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Pressure-induced superconductivity in itinerant antiferromagnet CrB2

The recent discovery of superconductivity up to 32 K in the pressurized MoB2 revives the interests in exploring novel superconductors in transition-metal diborides isostructural to MgB2. Although the Mo 4d-electrons participate in Cooper pairing, the electron-phonon coupling remains the dominant mechanism for the emergence of superconductivity in MoB2. To explore possible superconductivity driven by unconventional pairing mechanism, we turn our attention to an itinerant antiferromagnet CrB2. Here we report on the discovery of superconductivity up to 7 K in CrB2 via the application of external pressure. Superconductivity is observed after the antiferromagnetic transition at TN ~ 88 K under ambient pressure is completely suppressed. Then, the superconducting Tc increases monotonically with pressure and this evolution takes place without a structural transition in the sample. Since the proximity of superconductivity to an antiferromagnetic order, the quantum criticality and unconventional superconductivity may exist in CrB2. Current study would promote further studies and explorations of novel superconductors in other transition-metal diborides.


INTRODUCTION
Now it is generally accepted that superconductivity occurs via two microscopic mechanisms. One is the celebrated Bardeen-Cooper-Schrieffer (BCS) mechanism based on the electron-phonon coupling that can be applied to the large family of socalled conventional superconductors, among which MgB2 holds the record of the highest Tc at ambient pressure reported so far. Thus, the Tc ≈ 39 K of MgB2 has been considered to approach the McMillan limit [1]. The others beyond the BCS theory are usually named as unconventional superconductors, among which the well-known examples include the cuprates and iron-based high-Tc superconductors with the Tc values well above the McMillan limit [2,3]. Although the microscopic mechanism for these unconventional superconducting families remains elusive so far, one of the common features is that the superconductivity emerges in the vicinity of long-range antiferromagnetically ordered state [4][5][6][7]. The close proximity of superconductivity to a magnetic instability suggests that the critical spin fluctuations should play a crucial role on the formation of Cooper pairs. As such, to realize an antiferromagnetic quantum critical point (QCP) has been considered as an effective approach for exploring novel unconventional superconductors. Despite of the prevailing consensus on the magnetism-mediate mechanism, recent studies on several unconventional superconductors have also indicated the importance of electron-phonon coupling [8,9].
In this regard, it would be interesting to explore some candidate superconducting materials that can potentially combine cooperatively the phonon-and spin-mediate mechanisms.
Keeping this idea in mind, we recently initiated a high-pressure study on the transitionmetal diborides, which are isostructural to MgB2 but are expected to have the spin degree of freedom. We surprisingly discovered that the application of high pressure drives the paramagnetic MoB2 to a superconductor with Tc as high as 32 K, which is close to that of MgB2 and the highest among the transition-metal diborides even though reported the discovery of superconductivity in WB2 with the optimal Tc ~ 17 K at 90 GPa [11]. Although the 4d/5d-electrons participate in the formation of Cooper pairs in these two compounds, the influences of spin degree of freedom are not evident due to the absence of long-rage magnetic order at ambient conditions.
To this end, we turn our attention to a 3d transition-metal diborides, CrB2, which exhibits an itinerant-electron antiferromagnetic (AF) transition at TN ≈ 88 K at ambient pressure [12]. CrB2 has been known for a long time and it adopts the AlB2-type hexagonal crystal structure (space group: P6/mmm), isostructural to the well-known MgB2 [13]. Although some high-pressure study at very limited pressure range has been performed on this compound [14], none of the following questions have been addressed so far: (i) whether the AF order can be suppressed completely by higher pressure? (ii) if so, can the superconductivity emerge near the AF QCP? (iii) what is the role of spin and phonon in mediating Cooper pairs?
To address these issues, here we perform a comprehensive high-pressure study on the CrB2 single crystal over a wide pressure range. We find that the AF order of CrB2 can indeed be suppressed by pressure ~ 4 GPa and then bulk superconductivity appears at higher pressures. The superconducting Tc increases monotonically with pressure and reaches a maximum and unsaturated Tc of 7 K at ~ 100 GPa without showing structural transition. In addition to CrAs, CrB2 becomes the second Cr-based superconductor at high pressures.

EXPERIMENTAL SECTION
Single crystals of CrB2 were grown out of Al flux. Cr (99.5 %), B (99.9 %) and Al The high-pressure resistivity was measured with the standard four-probe method in a palm-type cubic anvil cell (CAC) apparatus at pressures < 12 GPa [15]. Glycerol was used as the pressure transmitting medium. The pressure values were estimated from the pressure-load calibration curve determined at low temperatures by monitoring the superconducting transition of lead. It should be noted that the pressure values inside the CAC exhibit slight variations upon cooling, which has been well characterized in our previous work [15].
Resistivity measurements under higher pressures (>12 GPa) were performed in a nonmagnetic diamond anvil cell (DAC). A cubic BN/epoxy mixture layer was inserted between BeCu gaskets and electrical leads. Electrical resistivity was measured using the dc current in van der Pauw technique in Physical Property Measurement System (Quantum Design). Pressure was measured at room temperature using the ruby scale by measuring the luminescence from small chips of ruby placed near the sample.
In-situ high pressure XRD measurements were performed at the beamline 15U at Shanghai Synchrotron Radiation Facility (λ = 0.6199 Å). Symmetric DAC with anvil culet size of 200 μm and Re gasket were used. Mineral oil was used as pressure transmitting medium and pressure was determined by the ruby luminescence method [16]. CeO2 was used to calibrate the sample-detector distance and the orientation parameters of the detector. The two-dimensional diffraction images were analyzed using the FIT2D program [17]. Rietveld refinements on crystal structures under high pressure were performed by General Structure Analysis System (GSAS) and graphical user interface EXPGUI package [18,19].

RESULTS AND DISCUSSION
At ambient pressure, the resistivity ρ(T) of CrB2 shows a metallic behavior and displays a pronounced kink anomaly at TN ≈ 88.5 K corresponding to the AF transition temperature. The AF order is further confirmed by magnetic measurements and is consistent with previous studies [20]. To study the effect of pressure on the AF transition of CrB2, we have measured the temperature-dependent resistivity ρ(T) under various hydrostatic pressures up to 12 GPa (Fig. 1a) by using the CAC. With increasing pressure, the magnitude of resistivity shifts down slightly in the whole temperature range, but TN manifested by the peak in dρ/dT is reduced quickly to ~ 66.2 K at 2 GPa and 42.3 K at 3.5 GPa, and almost vanishes over 4 GPa, as shown in Figs. 1b and 1c. The pressure dependence of TN shown in Fig. 1d indicates the presence of AF QCP near ~ 4 GPa. In contrary to our expectation, however, we did not observe superconductivity down to 1.4 K and up to 12 GPa, which are the temperature and pressure limit of our CAC apparatus. Whether the superconductivity will occur at lower temperatures in the studied pressure range deserves further studies.
To explore whether superconductivity can be achieved in CrB2 under higher pressure, we then measured ρ(T) of CrB2 single crystal by using DAC in an extended pressure range. Figure 2a   From the above measurements, we can conclude that the application of high pressure first suppresses the long-range AF order around Pc ~ 4 GPa and then induces superconductivity with Tc increasing continuously with pressure. As shown in Figure 4, a rough extrapolation of the Tc(P) to zero seems to match closely to Pc. In addition, the low-temperature ρ(T) change from Fermi liquid to non-Fermi liquid behaviors near Pc and then back to Fermi-liquid-like one, implying an intimated correlation between the AF QCP and the emergence of superconductivity. Unlike the common observations that Tc peaks out near the AF QCP, however, Tc(P) continuously climbs up with increasing pressure without showing signs of saturation up to at least 110 GPa. This observation implies that besides the possible magnetism-mediate pairing mechanism, other pairing mechanism may also exist in CrB2, which need to be clarified in the future.
Chromium (Cr) is the only element metal that shows spin-density-wave AF order above room temperature, and superconductivity has not been observed even when the AF order is suppressed by either high pressure or chemical doping[21-23]. Except for several binary Cr alloys [24][25][26][27][28], there have been very few Cr-containing compounds exhibiting superconductivity [29][30][31][32][33][34]. Besides CrAs, CrB2 is the second Cr-based superconductor under high pressures. For CrAs, the superconductivity emerges with the maximum Tc close to the AF QCP, while the optimal Tc for CrB2 occurs well above Pc. From a structural perspective, Cr-based superconductors up to now are limited to one-dimensional (A2Cr3As3 and ACr3As3 (A = K, Rb, Cs, Na)) or three-dimensional structures (CrAs, Pr3Cr10-xN11, Cr2Re3B). CrB2 shown here has a typical layered structure, which resembles the situation in the 2D cuprates and iron-pnictides. Current study thus sheds light on the exploration of novel superconductors in the Cr and other transition metal-based systems. Here, the Tcs are determined at the 90% of the normal state resistivity just above the onset superconducting transition temperature. The red lines represent the fits using the Ginzburg-Landau formula.