Tuning from Frustrated Magnetism to Superconductivity in Quasi-One-Dimensional KCr$_3$As$_3$ Through Hydrogen Doping

TL;DR

This study demonstrates that hydrogen intercalation effectively electron-dopes KCr$_3$As$_3$, suppressing magnetic order and inducing superconductivity, thereby establishing a charge-doped phase diagram for this quasi-one-dimensional material.

Contribution

It reveals hydrogen doping as a new method to tune magnetic and superconducting states in KCr$_3$As$_3$, providing the first charge-doped phase diagram for these materials.

Findings

Hydrogen intercalation increases H concentration in CrAs tubes.

Charge doping suppresses magnetic order and induces superconductivity.

Superconductivity correlates with increased hydrogen content, not crystallinity.

Abstract

We report the charge doping of KCr$_3$As$_3$ via H intercalation. We show that the previously reported ethanol bath deintercalation of K$_2$Cr$_3$As$_3$ to KCr$_3$As$_3$ has a secondary effect whereby H from the bath enters the quasi-one-dimensional Cr$_6$As$_6$ chains. Furthermore, we find that - contrary to previous interpretations - the difference between non-superconducting as-grown KCr$_3$As$_3$ samples and superconducting hydrothermally annealed samples is not a change in crystallinity but due to charge doping, with the latter treatment increasing the H concentration in the CrAs tubes effectively electron-doping the 133 compound. These results suggest a new stoichiometry KH$_x$Cr$_3$As$_3$, that superconductivity arises from a suppressed magnetic order via a tunable parameter and pave the way for the first charge-doped phase diagram in these materials.

Tables

Table 1. Table 1: Crystallographic properties of samples obtained from Rietveld refinements using both the XRD and NPD data at 300 K.

		S1	S2	S3	S4
Space group		$P{6}_3/m$
$R_{wp}$		4.13%	3.56%	2.46%	3.36%
$a$ ($\AA$)		9.0948(1)	9.0939(1)	9.1042(1)	9.1041(1)
$c$ ($\AA$)		4.1770(3)	4.18090(3)	4.19733(1)	4.19426(2)
$V$ (${\AA }^3$)		299.22(1)	299.43(1)	301.29(1)	301.06(1)
K($2c$)
	$U$	0.020(6)	0.010(5)	0.017(7)	0.018(1)
Cr($6h$)
	$x$	0.1488(2)	0.1498(1)	0.1520(2)	0.1512(1)
	$y$	0.1850(2)	0.1867(1)	0.1887(2)	0.1885(1)
	$U$	0.009(1)	0.004(1)	0.003(1)	0.005(2)
As($6h$)
	$x$	0.3438(1)	0.3430(1)	0.3459(1)	0.3461(1)
	$y$	0.0672(1)	0.0672(1)	0.0688(1)	0.0697(1)
	$U$	0.014(2)	0.013(2)	0.011(1)	0.009(2)
H($2b$)
	$occ.$	0.35(1)	0.34(1)	0.71(3)	0.65(1)
	$U$	0.015	0.015	0.015	0.015(3)

Full text

††preprint: APS/123-QED

I Introduction

Superconductors are unique in the heirarchy of quantum materials. Arguably the start of the field, superconductivity (as manifested in unconventional superconductors) was one of the phenomenon which suggested topological classifications, gave rise to the Resonating Valence Bond theory now used to describe quantum spin-liquids and grew into the basis of many active research areas including Majorana Fermions and qubit design Bednorz and Müller (1986); Anderson (1987); Wen (2017); Nakamura et al. (1999); Read and Green (2000); Linder and Robinson (2015). Yet unlike many of the topological quantum materials whose physics are theortically understood but which struggle to find physical realizations, unconventional superconductors exist, not infrequently, but have no settled upon microscopic theory leaving the search for new superconductors and the optimization of their properties unguided Wen (2017); Fernandes and Chubukov (2016); Lee et al. (2019); Balents (2010); Norman (2011) Instead, what has arisen is an empirical recipe born of a suprisingly universal phase diagram observed in unconventional superconductors where superconductivity is found near instabilities - namely of magnetic character Basov and Chubukov (2011).

It is therefore useful to find new unconventional superconductors which are structurally simple, have few orders (ideally none of which are coupled) and are easy to address theoretically (e.g. fewer relevant orbitals or with lower dimensionality) to refine this recipe and help discriminate between possible mechanisms. Much as the iron-pnictide superconductors have helped refine concepts proposed in the study of the cuprates such as cementing the importance of magnetism and demonstrating a tunability to the strength of electron-correlations in the normal state, a new family of non-centrosymmetric unconventional superconductors $A_{2}TM_{3}$As3 (with A = Na, K, Cs or Rb and TM = Cr or Mo) with a structural motif of CrAs tubes have begun offering new insights into unconventional superconductivity, but importantly in a quasi-one-dimensional structure Basov and Chubukov (2011); Allred et al. (2016); Bao et al. (2015a); Mu et al. (2018a, b); Tang et al. (2015); Zhao et al. (2018).

Though nascent, the study of these materials (specifically K2Cr3As3) has yielded helpful corroborations and refinements of our recipe. Neutron scattering, nuclear magnetic resonance, Raman scattering and density functional theory work have revealed both structural and magnetic instabilities - as seen in the pnictides Taddei et al. (2017, 2018); Zhi et al. (2015); Zhang et al. (2015); Wu et al. (2015); Jiang et al. (2015). However, in K2Cr3As3 these orders break different crystal symmetries, show no coupling, and have temperature dependences suggesting that only the magnetic order couples to superconductivity Taddei et al. (2017, 2018). Additionally, evidence of Luttinger liquid type physics, spin-triplet superconductivity and a nodal gap function have emerged indicating the broader novelty of this system Watson et al. (2017); Shao et al. (2018); Liu et al. (2016); Adroja et al. (2015); Sun et al. (2017); Yang et al. (2015). Yet, the more comprehensive study of this family has been inhibited by its extreme air sensitivity and its seeming unsuitability for doping studies. While recently success was found in continuously varying the A-site between K and Na, the effect is of internal pressure, ideally we would be able to charge dope and create phase diagrams exploring how the discovered instabilities behave as the Fermi-surface and potentially the superconducting transition are tuned Mu et al. (2019).

Promisingly, one member compound (KCr3As3) has been found which exhibits both a diminished electron count and stability in air. KCr3As3 (or ‘133’ as opposed to ‘233’ for K2Cr3As3) is created by soaking the 233 stoichiometry in an ethanol bath Bao et al. (2015b). This process, according to previous reports, discretely removes one K from the structure increasing the crystal symmetry from $P\overline{6}m2$ to $P6_{3}/m$ while crucially retaining the quasi-one dimensional CrAs tubes (albeit now with centrosymmetry) Bao et al. (2015b). Though the K content cannot be continuously controlled and so does not allow a full charge doped phase diagram, KCr3As3 could afford some insight as to what endmembers of such a phase diagram might look like.

Yet, results from KCr3As3 (and the broader ACr3As3  with A = K, Rb and Cs family) have proven surprisingly contentious. Initial reports suggested no superconductivity and spin-glass like magnetism Bao et al. (2015b); Tang et al. (2015). However, more recent work which performed a post-bath hydrothermal anneal in ethanol reported bulk superconductivity with non-magnetic behavior Mu et al. (2017); Liu et al. (2017). Similarly, theory work has resulted in significant disagreement with reports of long range magnetic ground states, spin-density wave Fermi-surface nesting mediated superconductivity and more recently of a structural instability which precludes spin-fluctuation driven superconductivity Cao et al. (2015); Zhang et al. (2019); Xing et al. (2019).

An ostensible explanation for these conflicting results has been found in crystalline disorder - in Ref. Mu et al., 2017 it is argued that superconductivity arises in KCr3As3 upon post-treatment with a hydrogthermal anneal due to an improvement of the crystallinity. While in theory work, Ref Cao et al., 2015 suggests its predicted long range magnetic order is not seen experimentally due to disorder in the physical material and Ref Xing et al., 2019 suggests that similar to K2Cr3As3 the predicted structural instability may have no long range coherence Taddei et al. (2018). In attempting to sort through these possibilities in the work reported here, we stumbled upon a different and quite intriguing origin to these discrepancies.

Here we report a systematic study of KCr3As3 and its various reported growth techniques using neutron and x-ray diffraction together with Density Functional Theory (DFT). We reveal a previously missed effect of the ethanol bath deintercalation - H doping. Using structural solution methods we find that H intercalates into the CrAs tubes, partially charge compensating for the hole-doping effect of the deintercalated K. Furthermore, we show that the difference between non-superconducting spin-glass and non-magnetic superconducting samples is not in the sample crystallinity but is rather in the amount of H intercalated. Our DFT work shows that the intercalated KHxCr3As3 structure is stable, electron-doped relative to KCr3As3 and consequently has an electronic structure more similar to K2Cr3As3 than that calculated for KCr3As3. Additionally, we find that the H stabilizes the $P6_{3}/m$ structure and places the compound proximate to an antiferromagnetic instability. These results lead to the exciting possibility of continuous tuning KHxCr3As3 from a spin-glass to a superconductor via charge doping with H - the long sought after tuning parameter. Furthermore this shifts interest to a new air stable composition opening up this system for widespread study. More contemplatively, it offers a realization of a frustrated magnetic to superconducting transition in a Q1D system which has long been suggested as a potential recipe for spin liquid physics Sigrist et al. (1994); Rice et al. (1993).

II Methods

High quality powder samples of K2Cr3As3 were grown as described previously Bao et al. (2015b); Taddei et al. (2017). To obtain samples of the 133 stoichiometry the deintercalation methods reported in Bao et al., 2015b and refined in Mu et al., 2017 were both used. Powder samples of K2Cr3As3 were placed in room temperature baths of high purity dehydrated ethanol for seven days before being rinsed thoroughly with ethanol and dried under vacuum. With the initial reaction complete, samples underwent one of three post treatments (i) left as grown, (ii) post-annealed under vacuum or (iii) annealed in a hydrothermal vessel with ethanol and then vacuum annealed. The powders obtained from all reactions were of gray color and were air stable on the order of days. The crystal structure using the initially reported stoichiometry is shown in Fig. 1(b). Further details on the synthesis procedures are included in the supplemental materials (SM)SM .

Neutron powder diffraction (NPD) data were collected using the HB-2A powder diffractometer and the NOMAD total scattering instruments of Oak Ridge National Laboratory’s High Flux Isotope Reactor and Spallation Neutron Source Calder et al. (2018). For the latter the data were reduced using the instrument developed software McDonnell et al. (2017); Neuefeind et al. (2012). High resolution synchrotron x-ray powder diffraction (XRD) data were collected at beamline 11BM-B of the Advanced Photon Source at Argonne National Laboratory. Structural analysis was performed using the Rietveld method as implemented in the FullProf, GSAS and EXPGUI software suites (see SM for more details) Rodríguez-Carvajal (1993); Larson and Von Dreele (2004); Von Dreele et al. (1982); Finger et al. (1994); SM .

Density Functional Theory (DFT) calculations were performed using projector-augmented plane-wave method, as implemented in the VASP code, with the Perdew-Burke-Ernzerhof generalized gradient approximation and checked using the general potential linearized augmented planewave method as implemented in the WIEN2k code Kresse and Joubert (1999); Kresse and Furthmüller (1996); Perdew et al. (1996); Singh and Nordström (2006); Blaha et al. (2001). For the basis set, an energy cutoff of 400 eV was used and a Brillouin zone was composed of $4\times 4\times 10$ k-point sampling. Starting from the experimentally determined lattice cell, the internal atomic positions were relaxed subject to hexagonal symmetry with the lattice parameters held at the experimental values. The phonons were calculated using the PHONOPY code Togo and Tanaka (2015). For more details on the calculations see the SM SM .

III Results and Discussion

To systematically compare the thermodynamic and crystallographic properties of samples grown using different techniques, we synthesized samples following the methodologies reported by Bao et al., in Ref. Bao et al., 2015b and Mu et al., in Ref. Mu et al., 2017. Thus three samples were used for initial characterization: (S1) as grown from an ethanol bath, (S2) with a post-bath vacuum anneal and (S3) with both post-bath hydrothermal and vacuum anneals (a fourth sample (S4) was also grown and will be discussed later in this text). Susceptibility measurements (Fig. 1 (a)) show we were successful in reproducing the previously reported change in properties with S1 and S2 being only trace-superconducting with signatures of spin-glass behavior (identified here as in Ref. Bao et al., 2015b by the low temperature divergence of the inverse susceptibility from linearity and a split between field-cooled and zero-field cooled measurements, see SM) and S3 showing bulk superconductivity.

Previously, the change from spin-glass to superconductor was attributed to the post-bath annealings’ effects on the crystallinity of the sample. Ostensibly, the more crystalline annealed samples hosted superconductivity which was otherwise destroyed by disorder in the as-grown samples Mu et al. (2017). To test this assertion NPD and XRD data were collected and are shown along with the best fit results of Rietveld refinements in Fig. 2. We note that even visually, the main qualitative difference between the samples is not in the peak shape but rather is in the background to signal ratio. Specifically, in the NPD data the bulk superconducting S3 has a remarkably larger background to signal ratio than any other sample.

To qualitatively check the crystallographic properties the NPD and XRD were used together for Rietveld refinements. From this analysis, the lattice parameters and measures of strain, crystallinity and crystal size can be assessed (plotted as a function of sample number along with the 233 compound labeled as sample 5 in Fig. 3). Considering the extracted anisotropic microscopic strain parameters ($L_{nn}$ where $n=1,2,3$ correspond to the $a,b\text{ and }c$ lattice directions respectively) and $L_{x}$ (which is proportional to the inverse of the crystallite size), little difference is seen in these peak broadening terms between samples indicating similar crystalline properties. Compared to the broadening parameters determined in previous work on the 233 compound (Ref. Taddei et al., 2017), it is seen that generally the deintercalation process leaves the 133 material with poor crystallinity, as reported. However, it is not significantly improved upon either vacuum or hydrothermal annealing, in contrast with previous reports which did not compare qualitative measures of the crystallinity Mu et al. (2017).

Looking at the crystal parameters rather than the crystallinity a clear trend relative to the superconducting volume fraction is seen (Fig. 3 (b)). Both the $a$ and $c$ lattice parameters dilate between the trace (S2) and bulk (S3) superconducting samples by 0.1 and 0.4 % respectively leading to a >0.4\%\ change in the unit cell volume (Fig. 3 (b)). On the other hand, between S1 and S2 no such change is seen with only a small $\sim$ 0.1% change in volume and no significant change in the superconducting volume fraction. Both of these behaviors are unexpected: if the post-annealing treatments improve the crystallinity the profile peak broadening terms should decrease and the unit cell relax - our data contradict both of these expectations Forrest et al. (2016); Becht et al. (2008). Therefore, we aimed to identify a different cause to the change in properties between the as-grown/vacuum-annealed samples and the hydrothermally annealed samples.

Noting that the background to signal increase is only seen in the NPD and is exacerbated by the hydrothermal anneal, we posit the presence of H in the samples. H is nearly invisible to x-rays but has a large incoherent cross-section to neutrons ($\sigma_{H_{Inc}}\sim 80.26$ $b$) Sears (2006). This manifests as a large background which increases at lower scattering angle (or high d-spacing) and would explain the difference between the XRD and NPD patterns Temleitner et al. (2015). As such, any H in KCr3As3 would be missed in XRD and energy dispersive x-ray spectroscopy indicating why such a feature has not been seen in previous structural work Bao et al. (2015b); Mu et al. (2017); Liu et al. (2017); Tang et al. (2015). One could propose several mechanisms which would introduce H to the samples during the two ethanol baths: as a minority organic compound not removed with ethanol washing, substituted onto the K site or intercalated into the 133 structure. Of these scenarios the latter two provide an explanation for the increase in background upon the hydrothermal anneal assuming the process either forces more H into the material through higher temperature, pressure or a longer exposure to ethanol.

To discriminate between these scenarios, a sample of 133 was grown following the procedure of S3 but using fully deuterated ethanol in both the room temperature bath and hydrothermal anneal. The resulting sample (S4) shows simlar superconducting properties to S3 (Fig. 1) indicating that the use of deuterated ethanol does not significantly change the sample properties or reaction pathway. Deuterium (D) does not have the large incoherent cross section of H ($\sigma_{D_{Inc}}\sim\ 2.05$ $b$) and so does not add as much background scattering. Furthermore, it has a large coherent cross-section of $\sigma_{D_{Coh}}\sim\ 7.6$ b making it a strong scatterer suitable for structural model refinements (the coherent cross-sections of K, Cr, As and H are 1.69, 1.66, 5.44 and 1.76 b respectively) Sears (2006). Therefore, if H does intercalate into the structure or substitute onto the K site using D would allow for structure solution and composition determination through changes in peak intensities, else if the H background comes from remnant organic compounds the background will be reduced in S4 but no other changes should occur.

The XRD and NPD of S4 are shown in Fig. 2. While the XRD looks similar to those of the other samples, the NPD exhibits a nearly 50% reduction in background (compared to S3) as well as a change in peak intensities where peaks at 3.9 and 4.6 $\mathrm{\AA}$ (the (020) and (110) reflections respectively) become background equivalent. The background reduction confirms the presence of H in the samples (rather than for instance the background arising from amorphous parts of the sample) while the change in peak intensities between the samples grown with H and D indicates that the H(D) is incorporated into the 133’s crystal structure rather than existing as remnant organic compounds from the ethanol bath.

To determine where D is incorporated, Rietveld analysis was performed using the NPD. Since the D containing samples show a reduction in peak intensity but not the appearance of new peaks the original $P6_{3}/m$ crystal symmetry was used and Rietveld refinements were performed trying models containing D on all of $P6_{3}/m$’s special Wyckoff positions. Of these, only models with D on the $2b$ (0,0,0) site reproduced the observed intensities as shown in Fig. 2 leading to high quality fits with satisfactorily small residuals (Table 1). We note that the new reflection conditions of these sites do not forbid either the (020) or (110) reflections by symmetry instead they become accidentally forbidden.

The new structural model is shown in Fig. 4. In it, D is intercalated into the center of the CrAs tubes with the $2b$ site located between the stacked CrAs layers creating a chain of D along the center of each tube. We find no evidence of K vacancies or of D substitution on the K site - changes neutron scattering is very sensitive to due to the considerable difference in the neutron scattering cross sections of K and D. However, our refinements show the D site is only partially occupied with a refined occupancy of 65% .

With the location of the intercalated D known, we can attempt Rietveld refinements using similar models for S1, S2 and S3, this is possible since despite its large incoherent cross-section H also has a coherent cross-section which is on the order of both K and Cr. Starting with S3, our refinements show remarkable agreement with S4, with a similar occupancy of $\sim 71$% . This corroborates our assumptions from the susceptibility that the synthesis procedure and resulting material is insensitive to the change from H to D. On the other hand, S1 and S2 are found to have significantly less H than S3 and S5. In both S1 and S2 the $2b$ site refines to have $\sim 35\%$ occupancy. As all samples show intercalated H - even the non-bulk superconductors - we revise the chemical formula of KCr3As3 to KHxCr3As3 to explicitly reflect the H content and its variability.

These results elucidate the effects of the different reaction stages and show each step to be quite reproducible. During the room temperature ethanol bath, K is deintercalated from the 233 starting material as reported previously however, as our analysis shows some H from the bath also intercalates into the CrAs tubes at positions between the CrAs layers. Whether this occurs to charge compensate for the deintercalated K is not known and should be the interest of future experimental and theoretical work - although this may be indicated by the inability to synthesize KCr3As3 directly from sintering Bao et al. (2015b); Mu et al. (2017). The similarities both in crystallinity and H content of S1 and S2 suggest that vacuum annealing does not have a significant effect at this stage, it does not improve the sample quality and causes no venting of the intercalated H. This latter result suggests the stability of KHxCr3As3 at least up to the low annealing temperatures (373 K) used in this work. Upon annealing in a hydrothermal ethanol bath, more H intercalates into the structure bringing the concentration from $\sim 35\%$ to $\sim 65-71\%$.

It is these last two effects which appear to induce superconductivity in KHxCr3As3. As was discussed earlier no change in crystallinity is seen between bulk and trace superconducting samples but rather a previously unexplained change in the lattice parameters. We we can now ascribe this to an increase in the amount of H intercalated into the CrAs tubes. Beyond causing the unit cell to expand, the intercalated H should also affect the charge count of the CrAs tubes. However, the chemistry of H is incredibly flexible and it is not evident from the diffraction data how the H contributes electronically therefore, we turn to DFT to help uncover the role of H in the structure Gupta et al. (2005); Khan and Raub (1976).

To study KHxCr3As3’s energetics and electronic structure, DFT calculations were performed using both the theoretical $x=0$ and $x=1$ stoichiometries. Starting with the energetics, calculations for KHCr3As3’s formation energy relative to KCr3As3 and H2 gas were performed. These found a negative formation energy of 92 kJ/mol H2 - similar to that of other stable hydrides such as MgH2 - indicating the intercalated structure is energetically favorable (see SM for more details) SM . Next, we investigated the electronic contribution of H by calculating KHCr3As3’s band structure (Fig. 5). We found that the intercalated H raises the Fermi energy ($E_{F}$)and causes only modest distortions to the bands near $E_{F}$. Therefore, we can say that in KHCr3As3 the H has metallic bonding and acts as an electron donor. As shown, it increases the electron count such that the electronic structure (and Fermi surface Fig. 6) are more similar to K2Cr3As3 than the pure KCr3As3 ostensibly indicating similar electronic behavior.

Inspired by previous reports of structural and magnetic instabilities we also performed calculatinos studying the phonon spectrum and proximity to magnetic instabilities Xing et al. (2019); Cao et al. (2015); Feng et al. (2019); Zhang et al. (2019). considering the predicted structural instability, our calculations show neither the unstable phonon modes seen in KCr3As3 or K2Cr3As3Taddei et al. (2018); Xing et al. (2019). Furthermore, we find relatively high energy nearly dispersionless H modes and thus suggest that H removes the unstable phonon modes leading KHCr3As3 to be structurally stable Rowe et al. (1974). Finally, we consider the hydride’s proximity to a magnetic instabilities. Previously, KCr3As3 has been suggested to either have a magnetic ground state which is frustrated by disorder, or no magnetic ground state due to a structural instability which removes favorable nesting conditions Cao et al. (2015); Xing et al. (2019). For KHCr3As3 we find weak itinerant instabilities to both ferromagnetic and antiferromagnetic A-type magnetic orders with a slight preference for the latter. However, we also find both to have itinerancy and low energy scales suggesting that the magnetic order could easily be suppressed to a paramagnetic state with spin-fluctuations by quantum fluctuations such as the case of Ni3Ga Aguayo et al. (2004). This last result is quite interesting as it restores the possibility of a spin-driven pairing mechanism which had previously been discounted in Ref. Xing et al., 2019.

These DFT and experiment results suggest rather provocatively that superconductivity arises in KHxCr3As3 upon electron doping, a process which until now has been achieved unintentionally during the hydrothermal anneal. In this scenario, the underdoped KH0.35Cr3As3 is a non-superconducting spin-glass phase as reported in Bao. et al.’s initial report Bao et al. (2015b). Upon electron doping to KH0.65Cr3As3 via hydrothermal annealing, the spin-glass phase is suppressed and the material becomes a bulk superconductor with $T_{c}$ $\sim 5$ K as reported by Mu et al., Mu et al. (2017). This has multiple significant implications: (i) the intercalated CrAs systems inhabit a familiar phase space where superconductivity arises out of a suppressed magnetic state, (ii) the possibility of creating the previously elusive charge doped phase diagram for the intercalated CrAs for not just KHxCr3As3 but also the Rb and Cs analogues all of which are grown in ethanol and show the spin-glass to superconductor crossover, (iii) the need to revisit previous ab initio calculations on the ACr3As3 materials which were based on the hydrogen free 133 stoichiometry, (iv) an avenue to study how superconductivity arises from a frustrated magnetic state in a quasi-one-dimensional system by charge doping a concept which has long held interest for spin- and Luttinger liquid type physics Sigrist et al. (1994); Rice et al. (1993). As an additional point of interest, one of the significant impediments to the study of the intercalated CrAs materials to this point has been the initial 233 stoichiometry’s extreme air sensitivity, with this work, we show that the 133 version offers a clear path forward to study interactions between magnetic and superconducting orders while also being air stable alleviating this barrier. Our results layout a rich path of future work, not least of which will be to gain fine control of the H content through the synthesis procedures - work which these authors have already begun to early positive indications.

IV Conclusions

In conclusion, we have shown via neutron and x-ray diffraction that during the wet chemistry K deintercalation of K2Cr3As3, H intercalates into the structure forming a partially occupied H chain centered in the CrAs tubes. We find that it is the concentration of H and not the crystalline quality of the material which determines the superconducting properties with the reported synthesis methods producing the non-bulk-superconducting spin-glass KH0.35Cr3As3 and the bulk-superconducting metallic KH0.65Cr3As3. Density functional theory analysis shows that the KHCr3As3 structure is energetically very favorable and that the intercalated H acts as a electron donor. The resulting electronic structure is similar to that of the K2Cr3As3 compound, does not have the structural instability predicted for KCr3As3 and has weak magnetic instabilities. These results suggest the need to revisit much of the theoretical work on these materials as well as multiple new research avenues to understand unconventional superconductivity and magnetic interactions in these novel quasi-one-dimensional systems.

Acknowledgements.

The part of the research that was conducted at ORNL’s High Flux Isotope Reactor and Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. The research is partly supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Science and Engineering Division. Work at the University of Missouri is supported by the U.S. DOE, BES, Award No. DE-SC0019114. The authors thank J. Neuefeind and J. Liu for help with data collection on NOMAD.