconductivity induced by the bulk condensate in YRuB$_2$

While the possibility of topological superconductivity (TSC) in hybrid heterostructures involving topologically nontrivial band structure and superconductors has been proposed, the realization of TSC in a single stoichiometric material is most desired for fundamental experimental investigation of TSC and its device applications. Bulk measurements on YRuB$_2$ detect a single superconducting gap of $\sim$ 1 meV. This is supported by our electronic structure calculations which also reveal the existence of topological surface states in the system. We performed surface-sensitive Andreev reflection spectroscopy on YRuB$_2$ and detected the bulk superconducting gap as well as another superconducting gap of $\sim$ 0.5 meV. From our analysis of electronic structure, we show that the smaller gap is formed in the topological surface states in YRuB$_2$ due to the proximity of the bulk superconducting condensate. Thus, in agreement with the past theoretical predictions, we present YRuB$_2$ as a unique system that hosts superconducting topological surface states.

cond-mat.supr-con↗