Discovery of a bi-critical point between antiferromagnetic and superconducting phases in pressurized single crystal Ca0.73La0.27FeAs2 Yazhou Zhou1*, Shan Jiang2*, Qi Wu1*, Vladimir A. Sidorov3, Jing Guo1, Wei Yi1, Shan Zhang1, Zhe Wang1, Honghong Wang1, Shu Cai1, Ke Yang 4, Sheng Jiang4, Aiguo Li4, Ni Ni2, Guangming Zhang5,6†, Liling Sun1,6† & Zhongxian Zhao1,6 1
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China 2
Department of Physics and Astronomy, UCLA, Los Angeles, CA90095, USA
3
Institute for High Pressure Physics, Russian Academy of Sciences, 142190 Troitsk, Moscow, Russia
4
Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China 5
State Key Laboratory for Low dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 6
Collaborative Innovation Center of Quantum Matter, Beijing, 100190, China
One of the most strikingly universal features of high temperature superconductors is the superconducting phase emerging in the close proximity of the antiferromagnetic phase, and the interplay between these two phases poses a long-standing challenge to condensed matter physicists1-8. It is commonly believed that，as the antiferromagnetic transition temperature is suppressed to zero, there appears a quantum critical point and the antiferromagnetic fluctuation around the quantum critical point is responsible for the development of unconventional superconductivity9,10. Here we report that, in contrast to this scenario, the discovery of a direct first-order phase transition with a bi-critical point between the antiferromagnetic metallic phase and superconducting phase in the pressurized high-quality single crystal Ca0.73La0.27FeAs2, which is the ‘parent compound’ of a newly discovered iorn pnictide superconductors with unusual intercalated layers11-13. By advanced in-situ high pressure measurements of electrical
resistivity, alternating current susceptibility, heat capacity and synchrotron x-ray diffraction, we observed that, upon increasing pressure, the superconducting phase with transition temperature 25.8 K suddenly appears at 2.99 GPa and then increases slightly after the antiferromagnetic transition temperature is continuously suppressed and disappears at 26.2 K and 2.80 GPa, from which the bi-critical point is identified at 2.90 GPa and 25.2 K. The discovery of the bi-critical point provides an alternative picture to understand the interplay between antiferromagnetism and superconductivity, and revives the unified theory of the antiferromagnetism and superconductivity proposed
twenty
years
ago
for
the
high
temperature
copper
oxide
11-13
expand
superconductors14,15. PACS: The newly discovered iron pnictide superconductors Ca1-xLaxFeAs2
the family members of Fe-based superconductors2,16-19. This type of superconductors has a novel crystal structure. X-ray diffraction shows that they crystalize in a monoclinic unit cell with -(FeAs)-(Ca/La)-As-(Ca/La)-(FeAs)- stacking along c axis11,12. In particular, the presence of the As-As zig-zag chains in the intercalated layers is distinct from that of the other iron pnictides. The high temperature superconductivity
exhibits in
the
doping
range
from x=0.15
to
x=0.25
(Ref.11,13,20,21). Nuclear magnetic resonance measurements found that the superconducting phase coexists with the antiferromagnetic (AFM) phase in the above doping range and the AFM transition temperature is enhanced with the increased doping concentrations22. However, above the doping level ~ 0.25, the sample
becomes a pure metal with a stripe like AFM long-ranged order21, which can be regarded as the ‘parent compound’ of this new type of superconductors. It is well-known that high pressure is a clean way to realize a continuously tuning of the crystal structure and the corresponding electronic structure without adding chemical complexity, being an ideal method to study the interplay between the AFM and superconducting phases23,24. In this study, by applying a Toroid (also known as Paris-Edinburgh-type) high-pressure cell with glycerin/water (3:2) liquid as the pressure transmitting medium, we first performed comprehensive in-situ hydrostatic pressure measurements of resistance, alternating current (ac) susceptibility and heat capacity on the high-quality single crystals Ca0.73La0.27FeAs2. In Fig.1a, the arrangement of the samples and the components for the measurements on lower part of the pressure anvil is shown (the details on the measurements is described in Method). Figure 1b displays the temperature dependence of electrical resistivity for pressures ranging from ambient pressure to 1.45 GPa. A resistivity drop at the temperature ~54 K (TM), signifying an AFM transition, is observed under ambient pressure, in fairly agreement with the reported results from neutron scattering measurements21. Above the TM, there is a weak structure transition from a monoclinic to a triclinic phase, which can be identified from the first derivative of the resistance with respective to the temperature21. It can be seen that the drop representing the TM in Fig.1b shifts to lower temperature upon increasing pressure. Furthermore, the high-pressure heat capacity results confirm the decrease of the TM with pressure (Fig.1c). The achieved
results are valuable because such a heat capacity measurement, a powerful method to detect a magnetic phase transition, is a challenging technique in high pressure studies (see Method). Then, with further increasing pressure, a pressure-induced superconducting phase with a transition temperature (TC) of 25.8 K is found at 2.99 GPa. The value of the TC slightly increases to 28.5 K with pressure up to 3.97 GP, and then decreases a bit at higher pressures (Fig.2a). The values of the TC are determined by the onset temperatures of the resistivity drops. The zero resistant superconducting state is observed at 3.29 GPa and above (inset of Fig.2a). To further confirm the pressure-induced
superconductivity,
we
performed
in-situ
high
pressure
alternating-current (ac) susceptibility measurements on another sample in the same pressure cell, which is placed next to the sample for the resistivity measurements (Fig.1a). The results are shown in Fig. 2b, where the TC (ac) is determined by the intersection of the lines through the steep and the zero slopes. It can be seen that the trend of the TC (ac) change is in agreement with that of resistivity results. Moreover, by comparing the amplitude of diamagnetic throw of the sample with that of the Pb (employed as a reference, placed next to the sample in the same coil, as shown in Fig.1a) with the similar shape and mass to the sample25, we can estimate the relative change of the superconducting volume in the pressurized Ca0.73La0.27FeAs2, from~ 60 % at pressure of 2.99 GPa to ~97% at 3.53 GPa (Inset of Fig.2b), implying that the pressure-induced superconductivity is abruptly turned on from the AFM state at the critical pressure. Because the Toroid high pressure cell can maintain the hydrostatic
pressure conditions for the measurement as high as ~5 GPa26, we performed our measurements below this pressure. Our
high-pressure
x-ray
diffraction
measurements
indicate
that
no
pressure-induced crystal structure phase transition occurs throughout the pressure range investigated (Fig. S2, Supplementary Information). Thus it can be regarded that the pressure-induced suppression of AFM and emergence of superconductivity are caused by the electron-electron interactions. Next, we summarize the pressure dependence of the characteristic temperatures TM and TC in the phase diagram of Figure 3a, in which there are two distinct low temperature regions representing the AFM phase and superconducting phase. In the AFM phase region, the TM is remarkably suppressed with increasing pressure and terminated at 26.2 K and 2.80 GPa. Then the superconductivity appears at 2.99 GPa, where the TC (R) from the electrical resistance measurements and the TC (ac) from ac susceptibility results are determined to be 25.8 K and 24 K, respectively. The values of the TC are unexpectedly close to the terminating point of TM (26.2 K) at 2.80 GPa, which suggests that the transition from the AFM phase to the superconducting phase is a direct first-order phase transition with an intersected point between the TM and TC at a critical pressure. Such a special point is conventionally defined as a bi-critical point in phase transition theory, which is denoted by the red star in the phase diagram. In the superconducting phase region, the TC shows a weak response to pressure. Our deduced phase diagram is different from that obtained by the chemical doping in Ca1-xLaxFeAs2, whose superconducting phase coexists with the AFM phase22.
Finally, to characterize the first-order phase transitions below the bi-critical point, we analyze the heat capacity versus pressure at different temperatures extracted from our high-pressure heat capacity data, as shown in Fig.3b. Clear heat capacity jumps at the critical pressure PC=2.90 GPa can be observed below the critical temperature TBC=25.2 K. These jumps in the heat capacity, defined as C, directly reflect the first-order phase transition from the AFM phase to the superconducting phase. Remarkably, C is found to vary with temperature (Fig.3c). As the temperature decreases down to the TBC, C suddenly appears. Below TBC, C continuously reduces until it is undetectable at the temperatures lower than 8 K. Such a feature of the changes in the C indicates a first-order phase transition with a bi-critical point (Fig.3c). To our knowledge, this is the first time that a bi-critical point between an AFM phase and a superconducting phase is discovered in high temperature superconductors. Such a phenomenon was predicted nearly twenty years ago by the SO(5) theory of unifying the AFM and superconductivity of the high temperature copper oxide superconductors14,15, in which the chemical potential change is supposed to induce a first-order phase transition with a bi-critical point. But later on it was considered that the bi-critical point is impossible to realize in copper oxide superconductors experimentally or even in the numerical simulations for a simple Hubbard-type model15,27. In our system Ca0.73La0.27FeAs2, however, the reasons why such a bi-critical point can be observed are summarized as follows: (i) the metallic AFM with a unique intercalated layer structure and a corresponding peculiar electronic
structure; (ii) the high quality of the single crystal samples; (iii) our comprehensive advanced high pressure methods. Therefore, our discovery of the bi-critical point in this material provides a unique and solid experimental foundation to understand the interplay between the AFM and superconductivity. Around the bi-critical point, a new unified theory can be developed, paving a path to finalize the debate on the mechanism of high Tc superconductivity28,29.
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Acknowledgements The work was supported by the NSF of China (Grants No. 91321207, No. 11427805, No. U1532267, No. 11404384, No. 20121302227), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020300), the Russian Foundation for Basic Research (Grant No. 15-02-02040), the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (Grant No. DE-SC0011978) and the U. S. NSF DMREF (DMR-1435672).
Author Information † Correspondence and requests for materials should be addressed to L.L.S. (
[email protected]) and G.M.Z. (
[email protected]) *These authors are contributed equally.
Figure 1 Arrangement of samples and measurement components in a Toriod high pressure cell and determinations of the antiferromagnetic (AFM) transition temperature in pressurized single crystal Ca0.73La0.27FeAs2 through the electrical resistance and heat capacity measurements. (a) Top review of the arrangement of samples and the components for electrical resistance and ac susceptibility measurements on the lower part of the pressure anvil. (b) Temperature dependence of the resistivity at different pressures. (c) Heat capacity as a function of temperature at different pressures. The arrows in figure (b) and (c) indicate the AFM transition temperature TM, showing the decrease of the TM with pressure.
Figure 2 Characterizations of pressure-induced superconductivity in Ca0.73La0.27FeAs2. (a) Temperature dependence of the electrical resistivity for given pressures, showing a superconducting transition starting at 2.99 GPa. The low-temperature resistivity is zoomed in for a clearer view (inset). (b) Real part of the ac susceptibility as a function of temperature for different pressures, clearly demonstrating the diamagnetic signals. The inset shows the pressure versus estimated superconducting (SC) volume.
Figure 3 Electronic phase diagram and high-pressure heat capacity data. (a) Pressure-temperature phase diagram of the single crystal Ca0.73La0.27FeAs2. The acronym PM, AFM and SC stand for paramagnetic, antiferromagnetic and superconducting phases, respectively. The wine circles and custom diamonds represent the temperature of the AMF phase transition detected from both the
electrical resistance TM(R) and heat capacity TM(C) measurements under hydrostatic pressure condition. The green diamonds and blue circles stand for the superconducting transition temperature obtained from the resistance TC(R) and ac susceptibility TC(ac) measurements, respectively. The position of red star denotes the location of the bi-critical point, which is determined by an intersection of extrapolated lines of the pressure dependent TM and TC. The gray circles are the data extracted from the heat capacity results. (b) Pressure dependence of the heat capacity for given temperatures, showing the jumps at the boundary of the AFM-SC transition below the bi-critical point temperature (TBC). (c) The jump value (C) of the heat capacity in figure (b) as a function of temperature, illustrating that the C has a maximum at the temperature of the bi-critical point.
