CrossingLineNode Semimetals:
General Theory and Application to RareEarth Trihydrides
Abstract
Multiple line nodes in energyband gaps are found in semimetals preserving mirrorreflection symmetry. We classify possible configurations of multiple line nodes with crossing points (crossing line nodes) under pointgroup symmetry. Taking the spinorbit interaction (SOI) into account, we also classify topological phase transitions from crossinglinenode Dirac semimetals to other topological phases, e.g., topological insulators and pointnode semimetals. This study enables one to find crossinglinenode semimetal materials and their behavior in the presence of SOI from the band structure in the absence of SOI without detailed calculations. As an example, the theory applies to hexagonal rareearth trihydrides with the HoD structure and clarifies that it is a crossinglinenode Dirac semimetal hosting three line nodes.
I Introduction
The degeneracy (node) of the energy spectrum in the Brillouin zone is a topological object. Gapless semimetals are the realization of topological nodes in condensed matter physics Murakami (2007); Wan et al. (2011); Young et al. (2012); Fang et al. (2012); Wang et al. (2012); Chiu et al. (2016); Chan et al. (2016). Interestingly, semimetals hosting topological nodes exhibit novel transport and response phenomena for external electromagnetic fields Koshino and Ando (2010); Zyuzin et al. (2012); Zyuzin and Burkov (2012); Hosur and Qi (2013). For instance, in Weyl semimetals, which have point nodes in the Brillouin zone, electric current flows perpendicular to an electric field (anomalous Hall effect) and parallel to a magnetic field (chiral magnetic effect Fukushima et al. (2008)) due to the topological structure of the nodes.
Since topological invariants crucially depend on the spatial dimension Schnyder et al. (2008); Kitaev (2009); Ryu et al. (2010); Matsuura et al. (2013), node structures other than point nodes are expected to induce topological responses distinct from those in Weyl semimetals. The line node Burkov et al. (2011); Chiu and Schnyder (2014); Fang et al. (2015); Gao et al. (2016); Fang et al. (2016); Yu et al. (2016); Carbotte (2016); Lim and Moessner (2017); Roy (); Murakami et al. () is one of these intriguing topological electronic states. Many linenode semimetal materials Mullen et al. (2015); Chen et al. (2015, 2015); Kim et al. (2015a); Liu et al. (2016); Weng et al. (2016); Huang et al. (2016); Xie et al. (2015); Yamakage et al. (2016); Ezawa (2016); Zhu et al. (2016); Hirayama et al. (2017); Kawakami and Hu (); Xu et al. (2017); Geilhufe et al. (2017); gup () have been proposed and some measurements have actually seen line nodes in semimetals Bian et al. (); Schoop et al. (2015); Neupane et al. (2016); Wu et al. (2016); Hu et al. (a); Okamoto et al. (2016); Takane et al. (2016); Emmanouilidou et al. (). Moreover, exotic magnetic transports Singha et al. (); Ali et al. (); Wang et al. (); Hu et al. (b) in linenode semimetals has been recently reported. In addition, superconductivity is also found in the noncentrosymmetric linenode semimetal PbTaSe Wang et al. (2016); Zhang et al. (); Chang et al. (2016); Pang et al. (2016); Guan et al. (). Linenode semimetals have great potential for diverse developments in materials science.
In contrast to point nodes, there are many types of configurations of line nodes, i.e., single, spiral Heikkilä et al. (2011); Heikkilä and Volovik (2011), chain Bzdušek et al. (), separate multiple Hirayama et al. (); Bian et al. (); Schoop et al. (2015); Neupane et al. (2016); Bian et al. (2016), nexus Heikkilä and Volovik (2015); Hyart and Heikkila (); Zhu et al. (), and crossing Weng et al. (2015); Zeng et al. (); Kim et al. (2015b); Yu et al. (2015); Du et al. () line nodes.
In this work, we focus on crossinglinenode semimetals, as shown in Fig. 1, and study a general theory for it from the viewpoint of crystalline symmetry. The configuration of the crossing line nodes is uniquely determined for a given level scheme of conduction and valence bands under a pointgroup symmetry. The spinorbit interaction (SOI) may open a gap in the line nodes but the crossing points possibly remain gapless, i.e., a Dirac semimetal may be realized. We also clarify whether the resulting states are Dirac semimetals or (topological) insulators. Applying the obtained results, one can find Dirac semimetals and topological insulators from linenode semimetals and can derive their topological indices from the band calculation in the absence of SOI.
As an example, we apply the present theory to a hexagonal hydride, YH [space group (No. 165)], with the HoD structure Mansmann and Wallace (1964). YH has been focused on as a switchable mirror Huiberts et al. (1996), i.e., the metalinsulator transition takes place at from a reflecting cubic crystal to a transparent hexagonal one. From optical measurements Griessen et al. (1997); van Gogh et al. (1999); Lee and Shin (1999); van Gogh et al. (2001), the gap has been evaluated to be 2.8 eV or slightly smaller. On the other hand, early band calculations predicted that the hexagonal YH is a semimetal rather than an insulator Wang and Chou (1993); Dekker et al. (1993); Wang and Chou (1995). Subsequent studies discussed another lower symmetric structure Kelly et al. (1997), weak van Gelderen et al. (2000); Miyake et al. (2000); van Gelderen et al. (2002) and strong Eder et al. (1997); Ng et al. (1997, 1999) correlation effects giving rise to a finite gap in YH. Although the actual material is insulating, we study the gapless electronic structure of the YH without correlation effects, as a representative of HoDstructure materials, and its topological properties in detail since the electronic structure has been established so it is useful for further investigations. The YH with HoD structure is shown to be a semimetal hosting three crossing line nodes. A tiny energy gap ( meV) is induced in the line nodes by SOI. This gap is characterized by the topological indices of (1;000).
Ii Crossing line nodes protected by point group symmetries
In general, a band crossing located on highsymmetry planes/lines is stable toward band repulsion if each energy band belongs to different eigenstates of crystalline symmetry. In particular, in mirrorreflection symmetric systems without SOI, a band crossing forms a stable Dirac line node (DLN) when it lies on a mirrorreflection plane and two energy bands have different mirrorreflection eigenvalues. Generalizing this approach to all point groups, we investigate crossing line nodes protected by point groups: , , , , , and () and their possible topological phase transitions to topological insulators and Dirac semimetals.
Here, we consider a level scheme consisting of onedimensional (1D) irreducible representations (IRRs) of the lowest conduction and highest valence bands. We focus on mirrorreflection symmetryprotected DLNs encircling timereversal invariant momenta (TRIM). According to the Schoenflies symbols, mirror reflections are labeled as , , and , which represent horizontal, vertical, and diagonal mirrorreflection operations in point groups, respectively. When conduction and valence bands cross on a ()symmetric plane, the band crossing is stable if 1D IRRs and have different eigenvalues of from each other, i.e., the character of is in . Furthermore, the number of crossing lines corresponds to the number of equivalent planes. For example, in symmetric systems, possible crossinglinenode configurations are , , and for , , and , respectively, where labels line nodes protected by () symmetry. Table 1 shows possible crossing line nodes for each point group, and the correspondence with the level schemes is shown in Appendix A. The symmetryadapted effective Hamiltonian for 1D IRRs are also described in Appendix B. The study of crossing line nodes for 1D IRRs can be generalized to crossing line nodes for higher dimensional IRRs. In that case, it is necessary to take into account the effect of multibands. Nevertheless, when we choose a basis diagonalizing , the mechanism for protecting line nodes is the same as in the 1D IRR case: namely, a line node on a symmetric plane is stable if two bands forming the line node have the different eigenvalues of . In particular, a level scheme consisting of 2D (3D) IRRs leads to two (three) line nodes at most on a symmetric plane. Possible line node configurations for 2D and 3D IRRs are listed in Table 7 in Appendix.
PG  Line nodes  SOI 

,  TI  
TI  
(, , )  I  
,  TI  
I  
TI  
(, )  ,  DP 
I  
, , ,  TI  
NI  
,  TI  
I  
(, )  , , ,  DP 
NI  
,  TI  
I  
TI  
,  DP  
TI 
Iii Effect of SOI
In systems with SOI, mirrorreflection symmetryprotected line nodes are generally unstable Yamakage et al. (2016) except for nonsymmorphic systems Fang et al. (2015); Liang et al. (2016) since the mirrorreflection eigenvalues for spin up and down are different, i.e., with spin up hybridizes with with spin down. This instability potentially leads to different topologically nontrivial phases such as Dirac/Weyl semimetals and topological insulators. The criteria for realizing these topological phases depend intrinsically on the level schemes and the number of line nodes encircling a TRIM, as we shall show in the following.
In the presence of SOI, the energy bands are labeled by the double representations, and 1D IRRs without SOI all become 2D IRRs after taking the product with the spin representation . Therefore, after including SOI, the crossing points of multiple line nodes on the symmetric line remains as a Dirac point if each crossing energy band belongs to different double representations within , i.e., when in are compatible with the 1D IRRs of , and and are different. Note that the symmetryprotected Dirac points occur independently of the presence of spatialinversion symmetry. The same criterion is applicable to higher dimensional IRRs if is decomposed into 2D IRRs, and two different 2D IRRs cross on a symmetric line. However, we do not completely predict the presence of Dirac points from the level schemes since the multibands are labeled again after including the SOI. Off the symmetric line, antisymmetric SOI may turn line nodes into Weyl points. The presence/absence of the Weyl points depends totally on the form of the SOI. It is beyond the scope of the paper to discuss such Weyl points.
If the SOI opens a gap on line nodes or an effect of breaking the crystalline symmetry destabilizes the Dirac point, the timereversalinvariant systems potentially become topological insulators, depending on the band topology of the occupied states. For centrosymmetric systems with point groups , , , , and (), we can adapt the parity criterion proved in Ref. Kim et al., 2015b for the crossing line nodes, which allows us to determine the topological number of the topological insulator from the number of DLNs in the system without the SOI: (see Appendix C for more details)
(1)  
(2) 
where is the number of DLNs encircling the TRIM for the th primitive reciprocal lattice vector.
On the other hand, for noncentrosymmetric systems, we can partially determine the topological numbers from the number of DLNs by adapting the mirrorparity criterion proved in Ref.Yamakage et al., 2016, which is applicable to the DLN of , , and , of , and of and . For these cases, the strong index is given by Eq. (1). The weak indices and are given by Eq. (2). The third weak index is also determined from Eq. (2), except for and . For example, when a single DLN encircles a TRIM in the absence of SOI, the topological numbers are given by () for of and ; () for of , of , and of and , where is determined for , , and due to the presence of an additional mirrorreflection symmetry. Other noncentrosymmetric systems are outside the scope of the mirrorparity criterion and depend on the details of the SOI.
Material  LN  w/ SOI  TRIM  PG  Ref. 

MT carbon  DP  Weng et al.,2015  
LaN  DP  Zeng et al.,  
CuNPd  DP  Kim et al.,2015b; Yu et al.,2015  
CaTe  DP  Du et al.,  
YH  TI  this work 
The obtained results enable us to predict the Dirac points and topological invariants in the presence of SOI from the band structures in the absence of SOI, without calculating the inversion/mirrorreflection parities of the wave functions. As an example, in Table 2, we show the results for four materials proposed in the literature.
Iv Application to rareearth trihydrides
Applying the general theory, we show that a hexagonal rareearth trihydride with the HoD structure is a crossinglinenode semimetal with three line nodes. As a representative of the HoDstructure materials, we consider the hexagonal YH. Results for LuH and ferromagnetic GdH are shown in Appendix F. In the present work, the band structure is calculated using the WIEN2k code Blaha et al. (2001). We used the fullpotential linearized augmented planewave method within the generalized gradient approximation. 10 10 8 point sampling was used for the selfconsistent calculation.
The gapless band structure in the hexagonal YH was originally proposed by Dekker et al. Dekker et al. (1993) and is verified by our calculation, as shown in Fig. 2.
Nearly gapless band dispersions are found on the M, K, and A lines. The detailed calculation shown in Fig. 3(a) reveals that the band gap closes at 0.13 Å on the lines and at 0.14 Å on the lines while the gap opens by 4 meV on the K line. Moreover, the conduction and valence bands at the point are assigned to the and representations of , respectively.
From the general theory, the system must host three crossing line nodes in the A–A scheme. Three crossing line nodes are actually seen on the three mirror (MAL) planes. The location of the nodes is depicted in the inset of Fig. 2. On the K line, a tiny band gap opens since the KAH planes are not mirror planes. On other lowsymmetry lines, the band gap is also weakly generated, on the order of 1 meV. In other words, the system could behave as a Diracsurfacenode semimetal such as graphene networks Zhong et al. (2016) and Ba ( V, Nb, Ta; S, Se) Liang et al. (2016), except for the lowenergy and lowtemperature regime (less than 1 meV).
It is worth mentioning that the Fermi surface of the holedoped system mainly consists of the 1 orbitals of the H atoms (see the right panel of Fig. 2). At eV, at which the carrier density is about cm, 90% of the total density of states comes from the 1 orbitals of H. This Fermi surface might lead to hightemperature superconductivity, as in hydrogen sulfide Drozdov et al. (2015); Akashi et al. (2015); Einaga et al. (2016). Indeed, YH has been predicted to be a superconductor below 40 K under 17.7 GPa Kim et al. (2009), although the crystal structure is not the HoD structure but the fcc under pressure Ahuja et al. (1997); Palasyuk and Tkacz (2005); Ohmura et al. (2006); Kume et al. (2007); Machida et al. (2007).
As mentioned above, the crossing line nodes realize and encircle the point, which has the pointgroup symmetry. The conduction and valence bands at the point are not degenerate, i.e., belong to the 1D IRRs of the point group. Then, our general theory shown in Table 1 and Eqs. (1) and (2) tells us that the SOI induces a gap on the crossing line nodes. The resulting gapped state is a strong topological insulator of (1;000). Notice that, strictly speaking, the system is semimetallic but the topological invariants are well defined since the direct gap opens at any momenta. The firstprinciples data, which are shown in Fig. 3(b), coincides with this prediction. The induced spinorbit gap is estimated to be on the order of 1 meV. The SOI of the Y atom is small because it is not a heavy element. The SOI of the H atom is, obviously, negligible. Note that the Dirac point on the A point, which is located 0.7 eV below the Fermi level, still remains even in the presence of SOI, due to the nonsymmorphic symmetry of Young et al. (2012).
Finally, we construct a lowenergy effective Hamiltonian in the vicinity of the point to describe the crossing line nodes and SOI, as follows: Here, denotes the Pauli matrix for the orbitals ( for the and orbitals, respectively). denotes the Pauli matrix for the spin. The parameters are determined to reproduce the crossing line nodes of the firstprinciples data: eVÅ, eVÅ, eVÅ. As seen in Fig. 2, the band structure is nearly isotropic and particlehole symmetric, hence the parameters approximately satisfy and . Calculating the surface states of the above effective model, we verify that YH is a strong topological insulator of . We focus on the surface. in the above Hamiltonian is regularized as and . The obtained lattice Hamiltonian is solved by using the recursive Green’s function technique Miyata et al. (2013, 2015), and the angleresolved density of states on the (001) surface is shown in Fig. 4.
The system is, as mentioned above, a semimetal but hosts gapless surface states around the point, which is projected from the point onto the surface, within the direct gap. This directly proves that the direct gap of YH is characterized by the topological indices (1;000).
V Summary
We studied a general theory classifying crossinglinenode semimetals under pointgroup symmetries. The classification tells us the configuration of crossing line nodes for a given level scheme of conduction and valence bands. This also enables us to determine whether the crossing line nodes are gapped by the SOI from the configuration of the nodes. This will be quite important for materials development, i.e., one can predict materials being topological insulators and semimetals by exploring the bandcalculation database in the absence of SOI, without any detailed calculations.
We found that the rareearth trihydride YH, as a representative of HoD–structure materials, is a crossingline node semimetal, which hosts three line nodes on the mirrorreflectioninvariant planes. Although YH is known to probably be an insulator by correlation effects, the present study encourages us to address materials with the HoD structure and promises to realize a new topological semimetal.
This study has extensively revealed the electronic states of crossing line nodes. There, on the other hand, remains an interesting issue: topological transports and responses in crossinglinenode semimetals. The configuration is distinct from those of other point, line, and surface nodal structures. Therefore, we expect new topological quantum phenomena in crossinglinenode semimetals, which should be clarified in future work.
Acknowledgements.
This work was supported by the GrantsinAid for Young Scientists (B, Grant No. 16K17725), for Research Activity Startup (Grant No. JP16H06861), and for Scientific Research on Innovative Areas “Topological Material Science” (JSPS KAKENHI Grants No. JP15H05851 and No. JP15H05853). S.K. was supported by the Building of Consortia for the Development of Human Resources in Science and Technology.Appendix A Tables of line node configurations for 1D IRRs
Tables 3, 4, and 5 show the correspondence between the level schemes and line node configurations building on the criteria, where and indicate line nodes protected by and the absence of a stable line node. After including the SOI, when crossing DLNs encircle a TRIM, they transform into normal (NI)/topological (TI) insulators or a Dirac point (DP). For the case of I, the SOI makes a gap on the crossing DLNs, but we cannot determine whether the system becomes a TI or NI from the point group symmetries.
Appendix B Symmetryadapted effective models
PG  Line nodes  

,  
()  
; ;  ; ;  
; ;  ; ;  
; ;  ; ;  
; ; ;  ; ; ;  
; ; ;  ; ; ;  
; ;  ; ;  
; ; ; ;  ; ; ; ;  
; ;  ; ;  
; ; ;  ; ; ;  
; ; ;  ; ; ;  
; ;  ; ;  
First of all, consider a level scheme consisting of 1D IRRs and . The lowenergy effective Hamiltonian is generally described by
(3) 
where are the identity and Pauli matrices in the orbital space and . We assume that the Hamiltonian (3) possesses timereversal symmetry, which demands that , , and . The group operation on this Hamiltonian is defined by
(4) 
where is a unitary matrix in terms of in the orbital space and represents a rotation matrix concerning in the momentum space. Since we focus on the 1D IRRs, becomes or . In particular, the mirrorreflection operations , , and are given as follows:

in , , , , and :
(5) 
in and ; in and (); in ():
(6) 
in and ; in and :
(7) 
in , , , and :
(8)
Assuming that the crossing energy bands appear around the ()symmetric planes, the crossing line is stable if on the symmetric planes because the term describes the band mixing between and and makes a gap. Thus, the stable DLNs requires that , leading to , where and are the momenta parallel to and perpendicular to the symmetric planes. This condition is consistent with the criterion in the main paragraph. Table 6 shows the symmetryadapted for each line node configuration.
Next, consider a level scheme consisting of 2D (3D) IRRs () [()]. To avoid cumbersome multiband effects, we ignore the level splitting and consider doubly (triply) degenerate conduction and valence bands as a starting point. In that case, the energy bands all form a DLN, and it is possible to decompose the effective Hamiltonian for 2D (3D) IRRs into two (three) effective Hamiltonians in terms of 1D IRRs. As an example of 2D IRRs, we discuss the level scheme () for . The symmetry operators are defined as
(9)  
(10) 
Then, the symmetryadapted effective Hamiltonian is given by
(11) 
where and . Here, , , , , , , and are material dependent parameters. The effective Hamiltonian can be described by the blockdiagonal form: , where is a effective Hamiltonian with . When and cross on the symmetric planes, we obtain six DLNs and label this line node configuration as , where represents thdegenerate DLNs protected by , i.e., line nodes appear on the symmetric plane. To check the effect of band splitting, we include it as a perturbation: with
(12) 
Since the three symmetric planes are equivalent, we focus on the symmetric plane of , on which the eigenvalues of are
(13)  
(14) 
The energy bands are plotted in Fig. 5. The small band splitting does not break the line node structure when [see Fig. 5 (a)]. On the other hand, for large , changes to due to the change in band structure [see Fig. 5 (b)]. Thus, although there exist at most two line nodes on a symmetric plane, we can engineer the line node configuration from to or by the band splitting . In a similar manner, we can construct symmetryadapted effective models for 3D IRRs. For example, consider the level scheme consisting of () of . In this case, the Hamiltonian is blockdiagonalized as