† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 11574377), the State Key Development Program for Basic Research of China (Grant Nos. 2018YFA0305700 and 2014CB921500), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH013), and the JSPS KAKENHI (Grant No. 15H03681).
We will build a cubic anvil cell (CAC) apparatus for high-pressure and low-temperature physical property measurements in the synergic extreme condition user facility (SECUF). In this article, we first introduce the operating principle, the development history, and the current status of the CAC apparatus, and subsequently describe the design plan and technical targets for the CAC in SECUF. We will demonstrate the unique advantages of CAC, i.e., excellent pressure homogeneity and large hydrostatic pressure capacity, by summarizing our recent research progresses using CAC. Finally, we conclude by providing some perspectives on the applications of CAC in the related research fields.
Recently, research interest has grown in condensed matter physics to explore novel emergent phenomena under high-pressure (HP) conditions. One celebrated example is the recent discovery of superconductivity (SC) with a record-high transition temperature of 203 K when pressurizing sulfur hydride to above 150 GPa.[1] The importance of HP is evidenced by the following aspects: first, pressure (P) is one of the fundamental thermodynamic parameters like temperature (T) that control the states of matter. It was predicted that condensed matters would on average undergo five phase transformations upon compression to 100 GPa. As such, the application of HP provides an effective means to access many new states of matter and can thus expand the universe of matters substantially. Next, pressure can effectively reduce the interatomic distances, enlarge the orbital overlap integral, and strengthen the electron transfer, and thus tip the balance of competing interactions in strongly correlated electron systems. In this regard, HP is one of the key tuning parameters to realize the quantum critical point, which has been considered as a universal phenomenon for many difficult problems in modern condensed matter physics.[2] Consequently, the application of HP would allow one to disentangle the key factors governing the physical properties of interest. It can not only give a critical check on some theoretical proposals but also provide important implications for practical applications. For example, the observation of positive pressure effect on the superconducting transition temperature (Tc) would prompt one to further enhance Tc at the ambient pressure via substituting smaller species, or the chemical pressure.[3] With the advances of HP techniques, researchers have thus focused more on in situ measurements of physical properties under HP conditions.
As many intriguing phenomena in condensed matter physics typically occur at low temperatures, it is necessary to combine HP techniques with a cryostat and/or magnet. Currently, piston cylinder cell (PCC)[4,5] and diamond anvil cell (DAC)[6,7] are widely used for high-pressure and low-temperature (HPLT) physical property measurements. The PCC can provide an excellent hydrostatic pressure condition and offer a relatively large sample space suitable for various physical property measurements. However, its maximum pressure of ∼ 3 GPa is insufficient for many studies pertaining to condensed matters with a bulk modulus on the order of ∼ 100 GPa. In contrast, the DAC can reach pressures above 300 GPa, and is exceptionally powerful in combination with synchrotron x-ray-based techniques owing to its transparent character. However, the sample space is very limited, thereby causing it difficult to measure the physical properties that require electrical contacts. Consequently, a solid or no-pressure transmitting medium is typically utilized when measuring the resistivity with DAC.[8,9] This will inevitably introduce a strong pressure inhomogeneity and shear strain, and thus hides the intrinsic pressure responses.
Among the various HP techniques, the cubic anvil cell (CAC)[10] stands out as an excellent supplementary that can solve the above-mentioned shortcomings of PCC and DAC. It can reach hydrostatic pressures up to 15 GPa with a relatively large sample space exceeding 1 mm3. Given the merits of excellent pressure homogeneity and large pressure capacity, we will build a CAC apparatus in the synergic extreme condition user facility (SECUF). In Section
Because the CAC has to be placed in a cryostat and/or magnet, the design of CAC involves some special requirements. In the following, we describe briefly its operating principle, development history, and current status.
As illustrated in Fig.
Currently, most CACs employ 4 mm × 4 mm square-top anvils and a 6 mm-edge-length cubic gasket together with the Teflon capsule with an outer diameter of 2 mm (see Fig.
The CAC apparatus for in situ HPLT measurements was originally developed by N. Mori in the 1990s at the Institute for Solid State Physics (ISSP), University of Tokyo. A detailed description of the CAC apparatus can be found elsewhere.[10] As shown in Fig.
However, the massive cell body of the conventional CAC apparatus prevents an efficient cooling to well below 2 K and also renders it difficult to integrate with a strong magnetic field. These issues were overcome by the recent development by Y. Uwatoko using a clamp-type, “palm”-sized CAC.[13] The operation principle is almost identical to the conventional type, except that the applied loading force is locked by tightening the nut of the beryllium-copper clamp cell, which can sustain a maximum loading force of 100 ton for the current design. As illustrated in Figs.
To further increase the pressure capacity of the palm CAC, we designed an integrated-fin gasket (see Fig.
Given the excellent pressure homogeneity and large hydrostatic pressure capacity, a CAC apparatus has been approved in the SECUF project. We shall follow the most recent developments of the CAC in the ISSP, University of Tokyo and aim to achieve the state-of-the-art technical targets. We will employ the palm CAC with the integrated-fin gasket and 2.5-mm WC anvils to reach the maximum hydrostatic pressure of 15 GPa. We shall integrate the palm CAC with two different cryostats for different research purposes: (i) a liquid 4He sample-in-vapor cryostat equipped with a 9 T room-temperature (RT) bore cryogen-free superconducting magnet for routine HPLT measurements from RT down to 1.5 K under 9 T; (ii) a cryogen-free 3He/4He dilution refrigerator equipped with 12 T cold-bore superconducting magnet for ultralow temperature studies. We will perform the following physical property measurements: electrical resistivity, magnetoresistance and Hall resistivity, dielectric constant, ac magnetic susceptibility, and ac specific heat.
By taking advantage of the CAC apparatus, we have recently obtained some important results in the research of unconventional SC. In the following, we present three examples to demonstrate the critical role of CAC.
One of the common features of unconventional superconducting systems such as the heavy-fermion, high-Tc cuprate and iron-pnictide superconductors is that SC emerges on the border of long-range magnetically ordered state. The close proximity of SC to a magnetic instability suggests that the critical spin fluctuations would be crucial for mediating the Cooper pairs. In addition to doping charge carriers, the application of HP is an effective approach for searching novel classes of unconventional superconductors near the magnetic quantum critical point (QCP). After we discovered SC with Tc ≈ 2 K in CrAs by suppressing its helimagnetic order via the application of hydrostatic pressure of ∼ 1 GPa,[17] we explored the pressure-induced SC in MnP by following the similar strategy.[18]
At the ambient pressure, MnP is an itinerant-electron helimagnet with an orthorhombic B31-type structure similar as CrAs. In the absence of magnetic field, MnP undergoes two successive magnetic transitions upon cooling:[19] a transition from the paramagnetic (PM) to ferromagnetic (FM) state at Tc = 291 K, and subsequently the second transition to a double helical state at Ts ≈ 50 K.[19] Earlier HP studies on MnP have revealed that both Tc and Ts decrease with pressure, but the highest hydrostatic pressure of ∼ 3 GPa in the previous studies was far from the magnetic QCP.[20]
Figure
Among the iron-based superconductors, the structurally simplest β-FeSe and its derived materials have attracted tremendous attention recently owing to its peculiar electronic properties and the great tunability of Tc.[23,24] At the ambient pressure, FeSe undergoes a tetragonal-to-orthorhombic structural transition at Ts = 90 K, below which it develops a significant electronic anisotropy or nematicity. Unlike the iron-pnictides superconductors, however, no magnetic order appears within the nematic phase of FeSe, and the SC with Tc ≈ 9 K occurs without chemical doping or HP. Meanwhile, it was found that HP can induce a static magnetic order at Tm around 1 GPa[25] and promote a high-Tc SC with a Tc of up to 37 K around 7 GPa.[26] These facts render FeSe an ideal platform to study the interplay of SC with magnetism and nematicity.
Despite the extensive HP investigations on FeSe, unfortunately, all previous studies failed to construct a completed T–P phase diagram for FeSe owing to technical limitations; i.e., the PCC with a maximum pressure of ∼ 3 GPa is insufficient to reach the highest Tc around 7 GPa,[27] whereas the presence of significant pressure inhomogeneity in the DAC with a solid pressure medium hinders the evolution tracking of the nematic and magnetic orders.[26] In contrast, the excellent hydrostaticity and large pressure capacity render CAC an ideal choice for this study.
By performing HP resistivity and ac magnetic susceptibility up to 15 GPa with the palm CAC on high-quality FeSe single crystals, we constructed its most comprehensive T–P phase diagram[28] (see Fig.
Previous studies on the FeSe-derived high-Tc (> 30 K) superconductors, including AxFe2−ySe2 (A = K, Cs, Rb, Tl), (Li,Fe)OHFeSe, and monolayer FeSe film, have shown a distinct Fermi surface (FS) topology consisting of only electron pockets near the Fermi level.[23] In contrast, the FS of FeAs-based superconductors typically consists of hole and electron pockets near the Brillouin zone center and corners, respectively. Thus, the distinct FS topology between FeAs- and FeSe-based materials has challenged the current theories on a unified understanding of the mechanism of iron-based superconductors.
In contrast to the electron-doping approaches, the application of HP does not introduce extra electron carriers, yet can still enhance the Tc of bulk FeSe to ∼ 38 K near 6 GPa. To understand the mechanism of pressure-induced high-Tc SC in FeSe, it is important to obtain the information about the evolution of FS under HP—a regime in which angle-resolved photoemission spectroscopy experiments are impractical, and where quantum oscillation measurements are challenging. Alternatively, we measured the Hall effect under hydrostatic pressures up to 8.8 GPa with the CAC to gain further insights into the electronic structure evolution of FeSe at HP.[29]
As shown in Fig.
Although the bulk FeSe displays a relatively low Tc = 8.5 K within the nonmagnetic nematic phase below Ts = 90 K, a high-Tc SC with Tc above 30–40 K have been successfully achieved via intercalating some alkali–metal ions, ammonia, or organic molecules in between the adjacent FeSe layers, such as in AxFe2−ySe2 (A = K, Rb, Cs, Tl),[30] Ax(NH3)yFeSe,[31] and (Li, Fe)OHFeSe.[32] We refer these high-Tc superconductors derived directly from FeSe as the SC-I phase. The superconducting mechanism for these SC-I phases has been subjected to extensive investigations, and the observed common FS topology consisting of only electron pockets in the Brillouin zone corners suggests that the electron doping plays an essential role for achieving a high Tc, in agreement with the gate-voltage regulation experiments on the FeSe flakes.[33]
Starting from the SC-I phase in AxFe2−ySe2, it was reported that pressure can induce the sudden reemergence of a second superconducting phase (denoted as SC-II hereafter) with a higher Tc of up to 48.7 K above ∼ 10 GPa.[34] A similar SC-II phase has also been observed in Cs0.4(NH3)yFeSe under HP.[35] Although the reemergence of the SC-II phase with a higher Tc is intriguing and a different pairing symmetry has been proposed theoretically,[36] the intrinsic superconducting- and normal-state properties have been poorly characterized thus far owing to some sample and technical difficulties. For example, AxFe2−ySe2 superconductors are prone to phase separation accompanied by the intergrowth of the antiferromagnetic insulating A2Fe4Se5 phase. Moreover, HP techniques capable of both large pressure capacity and good hydrostaticity are required to obtain reliable superconducting- and normal-state properties. Therefore, these complexities have hampered the proper understanding of the intriguing SC-II phase of these FeSe-derived materials.
To approach this intriguing problem, we focus on the recently discovered (Li1−xFex)OHFeSe,[32] which is free from phase separation, relatively stable in air, and more importantly, can be obtained in high-quality single crystals via a specially designed hydrothermal ion-exchange method.[37] Using the palm CAC, we recently performed detailed magnetotransport measurements on (Li1−xFex)OHFeSe single crystals under hydrostatic pressures up to 12.5 GPa.[38]
The primary results are summarized in Fig.
In summary, the CAC apparatus has some unique advantages over other HP techniques for low-temperature physical property measurements. First, the three-axis compression geometry together with the sample in liquid medium ensures excellent pressure homogeneity, and thus allows one to obtain the intrinsic pressure effects. Next, the CAC can reach a high hydrostatic pressure up to 15 GPa, which is an important pressure range to alter the ground states of condensed matters having a bulk modulus on the order of 100 GPa. In addition, the sample space of the CAC is relatively large to facilitate the accurate measurements of different physical properties such as resistivity, magnetic susceptibility, and ac specific heat.
Given the excellent hydrostatic pressure condition and large pressure capacity, we plan to build a CAC apparatus in the SECUF project and aim to achieve the state-of-the-art technical targets, i.e., the highest hydrostatic pressure of 15 GPa, the lowest temperature of <50 mK, and the strongest magnetic field of 12 T. Under these multiple extreme conditions, we will be able to perform the following measurements: electrical resistivity, magnetoresistance and Hall resistivity, dielectric constant, ac magnetic susceptibility, and ac specific heat.
Upon completion, the CAC apparatus will be open to both domestic and overseas users for collaborative research in different subjects. The CAC is especially useful in the field of strongly correlated electron systems: it can be used to tune the competing interactions to realize different ground states,[39] to explore exotic phenomena such as unconventional SC associated with pressure-induced QCP,[17,18] and to manipulate the metal–insulator transition or other phase transitions.[11,40] In the emergent field of topological materials, the CAC can also be employed to engineer the band gap size or the band topology to access different topological states,[41,42] or even to realize topological phase transitions.[43]
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