Normal Metal Research Articles

High Inductance Density Glass Embedded Inductors for 3-D Integration

Compared with silicon-based inductors, the inductors with through glass via (TGV) technology have significantly improved in quality ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q)$</tex-math> </inline-formula> factor. On the basis of TGV technology, a new type of 3-D embedded inductor is designed for the application scenarios with high inductance density. Through this research, a new preparation process is proposed to reduce the diameter of glass porous. On the basis of fully metallizing the glass porous, larger inductance density is achieved. The fabrication procedure requires grooving inside the glass substrate. In order to ensure the normal metal wiring on the surface, the cavity is filled with organic matter. Finally, the high-density, small-porous inductor array achieves an inductance density of 67.9 nH/mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}$</tex-math> </inline-formula> , which is three times higher than the previous glass inductance density. At the same time, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> factor is 49 and the self-resonance frequency (SRF) is 5.6 GHz.

Sensitivity of a DC SQUID with a non-sinusoidal current-phase relation in its junctions

In ballistic superconductor–normal metal–superconductor Josephson junctions, such as those made from graphene or high mobility semiconductors, the current-phase relation may not have the common, sinusoidal form but can be skewed to have a peak supercurrent at a phase difference greater than π/2. Here, we use a numerical simulation that includes thermal noise to investigate the sensitivity of a DC superconducting quantum interference device (SQUID) with such junctions. The simulation uses a resistively and capacitively shunted junction model where the current-phase relation of each junction can be defined as an arbitrary function. The modulation, transfer function, noise, and sensitivity of a SQUID are calculated for different types of current-phase relation. For the examples considered here, we find that the flux sensitivity of the SQUID is always degraded by forward skewing of the current-phase relation, even in cases where the transfer function of the SQUID has been improved.

Application of the pseudo-Voigt function to the analysis of single-particle tunneling spectra of [formula omitted]-wave superconductors

In this study, we focused on the peak–dip–hump structure, which is a hallmark of cuprate superconductivity and is often used to explore the relationship between strong electron–phonon coupling and superconductivity. Specifically, we examined the peak–dip–hump structure and other features of cuprate tunneling spectra in d-wave superconductor–insulator–normal metal (dSC–I–N) junctions. To demonstrate the peak–dip–hump structure and other features of cuprate tunneling spectra, we applied the pseudo-Voigt function as an energy-dependent gap. We also analyzed two important experimental features of the tunneling spectra, namely the temperature and doping evolution of the differential conductance. Finally, we compared our calculation results with experimental data and found a good quantitative agreement between theory and experimental data in the considered cases.

Renormalization of antiferromagnetic magnons by superconducting condensate and quasiparticles

The ability to modify and tune the spin-wave dispersion is one of the most important requirements for engineering of magnonic networks. In this study we demonstrate the promise of synthetic thin-film hybrids composed of an antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal for tuning and modifying the spin-wave dispersion in antiferromagnetic insulators. The key ingredient is the uncompensated magnetic moment at the AF/S(N) interface, which induces an effective exchange field in the adjacent metal via the interface exchange interaction. The exchange field spin polarizes quasiparticles in the metal and induces spinful triplet Cooper pairs screening the magnon. The quasiparticle and Cooper pair polarization renormalizes the magnon dispersion. The renormalization results in the splitting of the otherwise degenerate AF magnon modes with no need to apply a magnetic field. It is also proposed that measurements of the renormalized dispersion relations can provide the amplitude of the effective exchange field induced by the AF in the adjacent metal.

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor.

In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum1,2. In superconductors lacking (local)inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin-orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram3-5. However, when the Zeeman effect is suppressed because of spin locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries6-8. Here we report the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite-momentum Cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of a normal metal, a uniform Ising superconducting phase and a six-fold orbital FFLO state. This study highlights an alternative route to achieving finite-momentum superconductivity and provides a universal mechanism to preparing orbital FFLO states in similar materials with broken inversion symmetries.

Tailing the Magnetoelectric Properties of Cr2Ge2Te6 by Engineering Covalently Bonded Cr Self-Intercalation: Ferromagnetic Half-Metal

Two-dimensional intrinsic ferromagnetic half-metals (HMs) are important for spintronics. Based on a systematic research of CrGeTe3 (CGT) bilayers and multilayers with the self-intercalated (SI) Cr (CrSI) atom, we find that self-intercalation can enhance interlayer magnetic coupling, which results in a super-exchange interaction between interlayers. The CGT bilayer keeps half-metallicity with the interlayer ferromagnetic (FM) order after Cr’s self-intercalation, independent of CrSI atoms’ concentration and stacking orders. Moreover, SI-CGT multilayers with interlayer FM order transform from HMs into normal spin-polarized metals, as the states at the Fermi level increase. Moreover, the magnetic anisotropy energies (MAEs) of SI-CGT-AA and SI-CGT-AB are −0.160 and −0.42 meV/f.u., respectively, which are modulated by the CrSI atoms. The MAEs of SI-CGT-AA and SI-CGT-AB are different, as the hybridization interactions between Cr’s d orbitals are different from each other. SI-CGT bilayers’ magnetic easy axis (EA) switches from the original [001] of CGT to the [100] direction, independent of the stacking orders. It is inferred that the MAE is mainly contributed by the hybridization between Te’s px and py, py and pz orbitals, and that this hybridization interaction is obviously weakened as CrSI atom is inserted. The SI-CGT bilayers and multilayers show good dynamic and thermal stability at 300 and 500 K. These findings offer a promising way to manipulate the interlayer exchange interaction and magnetoelectric properties of CGT multilayers and other vdW magnets.

The penta-hexa silicene: A promising candidate for intrinsic room temperature magnetic semiconductor

Performing ab initio calculations, we investigate electronic and magnetic properties of a silicon allotrope (PH-silicene) composed entirely by six silicon pentagons and two silicon hexagons. The dynamically and mechanically stable PH-silicene hosts two-dimensional honeycomb spin structures, which can be antiferromagnetic, ferromagnetic, or ferrimagnetic depending on the applied tensile strain and/or number of stacked layers. In particular, the transition temperature of an in-plane antiferromagnetic ground state and a strain-induced ferromagnetic state of monolayer PH-silicene is found to be around 533 and 80 K, respectively. This unusual metal-free magnetism can be explained by the d0 charge transfer mechanism. On the other hand, we show that the PH-silicene is an indirect semiconductor with the bandgap of 0.585 eV. When stacking up to 4-layers, they vary from the semiconductor, the semimetal to the normal metal. Our findings suggest PH-silicene as a promising candidate for the room temperature magnetic semiconductor and will pave a way for silicon based spintronic devices.

Topological metals constructed by sliding quantum wire arrays

A general strategy of alternated slide construction to craft topological metals is proposed, where there is a relative slide between the odd and even chains in the trivial spinless quantum wire array. Firstly, taking the three-leg ladder as an example, we find that alternated slide can induce a topological phase transition from the normal metal to topological metal phases, which are protected by inversion symmetry. Remarkably, topological metal without nontrivial edge states is found, and the bulk-boundary correspondence breaks down. Secondly, the two-dimensional quantum wire arrays with alternated slide manifests similar physical behaviors. Two types of topological metal phases emerge, where there are gapless bulk bands with and without nontrivial edge states. These results could be confirmed by current experimental techniques.

Strong-Coupling Behavior of the Critical Temperature of Pb/Ag, Pb/Cu and Pb/Al Nanocomposites Explained by Proximity Eliashberg Theory

The experimental critical temperature of the systems of superconducting (Pb) and normal (Ag, Cu and Al) nanoparticles, with a random distribution and sizes less than their respective coherence lengths, is governed by the proximity effect, as shown by the experimental data. At first glance, the behavior of the variation in the critical temperature in function of the ratio of volume fractions of the superconducting and the normal metal components seems to suggest a weak coupling behavior for the superconductor. In reality, upon a more careful analysis, using Eliashberg’s theory for the proximity effect, the system instead shows a strong coupling nature. The most interesting thing is that the theory has no free parameters and perfectly explains the behavior of the experimental data just with the assumption in the case of the nanoparticles Ag and Cu, that the value of the density of states at the Fermi level of silver and copper is equal to the value of lead.

Effects of spin-orbit coupling on proximity-induced superconductivity

We investigate the effect of spin orbit coupling on proximity induced superconductivity in a normal metal attached to a superconductor. Specifically, we consider a heterostructure where the presence of interfaces gives rise to a Rashba spin orbit coupling. The properties of the induced superconductivity in these systems are addressed within the tunneling Hamiltonian formalism. We find that the spin orbit coupling induces a mixture of singlet and triplet pairing and, under specific circumstances, an odd frequency, even parity, spin triplet pairs can arise. We also address the effect of impurity scattering on the induced pairs, and discuss our results in context of heterostructures consisting of materials with spin-momentum locking.

Modulation of Contact Resistance of Dual-Gated MoS2 FETs Using Fermi-Level Pinning-Free Antimony Semi-Metal Contacts.

Achieving low contact resistance (RC ) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate-voltages (VTG and VBG ). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG , in sharp contrast to Ti contacts thatonly modulate RC by varying VBG . The anomalous behavior is attributedto the strongly modulated pseudo-junction resistance (Rjun ) by VTG , resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG . Technology computer aided design simulations further confirm the contribution of VTG to Rjun , which improves overall RC of Sb-contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual-gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG . The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.

Dynamics of weak magnetic coupling by x-ray ferromagnetic resonance

We investigate the interaction between two magnetic layers separated with a normal metal insertion layer (Ti, Pt, and Ru) using x-ray ferromagnetic resonance (XFMR). We measure the amplitude and phase of the ferromagnetic resonance of both layers. Our results indicate that a ferromagnetic exchange coupling between two layers is a dominant coupling mechanism for a thick insertion metal layer. Based on the exchange coupling model, we extract the smallest value of the indirect exchange coefficient of 1.2 μJ/m2, which corresponds to an exchange field of about 0.36 mT. While this value is difficult to measure with other experimental tools, we were able to measure the small value because XFMR detects a resonance phenomenon of a thin layer generated by an oscillating indirect exchange and the Oersted fields with a phase and layer resolved observation.

Nonequilibrium Josephson diode effect in periodically driven SNS junctions

In typical Josephson junctions, the Josephson current is an odd function of the superconducting phase difference. Recently, diode effect in Josephson junctions is observed in experiments wherein the maximum and the minimum values of the Josephson current in the current-phase relation do not have the same magnitude. We propose a superconductor-normal metal-superconductor (SNS) junction where Josephson diode effect manifests when the normal metal region is driven. Time reversal symmetry and inversion symmetry need to be broken in the SNS junction for the diode effect to show up. We calculate long time averaged current and show that the system exhibits diode effect for two configurations of the driven SNS junction - one in which inversion symmetry is broken in the undriven part of the Hamiltonian and the other wherein both the symmetries are broken by the driving potential. In the latter configuration, a nonzero current known as anomalous current appears at the junction in absence of phase bias. In the proposed setup, the diode effect vanishes in the adiabatic limit.

Gate-Tunable Negative Differential Conductance in Hybrid Semiconductor–Superconductor Devices

Negative differential conductance (NDC) serves as a crucial characteristic that reveals various underlying physics and transport process in hybrid superconducting devices. We report the observation of gate-tunable NDC outside the superconducting energy gap on two types of hybrid semiconductor–superconductor devices, i.e., normal metal–superconducting nanowire–normal metal and normal metal–superconducting nanowire–superconductor devices. Specifically, we study the dependence of the NDCs on back-gate voltage and magnetic field. When the back-gate voltage decreases, these NDCs weaken and evolve into positive differential conductance dips; and meanwhile they move away from the superconducting gap towards high bias voltage, and disappear eventually. In addition, with the increase of magnetic field, the NDCs/dips follow the evolution of the superconducting gap, and disappear when the gap closes. We interpret these observations and reach a good agreement by combining the Blonder–Tinkham–Klapwijk (BTK) model and the critical supercurrent effect in the nanowire, which we call the BTK-supercurrent model. Our results provide an in-depth understanding of the tunneling transport in hybrid semiconductor–superconductor devices.

Magnetic-field-controlled spin valve and spin memory based on single-molecule magnets

A single-molecule magnet is a long-sought-after nanoscale component because it can enable us to miniaturize nonvolatile memory storage devices. The signature of a single-molecule magnet is switching between two bistable magnetic ground states under an external magnetic field. Based on this feature, we theoretically investigate a magnetic-field-controlled reversible resistance change active at low temperatures in a molecular magnetic tunnel junction, which consists of a single-molecule magnet sandwiched between a ferromagnetic electrode and a normal metal electrode. Our numerical results demonstrate that the molecular magnetism orientation can be manipulated by magnetic fields to be parallel/antiparallel to the ferromagnetic electrode magnetization. Moreover, different magnetic configurations can be “read out” based on different resistance states or different spin polarization parameters in the current spectrum, even in the absence of a magnetic field. Such an external magnetic field-controlled resistance state switching effect is similar to that in traditional spin valve devices. The difference between the two systems is that one of the ferromagnetic layers in the original device has been replaced by a magnetic molecule. This proposed scheme provides the possibility of better control of the spin freedom of electrons in molecular electrical devices, with potential applications in future high-density nonvolatile memory devices.

Topological Superconductivity Mediated by Skyrmionic Magnons.

Topological superconductors are associated with the appearance of Majorana bound states, with promising applications in topologically protected quantum computing. In this Letter, we study a system where a skyrmion crystal is interfaced with a normal metal. Through interfacial exchange coupling, spin fluctuations in the skyrmion crystal mediate an effective electron-electron interaction in the normal metal. We study superconductivity within a weak-coupling approach and solve gap equations both close to the critical temperature and at zero temperature. Special features in the effective electron-electron interaction due to the noncolinearity of the magnetic ground state yield topological superconductivity at the interface.

Low-frequency spin dynamics in diluted magnetic semiconductors

The low-frequency dynamical magnetic properties in paramagnetic diluted magnetic semiconductors are addressed in the framework of the dynamical mean-field theory applied for the Kondo lattice model. In the infinite-dimensional limit, a set of self-consistent equations is derived so the single-particle Green's function and its self-energy can be evaluated numerically. In terms of the Green's function and self-energies, the local dynamical spin susceptibility function and then the spin-relaxation rate are explicitly expressed based on the Baym-Kadanoff approach. It is found that the spin fluctuations become dominated, indicated by the sharp peak appearing at the low frequency of the spin dynamical susceptibility function in the case of large magnetic coupling and temperature close to the paramagnetic-ferromagnetic transition point. The low-frequency spin dynamic in the systems is also addressed in the signatures of the spin-relaxation process. In the case of large temperature and small magnetic coupling, the spin-relaxation rate releases the scenario of the Korringa process specifying the weak correlation systems likely normal metals. Otherwise, i.e., at small temperature and large magnetic coupling, we find exponential behavior of the spin-relaxation rate versus temperature. Moreover, at a temperature approaching the paramagnetic-ferromagnetic transition point, one finds sharp suppression of the spin-relaxation rate or speeding up of the spin-relaxation time. These scenarios are attributed to the appearance of the magnetic coherence bound state or the spin clusters in diluted magnetic semiconductors due to the strongly magnetic correlations.

Implementation of SNS thermometers into molecular devices for cryogenic thermoelectric experiments

Thermocurrent flowing through a single-molecule device contains valuable information about the quantum properties of the molecular structure and, in particular, on its electronic and phononic excitation spectra and entropy. Furthermore, accessing the thermoelectric heat-to-charge conversion efficiency experimentally can help to select suitable molecules for future energy conversion devices, which—predicted by theoretical studies—could reach unprecedented efficiencies. However, one of the major challenges in quantifying thermocurrents in nanoscale devices is to determine the exact temperature bias applied to the junction. In this work, we have incorporated a superconductor–normal metal–superconductor Josephson junction thermometer into a single-molecule device. The critical current of the Josephson junction depends accurately on minute changes in the electronic temperature in a wide temperature range from 100 mK to 1.6 K. Thus, we present a device architecture which can enable thermoelectric experiments on single molecules down to millikelvin temperatures with high precision.

Resistivity of various metals described in a wide temperature range with a universal theoretical equation

A generic conductivity equation, developed in our previous work without any restrictions to specific materials, is borrowed to evaluate how conductivity/resistivity changes with temperatures in a very wide temperature range from almost zero to several hundreds of Kelvins. Two scenarios are investigated, the parameters A and B in the conductivity equation are both dependent and independent of temperature. Derived equations show that conductivity/resistivity has a Taylor expansion relationship with temperature. They can fit resistivity data very well at both low and high temperature regions with an adequate volume-temperature expansion relationship. The predictions are in line with experimental observations and better than the latest empirical equations. Our findings provide a unified theoretical description of normal, bad, and strange metals.

High performance laser-driven flyers based on a refractory metamaterial perfect absorber.

Laser-driven flyers (LDFs), which can drive metal particles to ultra-high speeds by feeding high-power laser, have been widely used in many fields, such as ignition, space debris simulation, and dynamic high-pressure physics. However, the low energy-utilization efficiency of the ablating layer hinders the development of LDF devices towards low power consumption and miniaturization. Herein, we design and experimentally demonstrate a high-performance LDF based on the refractory metamaterial perfect absorber (RMPA). The RMPA consists by a layer of TiN nano-triangular array, a dielectric layer and a layer of TiN thin film, and is realized by combing the vacuum electron beam deposition and colloid-sphere self-assembled techniques. RMPA can greatly improve the absorptivity of the ablating layer to about 95%, which is comparable to the metal absorbers, but obviously larger than that of the normal Al foil (∼10%). This high-performance RMPA brings a maximum electron temperature of ∼7500 K at ∼0.5 µs and a maximum electron density of ∼1.04 × 1016 cm-3 at ∼1 µs, which are higher than that the LDFs based on normal Al foil and metal absorbers due to the robust structure of RMPA under high-temperature. The final speed of the RMPA-improved LDFs reaches to about 1920 m/s measured by the photonic Doppler velocimetry system, which is about 1.32 times larger than the Ag and Au absorber-improved LDFs, and about 1.74times larger than the normal Al foil LDFs under the same condition. This highest speed unambiguously brings a deepest hole on the Teflon slab surface during the impact experiments. The electromagnetic properties of RMPA, transient speed and accelerated speed, transient electron temperature and density have been systematically investigated in this work.