Magnetic field decouples nodeless surface and nodal bulk orders

TL;DR

PdTe hosts coexisting topological surface states with a nodeless $s$-wave-like gap and a nodal bulk superconducting gap. Magnetic-field-dependent PCAR spectroscopy, analyzed with a two-channel BTK model, reveals an abrupt transfer of Andreev weight from a surface channel with gap $\Delta_s$ to a bulk channel with a nodal gap $\Delta_b$, triggered at a surface-vortex entry field $H_{en}$ around $\sim 0.35\ \mathrm{kG}$ and culminating in surface suppression at $H_{PC} \sim 4.5\ \mathrm{kG}$. The measurements show strong hysteresis due to vortex dynamics, with the surface channel remaining suppressed on field down-sweeps and recovering only at a small opposite field $H_{ex}$; this demonstrates a field-tunable decoupling of surface and bulk superconductivity in a multichannel system. Together, these results illuminate how gap topology and dimensionality govern the resilience of surface superconductivity in the presence of bulk nodal pairing, and they establish a spectroscopic pathway to disentangle coexisting order parameters in topological superconductors.

Abstract

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.

Figures (13)

• Figure 1: PCAR spectroscopy: (a) A representative PCAR spectrum (dots) measured at 470 mK with BTK fit (solid line). (b) Temperature evolution of the PCAR spectrum. A 2D image with single-gap BTK fits is shown in Supplementary Figure 2a. (c) $\Delta$ vs $T$ plot along with the BCS prediction (solid line). $Inset:$ Variation of $\Gamma$ with temperature, obtained from single gap BTK fit.
• Figure 2: Magnetic field evolution and hysteresis: Spectra with field sweep (a) up from 0 to 5.1 kG, (b) down from 5 kG to 0, ($Inset:$ Spectrum recorded at $H=0$ kG during sweep down: OZFS not recovered.), (c) up from 0 kG to -5 kG (d) down from -5 kG to 0. ($Inset:$ Spectrum recorded at $H=0$ kG during sweep down: OZFS not recovered), up from 0 kG to 5 kG. (f)The left image shows how the spectral evolution with an applied magnetic field ramping from 5 kG to -5 kG, while the right image shows the ramp from -5 kG to 5 kG. The black line corresponds to the zero-field spectra.
• Figure 3: Field-driven transfer of Andreev weight from surface to bulk in PdTe.(a) 3D schematic: illustration of the bulk gap has nodal lobes (colored clover), and the surface nodeless gap. (b) Vortex-free (Meissner) contact geometry: injected quasiparticles predominantly probe the surface channel. (c) After first vortex entry beneath the contact: the surface response is locally suppressed and the bulk nodal channel becomes dominant. (d) PCAR spectrum at $H=0$ G with two-gap BTK fit; the surface gap $\Delta_s$ carries most of the weight ($w_s \approx 0.91$). (e) PCAR spectrum at $H=0.4$ kG. Andreev enhancement is quenched and the fit is bulk-dominated with $\langle\Delta_b\rangle_{\!\perp}$ with a markedly reduced surface weight ($w_s \approx 0.13$).
• Figure 4: (a) Temperature evolution of PCAR spectra at $H=0.4$ kG. A 2D image with single-gap BTK fits is shown in Supplementary Figure 2b (b) Spectrum at $H=0.4$ kG fitted with a two-gap BTK model (green) compared to a single-gap fit (red). (c) Temperature dependence of $\Delta_{\mathrm{s}}$ (green) and $\langle\Delta_{\mathrm{b}}\rangle_{\!\perp}$ (red) from two-gap fits. (d) Magnetic-field dependence of $\Delta_{\mathrm{s}}$ and $\langle\Delta_{\mathrm{b}}\rangle_{\!\perp}$; inset shows coherence peak variation without significant gap change in $-0.5$ to $+0.5$ kG. (e) Field dependence of the weight factor for $\Delta_{\mathrm{s}}$; right inset shows zoom near zero field; left inset shows the field dependence of $\Gamma_{\mathrm{b}}$.
• Figure :