Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene

In conventional superconductors, Cooper pairing occurs between electrons of opposite spin. We observe spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom. Although all of these phases are metallic at zero magnetic field, we observe a transition to a superconducting state at finite B ‖ ≈ 150mT applied parallel to the two-dimensional sheet. Superconductivity occurs near a symmetry breaking transition, and exists exclusively above the B ‖ -limit expected of a paramagnetic superconductor with the observed transition temperature T C ≈ 30mK, consistent with a spin-triplet order parameter.

Steric Quenching of Mn(III) Thermal Spin Crossover: Dilution of Spin Centers in Immobilized Solutions

Structural and magnetic properties of a new spin crossover complex [Mn(4,6-diOMe-sal2323)]+ in lattices with ClO4−, (1), NO3−, (2), BF4−, (3), CF3SO3−, (4), and Cl− (5) counterions are reported. Comparison with the magnetostructural properties of the C6, C12, C18 and C22 alkylated analogues of the ClO4− salt of [Mn(4,6-diOMe-sal2323)]+ demonstrates that alkylation effectively switches off the thermal spin crossover pathway and the amphiphilic complexes are all high spin. The spin crossover quenching in the amphiphiles is further probed by magnetic, structural and Raman spectroscopic studies of the PF6− salts of the C6, C12 and C18 complexes of a related complex [Mn(3-OMe-sal2323)]+ which confirm a preference for the high spin state in all cases. Structural analysis is used to rationalize the choice of the spin quintet form in the seven amphiphilic complexes and to highlight the non-accessibility of the smaller spin triplet form of the ion more generally in dilute environments. We suggest that lattice pressure is a requirement to stabilize the spin triplet form of Mn3+ as the low spin form is not known to exist in solution.

Controllable chiral Majorana modes by noncollinear magnetic configuration in a topological Josephson junction

Recently, theory and experiment both have confirmed a Majorana zero mode to induce selective equal spin Andreev reflection (SESAR). Herein, we theoretically present controllable chiral Majorana modes (CMMs) by noncollinear magnetic configuration in a Josephson junction on a topological insulator with two ferromagnetic insulators (FIs) sandwiched in between two superconductors (SCs). It is shown that an additional phase shift is induced by the different chirality of the CMMs at the two FI/SC interfaces, whose magnitude is determined by misorientational angle θ, which can be administrated by the Andreev bound surface energies. The angle θ is found to result in the 0-π state transition and Φ0 supercurrent. Particularly, due to the SESAR, the coexistence of fully spin-polarized spin-singlet and -triplet correlations is exhibited with the exclusive fully spin-polarized spin-triplet (singlet) correlation corresponding to the ferromagnetic (F) [antiferromagnetic (AF)] configuration. For the two magnetizations only along y-axis, there exist no additional phase shift and topological supercurrent with fully spin-polarized correlations, especially, the supercurrent in the AF configuration is a lot larger than that in the F one, which is strongly dependent on the exchange field strength and FI length, thus even leading to 100% supercurrent magnetoresistance. The results can be employed to not only identify the topological SCs but also design a perfect topological supercurrent spin valve device.

Dynamical second-order noise sweetspots in resonantly driven spin qubits

Quantum dot-based quantum computation employs extensively the exchange interaction between nearby electronic spins in order to manipulate and couple different qubits. The exchange interaction, however, couples the qubit states to charge noise, which reduces the fidelity of the quantum gates that employ it. The effect of charge noise can be mitigated by working at noise sweetspots in which the sensitivity to charge variations is reduced. In this work we study the response to charge noise of a double quantum dot based qubit in the presence of ac gates, with arbitrary driving amplitudes, applied either to the dot levels or to the tunneling barrier. Tuning with an ac driving allows to manipulate the sign and strength of the exchange interaction as well as its coupling to environmental electric noise. Moreover, we show the possibility of inducing a second-order sweetspot in the resonant spin-triplet qubit in which the dephasing time is significantly increased.

Superconductivity coexisting with ferromagnetism in a quasi-one dimensional non-centrosymmetric (TaSe4)3I

Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5 K in the quasione dimensional crystals of non-centrosymmetric (TaSe4)3I (space group: P¯421c). The unique phase is a direct consequence of inversion symmetry breaking as the same material also stabilizes in a centro-symmetric structure (space group: P4/mnc) where it behaves like a non-magnetic insulator[1–4]. The coexistence here upfront contradicts the popular belief that superconductivity and ferromagnetism are two apparently antagonistic phenomena. Notably, here, for the first time, we have clearly detected Meissner effect in the superconducting state despite the coexisting ferromagnetic order. The coexistence of superconductivity and ferromagnetism projects non-centrosymmetric (TaSe4)3I as a host for complex ground states of quantum matter including possible unconventional superconductivity with elusive spin-triplet pairing[5–8].

Switchable Spiral Josephson junction: A superconducting spin-valve proposal

We propose a superconducting spin valve based on a Josephson junction with B20-family magnetic metal as barrier material. Our analysis shows that the states of this element can be switched by reorienting the intrinsic non-collinear magnetization of the spiral magnet. This reorientation modifies long-range spin-triplet correlations and thereby influences strongly the critical Josephson current. Compared to superconducting spin valves proposed earlier, our device has the following advantages: (i) it contains only one barrier layer, which makes it easier to fabricate and control; (ii) its ground state is stable, which prevents uncontrolled switching; (iii) it is compatible with devices of low-T Josephson electronics. This device may switch between two logical states which exhibit two different values of critical current, or its positive and negative value. I.e. 0-π switch is achievable on one simple Josephson junction.

Geometric entanglement of a photon and spin qubits in diamond

Geometric nature, which appears in photon polarization, also appears in spin polarization under a zero magnetic field. These two polarized quanta, one travelling in vacuum and the other staying in matter, behave the same as geometric quantum bits or qubits, which are promising for noise resilience compared to the commonly used dynamic qubits. Here we show that geometric photon and spin qubits are entangled upon spontaneous emission with the help of the spin − orbit entanglement inherent in a nitrogen-vacancy center in diamond. The geometric spin qubit is defined in a degenerate subsystem of spin triplet electrons and manipulated with a polarized microwave. An experiment shows an entanglement state fidelity of 86.8%. The demonstrated entangled emission, combined with previously demonstrated entangled absorption, generates purely geometric entanglement between remote matters in a process that is insensitive of time, frequency, and space mode matching, which paves the way for building a noise-resilient quantum repeater network or a quantum internet.

Evidence for anisotropic spin-triplet Andreev reflection at the 2D van der Waals ferromagnet/superconductor interface

Fundamental symmetry breaking and relativistic spin–orbit coupling give rise to fascinating phenomena in quantum materials. Of particular interest are the interfaces between ferromagnets and common s-wave superconductors, where the emergent spin-orbit fields support elusive spin-triplet superconductivity, crucial for superconducting spintronics and topologically-protected Majorana bound states. Here, we report the observation of large magnetoresistances at the interface between a quasi-two-dimensional van der Waals ferromagnet Fe0.29TaS2 and a conventional s-wave superconductor NbN, which provides the possible experimental evidence for the spin-triplet Andreev reflection and induced spin-triplet superconductivity at ferromagnet/superconductor interface arising from Rashba spin-orbit coupling. The temperature, voltage, and interfacial barrier dependences of the magnetoresistance further support the induced spin-triplet superconductivity and spin-triplet Andreev reflection. This discovery, together with the impressive advances in two-dimensional van der Waals ferromagnets, opens an important opportunity to design and probe superconducting interfaces with exotic properties.