arXiv subjects

Cheuk Yin Ng

Publications and source records attributed to Cheuk Yin Ng.

5 recordsLinked to original sources

arXiv2024

Evidence for the novel type of orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual superconducting state, defies high magnetic fields beyond the Pauli paramagnetic limit. It exhibits a spatial modulation of the superconducting order parameter in real space and is exceptionally rare. Recently, an even more exotic variant - the orbital FFLO state - was predicted and identified in the transition metal dichalcogenide superconductor 2H-NbSe2. This state emerges in thin samples with thicknesses below ~40 nm, at the boundary between two and three dimensions. The complex interplay between Ising spin orbit coupling and the Pauli paramagnetic effect can lead to a stabilization of the FFLO state in a relatively large range of the magnetic phase diagram, even well below the Pauli limit. In this study, we present experimental evidence of the formation of this orbital FFLO state in bulk 2H-NbSe2 samples. This evidence was obtained using high-resolution DC magnetization and magnetic torque experiments in magnetic fields applied strictly parallel to the NbSe2 basal plane. Both quantities display a crossover to a discontinuous first-order superconducting transition at the normal state boundary in magnetic fields of 4 T and above. This is usually seen as a sign that Pauli paramagnetic pair breaking effects affect the superconducting state. The magnetic torque reveals a small step-like reversible anomaly, indicating a magnetic field-induced thermodynamic phase transition within the superconducting state. This anomaly bears many similarities to the FFLO transitions in other FFLO superconductors, suggesting the potential existence of an orbital FFLO state in bulk 2H-NbSe2 samples. Additionally, we observe a pronounced in-plane 6-fold symmetry of the upper critical field in the field range above this phase transition, which has previously been interpreted as a hallmark of the orbital FFLO state in thin 2H-NbSe2.

cond-mat.supr-con↗

arXiv2022

Interfacial superconductivity and zero bias peak in quasi-one-dimensional Bi2Te3/Fe1+yTe heterostructure nanostructures

Bi2Te3/Fe1+yTe heterostructures are known to exhibit interfacial superconductivity between two non-superconducting materials: Fe1+yTe as the parent compound of Fe-based superconducting materials and the topological insulator Bi2Te3. Here, we present a top-down approach starting from two-dimensional (2D) heterostructures to fabricate one-dimensional (1D) Bi2Te3/Fe1+yTe nanowires or narrow nanoribbons. We demonstrate that the Bi2Te3/Fe1+yTe heterostructure remains intact in nanostructures of widths on the order of 100 nm and the interfacial superconductivity is preserved, as evidenced by electrical transport and Andreev reflection point contact spectroscopy experiments measured at the end of the nanowire. The differential conductance shows a similar superconducting twin-gap structure as in two-dimensional heterostructures, but with enhanced fluctuation effects due to the lower dimensionality. A zero-bias conductance peak indicates the presence of an Andreev bound state and given the involvement of the topological Bi2Te3 surface state, we discuss a possible topological nature of superconductivity with strong interplay with an emerging ferromagnetism due to the interstitial excess iron in the Fe1+yTe layer, developing in parallel with superconductivity at low temperatures.

cond-mat.supr-con↗

arXiv2022

Nodal and nematic superconducting phases in NbSe2 monolayers from competing superconducting channels

Transition metal dichalcogenides like 2H-NbSe2 in their two-dimensional (2D) form exhibit Ising superconductivity with the quasiparticle spins are firmly pinned in the direction perpendicular to the basal plane. This enables them to withstand exceptionally high magnetic fields beyond the Pauli limit for superconductivity. Using field-angle-resolved magnetoresistance experiments for fields rotated in the basal plane we investigate the field-angle dependence of the upper critical field (Hc2), which directly reflects the symmetry of the superconducting order parameter. We observe a six-fold nodal symmetry superposed on a two-fold symmetry. This agrees with theoretical predictions of a nodal topological superconducting phase near Hc2, together with a nematic superconducting state. We demonstrate that in NbSe2 such unconventional superconducting states can arise from the presence of several competing superconducting channels.

cond-mat.supr-con↗

arXiv2022

Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors

We compare the upper critical field of bulk single-crystalline samples of the two intrinsic transition metal dichalcogenide (TMD) superconductors, 2H-NbSe2 and 2H-NbS2, in high magnetic fields where their layer structure is aligned strictly parallel and perpendicular to the field, using magnetic torque experiments and a high-precision piezo-rotary positioner. While both superconductors show that orbital effects still have a significant impact when the layer structure is aligned parallel to the field, the upper critical field of NbS2 rises above the Pauli limiting field and forms a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while orbital effects suppress superconductivity in NbSe2 just below the Pauli limit. From the out-of-plane anisotropies, the coherence length perpendicular to the layers of 31 Å in NbSe2 is much larger than the interlayer distance, leading to a significant orbital effect suppressing superconductivity before the Pauli limit is reached, in contrast to the more 2D NbS2.

cond-mat.supr-con↗

arXiv2020

Superconductivity beyond Pauli's limit in bulk NbS2: Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state

We present magnetic torque, specific heat and thermal expansion measurements combined with a piezo rotary positioner of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields applied strictly parallel to its layer structure. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust against the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit.

cond-mat.supr-con↗