非共型な結晶中の電子がもたらす多彩な相転移の研究

概要

This dissertation reports the discovery and elucidation of a variety of phas_e transitions driven by electrons in nonsymmorphic c1-ystals. Nonsymmorphic c対stalsare solids with crystal structures belonging to nonsymmorphic space groups. In this dissertation, a~l the crystals treated in our study belong to the nonsymmorphic space group Pnma. In the nonsymmorphic space group, there are more than one equivalent point even at the most high-symmetric site in the unit ce~. In most case, this multiplicity of a crystallographic site comes from glide or screw operation as an essential symmetry operation of a space group. In such a space group, there are always specific kpoints (Bloch state) at the boundary of the Brillouin zone where the energy degeneracy of the electronic state is maintained. In principle, the more glide and screw operations exist in a space group of a crystal structure, the more such年 ointsthere will be for the electrons in the crystal. The nonsymmorphic space group Pnma, as its notation indicates, contains n・glide and a・glide operation as essential symmetry operations, as well as three two-fold screw axes. As a result, the nuinber of kpoints with the essential degeneracy of the electronic states at the Brillouin zone boundary is also large. Therefore, the electronic instability resulting from the large density of states of electrons at the Fermi level can be expected when such degenerate electronic bands touch the Fermi level in metallic systems. When the Fermi surface instability exists in a metallic system, a chan邸 ofexternal parameter, such as temperature or pressure, may lead to a p畑setransition to release the instability. Before our research started, many compounds which crystallize in Pnma space group have been already known for a variety of phase transitions, such as noncollinear. magnetism and unconventional superconductivity. However, there were not many systematic research and experiments on the synthesis and physical properties of materials from the view point of how nonsymmorphic symmetry affects phase transitions and P_hysical properties. In this paper, we introduce the nonmagnetic and magnetic phase transitions in the ternary systems NbCrP, TaCrP, and NbMnP with a three-dimensional orthorhombic TiNiSi・type crystal structure. Those transitions have been newly discovered and reported by our extended research for many compounds with Pnma symmetry. We also present the 75As・NMR Knight shift study on the pressure induced superconductivity in CrAs with orthorhombic MnP・type crystal structure with Pnma symmetry, which has attracted much attention as a very rare platform to understand the relation between helical magnetism and unconventional superconductivity.

In Part Ⅰ, a general introduction to the dissertation is written, describing the background of the research and the measurement principles of the experiments.

In Part—II, "'.'e will・ present.the discovery of畑 oidentical nonmagnetic structural phase transitions in NbCrP and TaCrP. The frrst・order phase transitions are discovered in our measurements for single crystalline samples which are obtained using the flux-method for the first time. We tried many flux-methods to make singl~ crystals and found that the single crystalline sample of NbCrP can be obtained by Sn-flux method while Ga-flux works for TaCrP. We performed resistivity, magnetization, and NMR measurements to study the mechanism of those transitions. Our collaborator's X・ray diffraction and band calculation were also performed to add the insights into those first-order transitions. The trigger of the structural transitions in those systems is thought to be the band Jahn-Teller effect which has been reported as the main cause of the nonmagnetic metal-insulator transitions in RuP and RuAs. RuP and RuAs crystallize in a three-dimensional orthorhombic MnP・type struct血rewith a space group of Pnma. Those metal-insulator transitions have been suggested) to originate from the Fermi surface instability coming fi・om, the degenerate flat bands with essential degeneracy protected by Pnma symmetry. Therefore, the n~nsymm~rphic symmetry of the crystals is thought to play an impo1-tant role in producing the electronic instability responsible for these phase transiti~ns. Although the transitions in NbCrP and TaCrP are not metal-insulator transitions, the decreased density_ of states resulting from smaller Fermi surfaces and the crystal structure with lower symmetry than Pnma is observed in ground states. The comparison of ternary compounds NbCrP and TaCrP in TiNiSi・type structure and binary RuAs and RuP in MnP・type structure will be helpful to understand when and how Pnma nonsymmorphic symmetry can trigger the nonmagnetic structural phase transition.

In Part-III, we will introduce the discovery of a magnetic phase transition in NbMnP. The noncollinear magnetic structure in the low temperature phase is discovered by the experiments for the single crystalline samples made by self-flux method for the first t1me. In ternary TiNiSi・type intermetallic compounds, several noncollinear magnetic materials have been reported, where the relationship between crystal structure and ma_gnetism is of interest. And we added NbMnP as a new example to study the relationship between noncollinear magnetism and TiNiSi・type structure. In binary MnP・type crystals, there are also many noncollinear magnetic materials with the complex magnetic structure, such as helical and elliptical ordering. The origin of those complex magnetic structures and their variou_s changes of the magnetic structure under exわrnalmagnetic fields is not understood well since one needs to consider many different interactions arising from their crystal natu.re, such as frustration effect caused by many neighboring bonds of magnetic ions and the presence of Dzyaloshinskii・Moriya interaction. It is an interesting topic whether noncollinear magnetism is also likely to appear in ternary TiNiSi-type systems, which is similar to binary MnP・type structure and share the same Pnma symmetry. The discovery of the noncollinear magnetic phase in NbMnP will advance our understanding of the relationship ・ between structural features and magnetism. A~other interesting expectation for NbMnP research is the observation of magnetoelectric effects which is expected for the q = 0 antiferromag~etic structure. For the occurring of a magnetoelectric effect, both space inversion symmetry and time reversal symmetry need to be broken. When a period of antiferromagnetic ordering in nonsymmorphic crystals ~s the same with that of the unit cell, the space inversion symmetry is broken instead of translational symmetry. Therefore, q = 0 antiferromagnetic structure in NbMnP satisfies the condition of magnetoelectric effects.

In Part・ IV, we will present the study using 75As・ NMR Knight shift measurements on the pressure ip.duced superconductor, CrAs. CrAs has been known as the helical ,magnet for a long time, and the recent discovery of unconventional superconductivity has attracted much attention. In addition to being the first Cr-containing unconventional superconductor, the anisotropy of the upper critical field suggests the possibility that CrAs is a spin triplet superconductor, which is rarely observed in d・electron systems. Therefore, the determination whether pressure induced superconductivity in CrAs is spin-triplet or not is very important for the research field of superconductivity. It is also •important to clarify what kind of magnetic correlation contributes to the emergence of the pressure induced superconductivity because CrAs is a rare example where superconductivity occurs in the vicinity of a helical magnetic phase. For iron・based superconductors, antiferromagnetic correlation with simple q is thought to contribute the cooper paring, but in CrAs, ferromagnetic correlation was also observed as well as antiferromagnetic correlation'in paramagnetic phase, which may relate ・to its helical magnetic structure. We performed 75As・NMR Knight shit measurements under pres~ure to reveal the connection between the magnetism and unconventional superconductivity in CrAs. The 75As・NMR Knight shift measurements in superconducting phase were done for the first-time and temperature dependence of Knight shift was successfully obtained. Our results suggest that the superconduc~ivity in CrAs is not the case for a typical s-wave superconductivity, In the context of nonsymmorphic symmetry of crystal, it is also interesting to ・understand how -Fermi surfaces constructed in Pnma symmetry affect unconventional superconductivity. The information of unconv:C;)ntional gap symmetry of CrAs will helpful to examine the recent theoretical suggestion that nonsymmorphic operations restrict the gap symmetry in orthorhombic crystal systems. Since uranium-based ferromagnetic superconductors also have Pnma symmetry, our study provides important insights into the study of unconventional superconductivity.