Nonsinusoidal Current-Phase Relationship in Josephson Junctions from the 3D Topological Insulator HgTe

Abstract

We use Superconducting QUantum Interference Device (SQUID) microscopy to characterize the current-phase relation (CPR) of Josephson Junctions from 3-dimentional topological insulator HgTe (3D-HgTe). We find clear skewness in the CPRs of HgTe junctions ranging in length from 200 nm to 600 nm. The skewness indicates that the Josephson current is predominantly carried by Andreev bound states with high transmittance, and the fact that the skewness persists in junctions that are longer than the mean free path suggests that the effect may be related to the helical nature of the Andreev bound states in the surface of HgTe.Topological insulators (TI) have a special band structure with important consequences for proximity-induced superconductivity. In 3-dimentional topological insulators (3D-TI), the inversion of the conduction and valence bands leads to conducting 2D surface states with energies that are linearly proportional to their momenta [1][2][3][4][5]. Spinmomentum locking protects the charge carriers at the surface against elastic backscattering [6,7]. These special properties are reflected in the superconducting proximity effect in an S/3D-TI bilayer or an S/TI/S junction, which may host Majorana fermions in a quasi-1D channel or vortex core [8][9][10]. Most previous works characterized current-voltage characteristics to determine the critical current's dependence on temperature, gate voltage, or magnetic field [11][12][13][14][15][16][17][18][19][20][21][22], while a few studies characterized the CPR [23,24].Here, we use a scanning SQUID microscope to perform contactless measurements of the diamagnetic response of Nb/HgTe bilayers and of the CPR of Nb/HgTe/Nb junctions. In contrast to previous CPR results [23,24], we find no evidence for bulk states, 2 and we observe that the CPRs of many junctions of different sizes consistently exhibit forward skewness.The CPR in an S/TI/S junction is a key diagnostic [8,[25][26][27][28][29][30][31][32]. Weak disorder in the TI far from the superconducting contacts theoretically does not affect the induced superconducting state [33,34]; therefore, Andreev bound states should form in hightransmittance surface channels [8,26,27,29,31]. A CPR with forward skewness -that is, a deviation from a perfect sinusoidal form -is a signature of such high-transmittance Andreev bound states [35][36][37].To our knowledge, there have not been direct observations of forward skewed CPRs in topological insulators [23,24], although the skewness has been indirectly inferred [24] from the Fraunhofer interference pattern. Previous CPR experiments in topological insulators [23,24] were complicated in part by bulk states, self-inductance effects, and bias voltage, factors that are eliminated in this work.Moreover, a skewed CPR can also result from ballistic transport [35]. Measurements in metallic break junctions showed that the CPR approaches the predictions for quantum point contacts in the ballistic limit [38]. In metallic atomic point contacts, the CPR was significantly skewed only in contacts wi...

Electrically Tunable Multiterminal SQUID-on-Tip

“…3b directly depicts the current-phase relation ( ) of junction 2. This correspondence holds for any form of current-phase relations 49 and therefore can provide a general tool, adding to existing techniques [52][53][54][55] , for direct probing of current-phase relations in various unconventional superconductor systems. In order to achieve a highly sensitive continuous operation with low flux noise 2 / = 2 3 / /|5 /5Φ |, one requires a high transfer function |5 /5Φ | and a low current noise 2 3 / at any value of the applied field.…”

Tunneling spectroscopy and Josephson current of superconductor-ferromagnet hybrids on the surface of a 3D TI

“…Recently, a non-sinusoidal current-phase relation has been reported in the 3D TI HgTe junction [47]. In the 3D TI heterojunctions like Nb/Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 /Nb, the temperature dependence of the critical current is almost linear in most of the range [46].…”

“…On the other hand, a variety of interesting phenomena about Josephson effect in TI materials have been discovered [41][42][43][44][45][46][47]. Recently, a non-sinusoidal current-phase relation has been reported in the 3D TI HgTe junction [47].…”

Probing Quantum Capacitance in a 3D Topological Insulator

“…While unstrained HgTe is a zero gap semiconductor with inverted band structure [8,9], the degenerate Γ8 states split and a gap opens at the Fermi energy E F if strained. This system is a strong topological insulator [10], explored by transport [6,7,11], angleresolved photoemission spectroscopy [12], photoconductivity, and magneto-optical experiments [13][14][15][16]; also, the proximity effect has been investigated [17]. Since these two-dimensional electron states (2DES) have high electron mobilities of several 10 5 cm 2 =V s, pronounced Shubnikov-de Haas (SdH) oscillations of the resistivity and quantized Hall plateaus commence in quantizing magnetic fields [6,7,11], stemming from both top and bottom 2DES.…”