Quasiparticle density of states of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ single crystals probed using tunneling spectroscopy in the $30–50\mathrm{mK}$ temperature range in high magnetic fields

Break-junction tunneling spectroscopy at temperatures $30–50\mathrm{mK}$ in high magnetic field is used to directly probe the quasiparticle density of states within the energy gap in a single crystal Bi2212 high-$T_c$ superconductor. The measured tunneling conductances $dI∕dV\left(V\right)$ in the subgap region have a zero flat region with no evidence for a linear increase of the density of states with voltage. A number of tunnel break junctions exhibited $dI∕dV\left(V\right)$ curves with a second energy gap structure at the average magnitude $2{\Delta }_{p-p}∕e=13\mathrm{mV}$. Our data cannot be explained by either a pure $s$ pairing or a pure $d_{x^2-y^2}$ pairing.

Figure 1

The scanning electron micrograph of a crystal fragment where the composition measurement points are denoted by crosses.

Figure 2

The temperature dependence of the in-plane resistivity ${\rho }_{ab}$ for one of the investigating underdoped crystals (#1, $p=0.125$). For comparison, we show also ${\rho }_{ab}\left(T\right)$ for an overdoped $(p=0.2)$ single crystal with the same $T_c=84\mathrm{K}$ (midpoint) as well as the data for a slightly underdoped sample with $T_c=87\mathrm{K}$ $(p=0.13)$.

Figure 3

$I\left(V\right)$ characteristics for the (a) $100\Omega$-, (b) $115\mathrm{k}\Omega$-, and (c) $6\mathrm{k}\Omega$-resistance tunnel junctions at $T=30$ and $50\mathrm{mK}$, in different magnetic fields (sample #1). The insets in (a) and the upper insets in (b) and (c) show the corresponding tunneling conductances data $dI∕dV\left(V\right)$ in the subgap region. The lower insets in (b) and (c) show the same $I\left(V\right)$ characteristics in the gap region. (d)–(f) the data points (squares) represent the density of states $N\left(E\right)$ obtained from the deconvolution of Eq. (1) (see text).

Figure 4

The effect of the magnetic field on the gaplike structure (marked by arrows) in the tunneling conductance $dI∕dV$ vs $V$ within the superconducting gap for different break junctions on three single crystals at $T=50\mathrm{mK}$ and $T=30\mathrm{mK}$. (a) and (b) two tunnel break junctions with resistance nearly $1\mathrm{k}\Omega$ fabricated with the same single crystal #7. The inset shows the full gap spectra at 0 and $26\mathrm{T}$. (c) and (d) the subgap regions of the spectra on an enlarged scale for two single crystals #5 and #1 (the resistances nearly $300\Omega$). For clarity, the curves in (b)–(d) have been shifted vertically with respect to the lower curves.

Figure 5

Differential conductances $dI∕dV$ as a function of the bias voltage $V$ for tunnel break junctions fabricated on single crystals #7 and #1 with double peaks in different magnetic fields at $T\simeq 50\mathrm{mK}$. For clarity, the curves have been shifted vertically with respect to the lower curves.