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Register a new userHiggs Amplitude Mode in BCS Superconductors Nb$_{1-x}$Ti$_{x}$N induced by Terahertz Pulse Excitation
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Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically-anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.
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We comment on the latest paper by M. Silaev [Phys. Rev. B {bf 99}, 224511 (2019)]
We present research on the superconducting properties of Nb$_{x}$Re$_{1-x}$ ($x$ = 0.13-0.38) obtained by measuring the electrical resistivity $rho(T)$, magnetic susceptibility $chi(T)$, specific heat $C_P(T)$, and London penetration depth $Deltalambda(T)$. It is found that the superconducting transition temperature $T_c$ decreases monotonically with an increase of $x$. The upper critical field $B_{c2}(T)$ for various $x$ can be nicely scaled by its corresponding $T_c$. The electronic specific heat $C_e(T)/T$, penetration depth $Deltalambda(T)$, and superfluid density $rho_{s}(T)$ demonstrate exponential behavior at low temperatures and can be well fitted by a one-gap BCS model. The residual Sommerfeld coefficient $gamma_0(B)$ in the superconducting state follows a linear field dependence. All these properties suggest an emph{s}-wave BCS-type of superconductivity with a very large $B_{c2}(0)$ for Nb$_{x}$Re$_{1-x}$ (0.13 $leq x leq$ 0.38).
When a continuous symmetry of a physical system is spontaneously broken, two types of collective modes typically emerge: the amplitude and phase modes of the order-parameter fluctuation. For superconductors, the amplitude mode is recently referred to as the Higgs mode as it is a condensed-matter analogue of a Higgs boson in particle physics. Higgs mode is a scalar excitation of the order parameter, distinct from charge or spin fluctuations, and thus does not couple to electromagnetic fields linearly. This is why the Higgs mode in superconductors has evaded experimental observations over a half century after the initial theoretical prediction, except for a charge-density-wave coexisting system. With the advance of nonlinear and time-resolved terahertz spectroscopy techniques, however, it has become possible to study the Higgs mode through the nonlinear light-Higgs coupling. In this review, we overview recent progresses on the study of the Higgs mode in superconductors.
We report on the first observation of a pronounced re-entrant superconductivity phenomenon in superconductor/ferromagnetic layered systems. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu(1-x)Ni(x). The superconducting transition temperature T_{c} drops sharply with increasing thickness d_{CuNi} of the ferromagnetic layer, until complete suppression of superconductivity is observed at d_{CuNi}= 4 nm. Increasing the Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at d_{CuNi}=13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer.
We propose and analyze a scheme for parametrically cooling bilayer cuprates based on the selective driving of a $c$-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time-dependent. We show how modulation at the difference frequency between the intra- and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching $c$-axis transport from a superconducting to resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing non-equilibrium superconductivity even above $T_c$, provided a finite pair density survives between the bilayers out of equilibrium.
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