Showing 1–50 of 58 results for author: Heil, C

Yttrium Superhydrides Revisited: Advanced Experimental and Theoretical Studies of YH$_6$, YH$_9$ and YH$_{10}$

Authors: Dmitrii V. Semenok, Pedro N. Ferreira, Di Zhou, Fabian Jőbstl, Andrey V. Sadakov, Kirill S. Pervakov, Burkhan I. Massalimov, Toni Helm, Ryosuke Akashi, Vladimir M. Pudalov, Viktor V. Struzhkin, Christoph Heil, Ivan A. Troyan

Abstract: Yttrium polyhydrides are benchmark materials in high-pressure superconductivity, yet several key properties of the Y-H system remain insufficiently characterized. Here we combine contact transport, contactless radio-frequency measurements, pulsed-field experiments, and first-principles calculations to reinvestigate YH$_6$, YH$_9$, and YH$_{10}$ in the pressure range 140-213 GPa. Yttrium hydrides Y… ▽ More Yttrium polyhydrides are benchmark materials in high-pressure superconductivity, yet several key properties of the Y-H system remain insufficiently characterized. Here we combine contact transport, contactless radio-frequency measurements, pulsed-field experiments, and first-principles calculations to reinvestigate YH$_6$, YH$_9$, and YH$_{10}$ in the pressure range 140-213 GPa. Yttrium hydrides YH$_6$ ($\textit{$T_c$}$ = 218-221 K) and YH$_9$ ($\textit{$T_c$}$ = 235-237 K) demonstrate narrow superconducting transitions ($\textit{$Δ$T$_c$}$ = 2-5 K), approaching the limit imposed by thermal fluctuations. Pulsed-field measurements on YH$_6$ up to 60 T establish an extended superconducting phase diagram with a linear slope $\textit{dB$_{c2}$/dT}$ = -0.52 T/K, pronounced transition broadening above 30 T, and negligible normal-state magnetoresistance. We report the radio-frequency AC susceptibility study of YH$_6$, providing evidence for superconductivity via high-frequency field screening in a contactless geometry. Experiments involving Pd incorporation, Pd thin-film sputtering, and Al alloying show strong suppression of high-temperature superconductivity, with no transitions detected above 78-120 K. Finally, using density-functional theory with the stochastic self-consistent harmonic approximation, superconducting density-functional theory, and full-bandwidth Migdal-Eliashberg calculations, we show that anharmonic effects substantially reduce the predicted $\textit{$T_c$}$ of cubic YH$_{10}$ to approximately 260-270 K. These results strongly disfavor room-temperature superconductivity in binary yttrium superhydrides. △ Less

Submitted 13 August, 2026; v1 submitted 11 August, 2026; originally announced August 2026.

Comments: Figure 4 has been expanded, Fig. S2 has been replaced, and the authors' contributions have been clarified

Ab initio thermodynamic statistical modeling of the miscibility gap and the metal-insulator phase transition in SrTi$_{1-x}$V$_x$O$_{3}$

Authors: Luka Wibmer, Chiara Ostendorf, Dominik Spath, Christoph Heil, Pedro N. Ferreira, Markus Aichhorn

Abstract: The substitutional alloy SrTi$_{1-x}$V$_x$O$_3$ interpolates between the band insulator SrTiO$_3$ and the correlated metal SrVO$_3$, exhibiting a composition-driven metal--insulator transition whose origin combines Mott physics with local chemical disorder. Previous first-principles studies relied on individual supercells, which cannot capture the thermally disordered solid solution, since configu… ▽ More The substitutional alloy SrTi$_{1-x}$V$_x$O$_3$ interpolates between the band insulator SrTiO$_3$ and the correlated metal SrVO$_3$, exhibiting a composition-driven metal--insulator transition whose origin combines Mott physics with local chemical disorder. Previous first-principles studies relied on individual supercells, which cannot capture the thermally disordered solid solution, since configurations of identical composition can display very different electronic properties. Here we treat the alloy within a generalized quasi-chemical approximation, a thermodynamically consistent statistical framework in which every property is obtained as an ensemble average over all symmetry-inequivalent clusters, weighted by occurrence probabilities that minimize the Gibbs mixing free energy. This provides a well-defined procedure to average over different supercells, and places the structural and electronic descriptions on an equal footing. From the mixing thermodynamics we obtain a miscibility gap with a critical temperature of 1443 K, consistent with experimental evidence. Combining the cluster ensemble with dynamical mean-field theory, we track the density of states at the Fermi level across the full composition range: whereas density-functional theory alone predicts a metal for all $x>0$, the correlated spectral function reproduces the transition, evolving from insulating below $x\approx0.3$ to metallic near $x=1$. Finally, classifying clusters as metallic or insulating and performing site percolation on a simple cubic lattice yields a sharp onset of system-spanning conduction near $x\approx0.4$. These results establish a thermodynamically consistent, configuration--averaged framework applicable to the broader class of correlated materials. △ Less

Submitted 8 July, 2026; originally announced July 2026.

Anharmonic lattice dynamics and superconductivity in strained bulk and surface niobium

Authors: Mihir Ranjan Sahoo, Roman Lucrezi, Pedro Nunes Ferreira, Chia-Nien Tsai, Matthew Julian, Rohit P. Prasankumar, Mahmoud I. Hussein, Christoph Heil

Abstract: Using first-principles calculations, we investigate how homogeneous strain and crystallographic surface orientation modify the vibrational and superconducting properties of niobium. For bulk Nb, tensile strain strongly softens the phonon spectrum and enhances the electron--phonon coupling, increasing the superconducting transition temperature from 9.5 K at equilibrium to 14.5 K at $\sim\!6\%$ latt… ▽ More Using first-principles calculations, we investigate how homogeneous strain and crystallographic surface orientation modify the vibrational and superconducting properties of niobium. For bulk Nb, tensile strain strongly softens the phonon spectrum and enhances the electron--phonon coupling, increasing the superconducting transition temperature from 9.5 K at equilibrium to 14.5 K at $\sim\!6\%$ lattice expansion. For the low-index Nb(001), Nb(110), and Nb(111) surfaces, harmonic phonon calculations exhibit imaginary modes, showing that anharmonic lattice effects are essential. To treat these effects efficiently, we train Nb-specific machine-learning interatomic potentials on bulk and slab first-principles configurations and use them to accelerate stochastic self-consistent harmonic approximation calculations, thereby obtaining anharmonically renormalized phonon modes that are combined with density-functional perturbation theory electron--phonon matrix elements to construct the Eliashberg spectral function. Among the clean free-standing slabs considered here, Nb(001) exhibits the strongest electron--phonon coupling and the highest calculated transition temperature of 10.0 K, while Nb(110) and Nb(111) show progressively reduced pairing strength. Finally, by analyzing the Eliashberg spectral function and the functional derivative $δT_\text{c}/δα^2F(ω)$, we identify the phonon energy ranges most effective for superconducting pairing. Our results show that strain, surface termination, and anharmonic phonon renormalization provide complementary and interrelated microscopic routes for tuning superconductivity in Nb. △ Less

Submitted 1 June, 2026; originally announced June 2026.

Comments: 14 pages, 10 figures

Fast Real-Axis Eliashberg Calculations: Full-bandwidth solutions beyond the constant density of states approximation

Authors: Alejandro Simon, James Shi, Dominik Spath, Eva Kogler, Reed Foster, Emma Batson, Pedro N. Ferreira, Mihir Sahoo, Phillip D. Keathley, Warren E. Pickett, Rohit Prasankumar, Karl K. Berggren, Christoph Heil

Abstract: Experimentally relevant signatures of superconductivity require access to real-frequency quantities, such as the spectral functions, optical response, and transport properties, yet Migdal-Eliashberg calculations are commonly performed on the imaginary axis and then analytically continued, a step that is numerically delicate and can obscure physically relevant spectral features. Here we present a p… ▽ More Experimentally relevant signatures of superconductivity require access to real-frequency quantities, such as the spectral functions, optical response, and transport properties, yet Migdal-Eliashberg calculations are commonly performed on the imaginary axis and then analytically continued, a step that is numerically delicate and can obscure physically relevant spectral features. Here we present a practical route to solving the finite-temperature Migdal-Eliashberg equations directly on the real-frequency axis, while retaining the effects from the full-bandwidth electronic structure. Our formulation accounts for particle-hole asymmetry through an energy-dependent electronic density of states, avoiding the constant density of states approximation often used in real-axis calculations, and includes a static screened Coulomb contribution. We introduce an efficient numerical technique to solve the Migdal-Eliashberg integrals whose computational cost scales linearly with the real-frequency grid, making high-resolution, full-bandwidth real-axis calculations feasible and providing direct access to the interacting Green's function and derived observables without analytic continuation. As an illustration, we apply the method to H$_{3}$S, where a van-Hove singularity near the Fermi level produces strong particle-hole asymmetry. The full-bandwidth solution yields noticeably different spectra than the constant density of states approximation and brings the superconducting gap and lineshapes into closer agreement with experiment, highlighting when band-structure details are essential. Furthermore, the methods presented here open the door to time-dependent, nonequilibrium simulations within Eliashberg theory. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Comments: 19 pages, 8 figures

Ultrafast dynamics and light-induced superconductivity from first principles

Authors: Alejandro Simon, James Shi, Eva Kogler, Reed Foster, Dominik Spath, Emma Batson, Pedro N. Ferreira, Mihir Sahoo, Rohit Prasankumar, Phillip D. Keathley, Karl K. Berggren, Christoph Heil

Abstract: Experiments on superconducting materials have unveiled unique emergent properties when they are driven far from equilibrium. However, a quantitative first-principles treatment that describes experimental observations is lacking. In this work, we develop an ab-initio model for the nonequilibrium response of optically irradiated superconducting films within the framework of conventional electron-pho… ▽ More Experiments on superconducting materials have unveiled unique emergent properties when they are driven far from equilibrium. However, a quantitative first-principles treatment that describes experimental observations is lacking. In this work, we develop an ab-initio model for the nonequilibrium response of optically irradiated superconducting films within the framework of conventional electron-phonon-mediated superconductivity, leveraging new numerical techniques to solve the Migdal-Eliashberg equations directly on the real-frequency axis. This enables us to quantitatively reproduce the optical response of superconducting films in pump-probe experiments and validate our approach on measurements of the differential reflectance of Pb and LaH$_{10}$ in response to a pump excitation. Similar calculations performed on the alkali-doped fulleride K$_3$C$_{60}$ reveal that a photo-induced superconducting state is generated after irradiation by an ultrafast mid-infrared pulse of sufficient intensity, as reported in prior experimental work. The enhancement in this framework is attributed to the excitation of quasiparticles to energies resonant with the strongest electron-phonon coupling in K$_3$C$_{60}$, in close analogy to the mechanism for enhancement of superconductivity under microwave irradiation, explaining the nature of the photo-induced superconducting state and elucidating the subsequent quasiparticle and phonon dynamics. Our results suggest that photo-induced superconductivity is accessible in more materials than previously recognized. We demonstrate this by performing calculations on calcium-intercalated graphite, CaC$_6$, and predict a similar photo-induced superconducting gap. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Comments: 15 pages, 11 figures

Inverse Isotope Effect in the Ternary Perovskite Hydride SrPdH/D$_{2.9}$: A Signature of Quantum Zero-Point Fluctuations

Authors: Wencheng Lu, Mihir Sahoo, Roman Lucrezi, Michael J. Hutcheon, Shubham Sinha, Pedro N. Ferreira, Chris J. Pickard, Qiang Zhang, Matthew N. Julian, Rohit P. Prasankumar, Christoph Heil, Timothy A. Strobel

Abstract: Guided by first-principles calculations, we demonstrate superconductivity in the ternary perovskite hydride SrPdH$_{3-x}$, synthesized at low pressure. Structural characterization via neutron diffraction reveals the near-stoichiometric composition SrPdD$_{2.9(2)}$ with 96\% deuterium site occupancy. Subsequent transport and magnetic susceptibility measurements establish onset superconducting trans… ▽ More Guided by first-principles calculations, we demonstrate superconductivity in the ternary perovskite hydride SrPdH$_{3-x}$, synthesized at low pressure. Structural characterization via neutron diffraction reveals the near-stoichiometric composition SrPdD$_{2.9(2)}$ with 96\% deuterium site occupancy. Subsequent transport and magnetic susceptibility measurements establish onset superconducting transitions at $T_\text{c} = \SI{2.1}{K} $ (H) and $T_\text{c} = \SI{2.2}{K} $ (D), exhibiting an inverse isotope effect that our first-principles calculations attribute predominantly to quantum zero-point motion. The excellent agreement between theory and experiment with respect to thermodynamic stability and superconducting properties provides important validation for theory-guided superconductor discovery. This work establishes superconductivity in the perovskite hydride structural prototype -- expanding the limited family of experimentally realized ternary hydride superconductors -- and demonstrates the importance of quantum nuclear motion on the accurate theoretical treatment of low-pressure hydride superconductors. △ Less

Submitted 27 February, 2026; originally announced February 2026.

High-pressure stabilization of Mg2IrH7: Structural proximity to high-Tc superconductivity

Authors: Shubham Sinha, Wencheng Lu, Mads F. Hansen, Michael J. Hutcheon, Trevor W. Bontke, Lewis J. Conway, Kapildeb Dolui, Chris J. Pickard, Christoph Heil, Piotr A. Guńka, Stella Chariton, Vitali Prakapenka, Liangzi Deng, Ching-Wu Chu, Matthew N. Julian, Rohit P. Prasankumar, Timothy A. Strobel

Abstract: Mg$_2$IrH$_6$ is a metastable complex metal hydride with a predicted superconducting transition temperature as high as 170 K at ambient pressure. Following the synthesis of isomorphic, insulating Mg$_2$IrH$_5$ at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements indica… ▽ More Mg$_2$IrH$_6$ is a metastable complex metal hydride with a predicted superconducting transition temperature as high as 170 K at ambient pressure. Following the synthesis of isomorphic, insulating Mg$_2$IrH$_5$ at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements indicate that cubic Mg$_2$IrH$_7$ is stabilized above ca. 40 GPa and coexists with a related hexagonal hydride with likely composition near Mg$_2$IrH$_5$. Electrical transport measurements show that the cubic Mg$_2$IrH$_7$ is insulating, in agreement with ab initio predictions, and persists during room-temperature decompression until $\sim$20 GPa before reverting back to the cubic Mg$_2$IrH$_5$. The experimental results confirm ground-state structure predictions in the Mg-Ir-H system, and the formation of two nearly identical phases with surrounding compositions opens new opportunities to access superconducting Mg$_2$IrH$_6$ through non-equilibrium processing pathways. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Sub 1 K Adiabatic Demagnetization Refrigeration with Rare-Earth Borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$, X = (Yb, Gd)

Authors: Marvin Klinger, Tim Treu, Felix Kreisberger, Christian Heil, Anna Klinger, Anton Jesche, Philipp Gegenwart

Abstract: Adiabatic demagnetization refrigeration (ADR) is regaining relevance for the refrigeration to temperatures below 1 K as global helium-3 supply is increasingly strained. While ADR at these temperatures is long established with paramagnetic hydrated salts, more recently frustrated rare-earth oxides were found to offer higher entropy densities and practical advantages since they do not degrade under… ▽ More Adiabatic demagnetization refrigeration (ADR) is regaining relevance for the refrigeration to temperatures below 1 K as global helium-3 supply is increasingly strained. While ADR at these temperatures is long established with paramagnetic hydrated salts, more recently frustrated rare-earth oxides were found to offer higher entropy densities and practical advantages since they do not degrade under heating or evacuation. We report structural, magnetic and thermodynamic properties of the rare-earth borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$ with X = (Yb, Gd). Except for Ba$_3$GdB$_9$O$_{18}$, which orders at 108 mK, the three other materials remain paramagnetic down to their lowest measured temperatures. ADR performance starting at 2 K in a field of 5 T is analyzed and compared to literature results. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 10 pages, 7 figures, 3 tables, submitted to MDPI/Applied Sciences

First-principles evidence for conventional superconductivity in a quasicrystal approximant

Authors: Pedro N. Ferreira, Roman Lucrezi, Sangmin Lee, Lucy Nathwani, Matthew Julian, Rohit P. Prasankumar, Warren E. Pickett, Chris J. Pickard, Philip Kim, Christoph Heil

Abstract: Quasicrystals (QCs) host long-range order without translational symmetry, a regime in which the very foundations of BCS theory are not straightforwardly applicable, yet experiments on QCs and their approximant crystals (ACs) point to conventional, $s$-wave, electron-phonon coupled superconductivity. Here we test the predictive power of the electron-phonon framework in a representative decagonal AC… ▽ More Quasicrystals (QCs) host long-range order without translational symmetry, a regime in which the very foundations of BCS theory are not straightforwardly applicable, yet experiments on QCs and their approximant crystals (ACs) point to conventional, $s$-wave, electron-phonon coupled superconductivity. Here we test the predictive power of the electron-phonon framework in a representative decagonal AC from first principles. Using state-of-the-art \textit{ab initio} methods, we compute the superconducting properties of the recently discovered AC Al$_{13}$Os$_4$ and quantitatively reproduce its bulk $T_\text{c}$. This constitutes, to our knowledge, the first \textit{ab initio} determination of $T_\text{c}$ for an AC and establishes that the electron-phonon framework is predictive in these systems as well. Using the generalized quasichemical approximation for alloy modeling in the decagonal Al--Os family, we predict tunable superconductivity in Al$_{13}$Os$_{4-x}$Re$_x$ and Al$_{13}$Os$_{4-x}$Ir$_x$; in particular, Al$_{13}$Re$_4$ is dynamically stable and estimated to have a $T_\text{c}$ about 30% above Al$_{13}$Os$_4$. Finally, we discuss the role of ACs as high-fidelity proxies for their parent QCs. Although long-range quasiperiodicity may introduce subtle electronic features, our findings indicate that the key ingredients for superconductivity are already encoded in the local structural motifs preserved by the AC. This places the Al--Os and Al--Re families among the most promising candidates for the highest-$T_\text{c}$ quasicrystalline superconductivity. △ Less

Submitted 1 April, 2026; v1 submitted 12 November, 2025; originally announced November 2025.

Stability and Superconductivity of Ternary Polyhydrides

Authors: Dmitrii V. Semenok, Di Zhou, Wuhao Chen, Alexander G. Kvashnin, Andrey V. Sadakov, Toni Helm, Pedro N. Ferreira, Christoph Heil, Vladimir M. Pudalov, Ivan A. Troyan, Viktor V. Struzhkin

Abstract: We review five years of experimental and theoretical attempts (2020-2025) to enhance the superconducting critical temperature ($\textit{T$_c$}$) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher $\textit{T$_c$}$ in ternary systems such as La-Y-H, La-Ce-H, and Ca-Mg-H, experiments consistently show that the maximum $\textit{T$_c$}$ i… ▽ More We review five years of experimental and theoretical attempts (2020-2025) to enhance the superconducting critical temperature ($\textit{T$_c$}$) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher $\textit{T$_c$}$ in ternary systems such as La-Y-H, La-Ce-H, and Ca-Mg-H, experiments consistently show that the maximum $\textit{T$_c$}$ in disordered ternary superhydrides does not exceed that of the best binary parent hydrides within experimental uncertainty. Instead, alloying primarily stabilizes high-symmetry polyhydride phases at lower pressures, enabling $\textit{T$_c$}$ = 200 K near 110-120 GPa, while also improving vortex pinning and upper critical fields. Magnetic dopants suppress $\textit{T$_c$}$, whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise $\textit{T$_c$}$, but can reduce the pressures required to stabilize high-$\textit{T$_c$}$ phases. We propose that fully ordered ternary hydrides, synthesized via controlled hydrogenation of intermetallic precursors, offer a promising route toward this goal. One of the most promising compounds of this kind is the recently discovered LaSc$_2$H$_{24}$. △ Less

Submitted 6 November, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

Journal ref: ANNALEN DER PHYSIK (2025): e00467

Antiferromagnetic ordering and critical behavior induced giant magnetocaloric effect in distorted kagome lattice Gd$_3$BWO$_9$

Authors: Zhuoqun Wang, Xueling Cui, Tim Treu, Jiesen Guo, Xinyang Liu, Marvin Klinger, Christian Heil, Nvsen Ma, Xianlei Sheng, Zheng Deng, Xingye Lu, Xiancheng Wang, Wei Li, Philipp Gegenwart, Changqing Jin, Kan Zhao

Abstract: We synthesize the high-quality Gd$_3$BWO$_9$ single crystal and investigate its lowtemperature magnetic and thermodynamic properties. Below $T\rm_{N}$ = 1.08 K, the anisotropic behavior of magnetic susceptibilities reveals that the Gd$^{3+}$ moments exhibit the dominant antiferromagnetic coupling along the $c$-axis, while displaying a ferromagnetic arrangement in kagome plane. With pronounced magn… ▽ More We synthesize the high-quality Gd$_3$BWO$_9$ single crystal and investigate its lowtemperature magnetic and thermodynamic properties. Below $T\rm_{N}$ = 1.08 K, the anisotropic behavior of magnetic susceptibilities reveals that the Gd$^{3+}$ moments exhibit the dominant antiferromagnetic coupling along the $c$-axis, while displaying a ferromagnetic arrangement in kagome plane. With pronounced magnetic frustration, in adiabatic demagnetization refrigeration experiments starting from initial conditions of 9 T and 2 K, Gd$_3$BWO$_9$ polycrystal reaches a minimum temperature of 0.151 K, significantly lower than its $T\rm_{N}$. Due to the high density of Gd$^{3+}$ ions ($S$=7/2), the maximum magnetic entropy change reaches over 50 J kg$^{-1}$ K$^{-1}$ under fields up to 7 T in Gd$_3$BWO$_9$, nearly 1.5 times as large as commercial sub-Kelvin magnetic coolant Gd$_3$Ga$_5$O$_{12}$(GGG). The H-T phase diagram of Gd$_3$BWO$_9$ under $H$//$c$ exhibits field-induced critical behavior near the phase boundaries. This observation aligns with the theoretical scenario in which a quantum critical point acts as the endpoint of a line of classical second-order phase transitions. Such behavior suggests the importance of further investigations into the divergence of magnetic Grüneisen parameter in the vicinity of critical field at ultralow temperatures. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Comments: This manuscript contains 5 figures, to appear in Phys. Rev. Mater soon

Journal ref: Phys. Rev. Mater. 9, 094407 (2025)

Vacancy-free cubic superconducting NbN enabled by quantum anharmonicity

Authors: Eva Kogler, Mihir R. Sahoo, Chia-Nien Tsai, Fabian Jöbstl, Roman Lucrezi, Peter I. C. Cooke, Birgit Kunert, Roland Resel, Chris J. Pickard, Matthew N. Julian, Rohit P. Prasankumar, Mahmoud I. Hussein, Christoph Heil

Abstract: Niobium nitride (NbN) is renowned for its exceptional mechanical, electronic, magnetic, and superconducting properties. The ideal 1:1 stoichiometric $δ$-NbN cubic phase, however, is known to be dynamically unstable, and repeated experimental observations have indicated that vacancies are necessary for its stabilization. In this work, we demonstrate that when the structure is fully relaxed and allo… ▽ More Niobium nitride (NbN) is renowned for its exceptional mechanical, electronic, magnetic, and superconducting properties. The ideal 1:1 stoichiometric $δ$-NbN cubic phase, however, is known to be dynamically unstable, and repeated experimental observations have indicated that vacancies are necessary for its stabilization. In this work, we demonstrate that when the structure is fully relaxed and allowed to distort under quantum anharmonic effects, a previously unreported stable cubic phase with space group $P\bar{4}3m$ emerges - 65 meV/atom lower in free energy than the ideal $δ$ phase. This discovery is enabled by state-of-the-art first-principles calculations accelerated by machine-learned interatomic potentials. To evaluate the vibrational and superconducting properties with quantum anharmonic effects accounted for, we use the stochastic self-consistent harmonic approximation (SSCHA) and molecular dynamics spectral energy density (SED) methods. Electron-phonon coupling calculations based on the SSCHA phonon dispersion yield a superconducting transition temperature of $T_\text{c}$ = 20 K, which aligns closely with experimentally reported values for near-stoichiometric NbN. These findings challenge the long-held assumption that vacancies are essential for stabilizing cubic NbN and point to the potential of synthesizing the ideal 1:1 stoichiometric phase as a route to achieving enhanced superconducting performance in this technologically significant material. △ Less

Submitted 4 July, 2025; originally announced July 2025.

Journal ref: Communications Materials 6, 283 (2025)

Prediction and Synthesis of Mg$_4$Pt$_3$H$_6$: A Metallic Complex Transition Metal Hydride Stabilized at Ambient Pressure

Authors: Wencheng Lu, Michael J. Hutcheon, Mads F. Hansen, Kapildeb Dolui, Shubham Sinha, Mihir R. Sahoo, Chris J. Pickard, Christoph Heil, Anna Pakhomova, Mohamed Mezouar, Dominik Daisenberger, Stella Chariton, Vitali Prakapenka, Matthew N. Julian, Rohit P. Prasankumar, Timothy A. Strobel

Abstract: The low-pressure stabilization of superconducting hydrides with high critical temperatures ($T_c$s) remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition-metal complex hydrides (hydrido complexes)-typically regarded as hydrogen storage materials-exhibit a large range of compou… ▽ More The low-pressure stabilization of superconducting hydrides with high critical temperatures ($T_c$s) remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition-metal complex hydrides (hydrido complexes)-typically regarded as hydrogen storage materials-exhibit a large range of compounds stabilized at low pressure with recent predictions for high-$T_c$ superconductivity. Motivated by this class of materials, we investigated complex hydride formation in the Mg-Pt-H system, which has no known ternary hydride compounds. Guided by ab initio structural predictions, we successfully synthesized a novel complex transition-metal hydride, Mg$_4$Pt$_3$H$_6$, using laser-heated diamond anvil cells. The compound forms in a body-centered cubic structural prototype at moderate pressures between 8-25 GPa. Unlike the majority of known hydrido complexes, Mg$_4$Pt$_3$H$_6$ is metallic, with formal charge described as 4[Mg]$^{2+}$.3[PtH$_2$]$^{2-}$. X-ray diffraction (XRD) measurements obtained during decompression reveal that Mg$_4$Pt$_3$H$_6$ remains stable upon quenching to ambient conditions. Magnetic-field and temperature-dependent electrical transport measurements indicate ambient-pressure superconductivity with $T_c$ (50%) = 2.9 K, in reasonable agreement with theoretical calculations. These findings clarify the phase behavior in the Mg-Pt-H system and provide valuable insights for transition-metal complex hydrides as a new class of hydrogen-rich superconductors. △ Less

Submitted 28 May, 2025; originally announced May 2025.

IsoME: Streamlining High-Precision Eliashberg Calculations

Authors: Eva Kogler, Dominik Spath, Roman Lucrezi, Hitoshi Mori, Zien Zhu, Zhenglu Li, Elena R. Margine, Christoph Heil

Abstract: This paper introduces the Julia package IsoME, an easy-to-use yet accurate and robust computational tool designed to calculate superconducting properties. Multiple levels of approximation are supported, ranging from the basic McMillan-Allen-Dynes formula and its machine learning-enhanced variant to Eliashberg theory including static Coulomb interactions derived from $GW$ calculations, offering a f… ▽ More This paper introduces the Julia package IsoME, an easy-to-use yet accurate and robust computational tool designed to calculate superconducting properties. Multiple levels of approximation are supported, ranging from the basic McMillan-Allen-Dynes formula and its machine learning-enhanced variant to Eliashberg theory including static Coulomb interactions derived from $GW$ calculations, offering a fully ab initio approach to determine superconducting properties, such as the critical superconducting temperature ($T_\text{c}$) and the superconducting gap function ($Δ$). We validate IsoME by benchmarking it against various materials, demonstrating its versatility and performance across different theoretical levels. The findings indicate that the previously held assumption that Eliashberg theory overestimates $T_\text{c}$ is no longer valid when $μ^*$ is appropriately adjusted to account for the finite Matsubara frequency cutoff. Furthermore, we conclude that the constant density of states (DOS) approximation remains accurate in most cases. By unifying multiple approximation schemes within a single framework, IsoME combines first-principles precision with computational efficiency, enabling seamless integration into high-throughput workflows through its $T_\text{c}$ search mode. This makes IsoME a powerful and reliable tool for advancing superconductivity research. △ Less

Submitted 18 June, 2025; v1 submitted 5 March, 2025; originally announced March 2025.

Journal ref: Computer Physics Communications Volume 315, October 2025, 109720

Ab initio modeling of nonequilibrium dynamics in superconducting detectors and qubits

Authors: Alejandro Simon, Reed Foster, Mihir Sahoo, James Shi, Emma Batson, Francesca Incalza, Matteo Castellani, Owen Medeiros, Christoph Heil, Karl K. Berggren

Abstract: Nonequilibrium quasiparticle (QP) and phonon dynamics are central to the operation of superconducting devices. Superconducting detectors, such as the superconducting nanowire single-photon detector, perform best when a large QP population is generated in response to small perturbations. Conversely, for superconducting qubits and topologically protected Majorana fermions, even relatively small QP d… ▽ More Nonequilibrium quasiparticle (QP) and phonon dynamics are central to the operation of superconducting devices. Superconducting detectors, such as the superconducting nanowire single-photon detector, perform best when a large QP population is generated in response to small perturbations. Conversely, for superconducting qubits and topologically protected Majorana fermions, even relatively small QP densities can lead to significant performance degradation, and thus, ideal materials are less susceptible to QP poisoning. However, existing models of these devices lack a rigorous description of the QP and phonon dynamics, relying on approximations and phenomenology. In this article, we combine kinetic equations with density functional theory to model the nonequilibrium dynamics of a superconducting film ab initio. To demonstrate the universality of our model, we illustrate two examples: (1) we develop a model for the detection of single photons in superconducting nanowires, and (2) we calculate the energy-relaxation rate of a transmon qubit due to the presence of excess QPs. Our examples demonstrate from first principles that NbN is well-suited for single-photon detection and that Ta transmon qubits possess reduced sensitivity to QP poisoning relative to other materials, which is likely in part responsible for their longer coherence times. In contrast to previous models, our ab initio approach makes these predictions without experimental input and thus can be used to accelerate progress in device development. Moreover, by considering the full-bandwidth electron-phonon coupling, our approach can incorporate strong-coupling effects. Our methods effectively integrate ab initio materials modeling with nonequilibrium theory of superconductivity to perform practical modeling of superconducting devices, providing a comprehensive approach that connects fundamental theory with device applications. △ Less

Submitted 17 May, 2025; v1 submitted 23 January, 2025; originally announced January 2025.

Comments: 16 pages, 8 figures

Journal ref: Phys. Rev. B 112, 174512 (2025)

Nevanlinna Analytic Continuation for Migdal-Eliashberg Theory

Authors: D. M. Khodachenko, R. Lucrezi, P. N. Ferreira, M. Aichhorn, C. Heil

Abstract: In this work, we present a method to reconstruct real-frequency properties from analytically continued causal Green's functions within the framework of Migdal-Eliashberg (ME) theory for superconductivity. ME theory involves solving a set of coupled equations self-consistently in imaginary frequency space, but to obtain experimentally measurable properties like the spectral function and quasipartic… ▽ More In this work, we present a method to reconstruct real-frequency properties from analytically continued causal Green's functions within the framework of Migdal-Eliashberg (ME) theory for superconductivity. ME theory involves solving a set of coupled equations self-consistently in imaginary frequency space, but to obtain experimentally measurable properties like the spectral function and quasiparticle density of states, it is necessary to perform an analytic continuation to real frequency space. Traditionally, the ME Green's function is decomposed into three fundamental complex functions, which are analytically continued independently. However, these functions do not possess the causal properties of Green's functions, complicating or even preventing the application of standard methods such as Maximum Entropy. Our approach overcomes these challenges, enabling the use of various analytic continuation techniques that were previously impractical. We demonstrate the effectiveness of this method by combining it with Nevanlinna analytic continuation to achieve accurate real-frequency results for ME theory, which are directly comparable to experimental data, with applications highlighted for the superconductors MgB$_2$ and LaBeH$_8$. △ Less

Submitted 14 October, 2024; v1 submitted 4 September, 2024; originally announced September 2024.

Journal ref: Computational Materials Today, Volume 4, December 2024, 100015

Probing the superconducting gap structure of ScRuSi via $μ$SR and first-principles calculations

Authors: K. Panda, A. Bhattacharyya, P. N. Ferreira, Rajib Mondal, A. Thamizhavel, D. T. Adroja, C. Heil, L. T. F. Eleno, A. D. Hillier

Abstract: In this study, we present a thorough investigation into the superconducting state of the ruthenium-based ternary equiatomic compound ScRuSi. Our analysis combines experimental techniques, including muon spin rotation/relaxation ($μ$SR) and low-temperature resistivity measurements, with theoretical insights derived from first-principles calculations. The low-temperature resistivity measurements rev… ▽ More In this study, we present a thorough investigation into the superconducting state of the ruthenium-based ternary equiatomic compound ScRuSi. Our analysis combines experimental techniques, including muon spin rotation/relaxation ($μ$SR) and low-temperature resistivity measurements, with theoretical insights derived from first-principles calculations. The low-temperature resistivity measurements reveal a distinct superconducting phase transition in the orthorhombic structure of ScRuSi at a critical temperature ($T_\text{C}$) of $2.5$ K. Further, the TF-$μ$SR analysis yields a gap-to-critical-temperature ratio of $2Δ/k_\mathrm{B}T_\mathrm{C} = 2.71$, a value consistent with results obtained from previous heat capacity measurements. The temperature dependence of the superconducting normalized depolarization rate is fully described by the isotropic $s$-wave gap model. Additionally, zero-field $μ$SR measurements indicate that the relaxation rate remains nearly identical below and above $T_\text{C}$. This observation strongly suggests the preservation of time-reversal symmetry within the superconducting state. By employing the McMillan-Allen-Dynes equation, we calculate a $T_\text{C}$ of $2.11$ K from first-principles calculations within the density functional theory framework. This calculated value aligns closely with the experimentally determined critical temperature. The coupling between the low-frequency phonon modes and the transition metal d-orbital states play an important role in governing the superconducting pairing in ScRuSi. The combination of experimental and theoretical approaches provides a comprehensive microscopic understanding of the superconducting nature of ScRuSi, offering insights into its critical temperature, pairing symmetry, and the underlying electron-phonon coupling mechanism. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Comments: 10 pages, 7 figures

Journal ref: Physical Review B 109, 224517 (2024)

Ab initio modeling of superconducting alloys

Authors: Pedro N. Ferreira, Roman Lucrezi, Ivan Guilhon, Marcelo Marques, Lara K. Teles, Christoph Heil, Luiz T. F. Eleno

Abstract: Designing new, technologically relevant superconductors has long been at the forefront of solid-state physics and chemistry research. However, developing efficient approaches for modeling the thermodynamics of superconducting alloys while accurately evaluating their physical properties has proven to be a very challenging task. To fill this gap, we propose an ab initio thermodynamic statistical met… ▽ More Designing new, technologically relevant superconductors has long been at the forefront of solid-state physics and chemistry research. However, developing efficient approaches for modeling the thermodynamics of superconducting alloys while accurately evaluating their physical properties has proven to be a very challenging task. To fill this gap, we propose an ab initio thermodynamic statistical method, the Extended Generalized Quasichemical Approximation (EGQCA), to describe off-stoichiometric superconductors. Within EGQCA, one can predict any computationally accessible property of the alloy, such as the critical temperature in superconductors and the electron-phonon coupling parameter, as a function of composition and crystal growth conditions by computing the cluster occurrence probabilities that minimize the overall mixing Gibbs free energy. Importantly, EGQCA incorporates directly chemical ordering, lattice distortions, and vibrational contributions. As a proof of concept, we applied EGQCA to the well-known Al-doped MgB$_2$ and to niobium alloyed with titanium and vanadium, showing a remarkable agreement with the experimental data. Additionally, we model the near-room temperature sodalite-like Y$_{1-x}$Ca$_x$H$_6$ superconducting solid solution, demonstrating that EGQCA particularly possesses a promising potential for designing in silico high-$T_{\text{c}}$ superhydride alloys. Our approach notably enables the high-throughput screening of complex superconducting solid solutions, intrinsically providing valuable insights into the interplay between synthesis, thermodynamics, and physical properties. △ Less

Submitted 26 June, 2024; v1 submitted 21 June, 2024; originally announced June 2024.

Journal ref: Materials Today Physics 48 (2024): 101547

Synthesis of Mg$_2$IrH$_5$: A potential pathway to high-$T_c$ hydride superconductivity at ambient pressure

Authors: Mads F. Hansen, Lewis J. Conway, Kapildeb Dolui, Christoph Heil, Chris J. Pickard, Anna Pakhomova, Mohammed Mezouar, Martin Kunz, Rohit P. Prasankumar, Timothy A. Strobel

Abstract: Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-$T_c$ phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg$_{2}$IrH$_{6}$ may help ove… ▽ More Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-$T_c$ phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg$_{2}$IrH$_{6}$ may help overcome this challenge with calculations of high-$T_c$ superconductivity (65 K$~<~T_c~<~$ 170 K) in a material that is stable at atmospheric pressure. In this work, the synthesis of Mg$_{2}$IrH$_{6}$ was targeted over a broad range of $P$-$T$ conditions, and the resulting products were characterized using X-ray diffraction (XRD) and vibrational spectroscopy, in concert with first-principles calculations. The results indicate that the charge-balanced complex hydride Mg$_{2}$IrH$_{5}$ is more stable over all conditions tested up to ca 28 GPa. The resulting hydride is isostructural with the predicted superconducting Mg$_{2}$IrH$_{6}$ phase except for a single hydrogen vacancy, which shows a favorable replacement barrier upon insertion of hydrogen into the lattice. Bulk Mg$_{2}$IrH$_{5}$ is readily accessible at mild $P$-$T$ conditions and may thus represent a convenient platform to access superconducting Mg$_{2}$IrH$_{6}$ via non-equilibrium processing methods. △ Less

Submitted 13 June, 2024; originally announced June 2024.

Comments: 18 pages, 17 Figures

Machine learning assisted prediction of organic salt structure properties

Authors: Ethan P. Shapera, Dejan-Krešimir Bučar, Rohit P. Prasankumar, Christoph Heil

Abstract: We demonstrate a machine learning-based approach which predicts the properties of crystal structures following relaxation based on the unrelaxed structure. Use of crystal graph singular values reduces the number of features required to describe a crystal by more than an order of magnitude compared to the full crystal graph representation. We construct machine learning models using the crystal grap… ▽ More We demonstrate a machine learning-based approach which predicts the properties of crystal structures following relaxation based on the unrelaxed structure. Use of crystal graph singular values reduces the number of features required to describe a crystal by more than an order of magnitude compared to the full crystal graph representation. We construct machine learning models using the crystal graph singular value representations in order to predict the volume, enthalpy per atom, and metal versus semiconducting phase of DFT-relaxed organic salt crystals based on randomly generated unrelaxed crystal structures. Initial base models are trained to relate 89,949 randomly generated structures of salts formed by varying ratios of 1,3,5-triazine and HCl with the corresponding volumes, enthalpies per atom, and phase of the DFT-relaxed structures. We further demonstrate that the base model is able to extrapolate to new chemical systems with the inclusion of 2,000 to 10,000 crystal structures from the new system. After training a single model with a large number of data points, extension can be done at significantly lower cost. The constructed machine learning models can be used to rapidly screen large sets of randomly generated organic salt crystal structures and efficiently downselect the structures most likely to be experimentally realizable. The models can be used either as a stand-alone crystal structure predictor or incorporated into more sophisticated workflows as a filtering step. △ Less

Submitted 14 February, 2024; originally announced February 2024.

Comments: 18 pages, 20 figures

Overcomplete Reproducing Pairs

Authors: Logan Hart, Christopher Heil, Ian Katz, Michael Northington V

Abstract: The Gaussian Gabor system at the critical density has the property that it is overcomplete in $L^2(\mathbf{R})$ by exactly one element, and if any single element is removed then the resulting system is complete but is not a Schauder basis. This paper characterizes systems that are overcomplete by finitely many elements. Among other results, it is shown that if such a system has a reproducing partn… ▽ More The Gaussian Gabor system at the critical density has the property that it is overcomplete in $L^2(\mathbf{R})$ by exactly one element, and if any single element is removed then the resulting system is complete but is not a Schauder basis. This paper characterizes systems that are overcomplete by finitely many elements. Among other results, it is shown that if such a system has a reproducing partner, then it contains a Schauder basis. While a Schauder basis provides a strong reproducing property for elements of a space, the existence of a reproducing partner only requires a weak type of representation of elements. Thus for these systems weak representations imply strong representations. The results are applied to systems of weighted exponentials and to Gabor systems at the critical density. In particular, it is shown that the Gaussian Gabor system does not possess a reproducing partner. △ Less

Submitted 7 November, 2023; originally announced November 2023.

Comments: 16 pages

Feasible route to high-temperature ambient-pressure hydride superconductivity

Authors: Kapildeb Dolui, Lewis J. Conway, Christoph Heil, Timothy A. Strobel, Rohit Prasankumar, Chris J. Pickard

Abstract: A key challenge in materials discovery is to find high-temperature superconductors. Hydrogen and hydride materials have long been considered promising materials displaying conventional phonon-mediated superconductivity. However, the high pressures required to stabilize these materials have restricted their application. Here, we present results from high-throughput computation, considering a wide r… ▽ More A key challenge in materials discovery is to find high-temperature superconductors. Hydrogen and hydride materials have long been considered promising materials displaying conventional phonon-mediated superconductivity. However, the high pressures required to stabilize these materials have restricted their application. Here, we present results from high-throughput computation, considering a wide range of high-symmetry ternary hydrides from across the periodic table at ambient pressure. This large composition space is then reduced by considering thermodynamic, dynamic, and magnetic stability, before direct estimations of the superconducting critical temperature. This approach has revealed a metastable ambient-pressure hydride superconductor, Mg$_2$IrH$_6$, with a predicted critical temperature of 160 K, comparable to the highest temperature superconducting cuprates. We propose a synthesis route via a structurally related insulator, Mg$_2$IrH$_7$, which is thermodynamically stable above 15 GPa and discuss the potential challenges in doing so. △ Less

Submitted 30 April, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: 30 pages, 30 figures, including SM

Journal ref: Phys. Rev. Lett. 132, 166001, 2024

Critical Role of Disorder for Superconductivity in the Series of Epitaxial Ti(O,N) Films

Authors: Fengmiao Li, Oliver Dicks, Myung-Geun Han, Solveig Aamlid, Giorgio Levy, Ronny Sutarto, Chong Liu, Hsiang-Hsi Kung, Oleksandr Foyevstov, Simon Godin, Bruce A. Davidson, Andrea Damascelli, Yimei Zhu, Christoph Heil, Ilya Elfimov, George A. Sawatzky, Ke Zou

Abstract: Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epita… ▽ More Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epitaxy (MBE). We discover that substituting nitrogen (N) for oxygen (O) in TiO, namely TiO(N), considerably increases the normal-state conductivity and the superconducting transition temperature Tc. The Tc of TiO(N) falling between those of TiO (about 0.5 K) and TiN (about 6 K) is contrary to their comparable Tc predicted by the Migdal Eliasberg theory. It is found that their resistivity vs temperature obeys the Mooij rule, known as the characteristic of metallic glasses. Density functional theory (DFT) calculations demonstrate that strong disorder severely scatters the Bloch electron waves at nonzero momenta, which consequently weakens electron-phonon coupling in TiO(N). △ Less

Submitted 24 November, 2024; v1 submitted 4 October, 2023; originally announced October 2023.

Full-bandwidth anisotropic Migdal-Eliashberg theory and its application to superhydrides

Authors: Roman Lucrezi, Pedro P. Ferreira, Samad Hajinazar, Hitoshi Mori, Hari Paudyal, Elena R. Margine, Christoph Heil

Abstract: Migdal-Eliashberg theory is one of the state-of-the-art methods for describing conventional superconductors from first principles. However, widely used implementations assume a constant density of states around the Fermi level, which hinders a proper description of materials with distinct features in its vicinity. Here, we present an implementation of the Migdal-Eliashberg theory within the EPW co… ▽ More Migdal-Eliashberg theory is one of the state-of-the-art methods for describing conventional superconductors from first principles. However, widely used implementations assume a constant density of states around the Fermi level, which hinders a proper description of materials with distinct features in its vicinity. Here, we present an implementation of the Migdal-Eliashberg theory within the EPW code that considers the full electronic structure and accommodates scattering processes beyond the Fermi surface. To significantly reduce computational costs, we introduce a non-uniform sampling scheme along the imaginary axis. We demonstrate the power of our implementation by applying it to the sodalite-like clathrates YH$_6$ and CaH$_6$, and to the covalently-bonded H$_3$S and D$_3$S. Furthermore, we investigate the effect of maximizing the density of states at the Fermi level in doped H$_3$S and BaSiH$_8$ within the full-bandwidth treatment compared to the constant-density-of-states approximation. Our findings highlight the importance of this advanced treatment in such complex materials. △ Less

Submitted 16 January, 2024; v1 submitted 29 September, 2023; originally announced October 2023.

Comments: 16 pages, 4 figures. Revised manuscript with minor changes. Supplemental Information available on the journal website (open access)

Journal ref: Commun Phys 7, 33 (2024)

$\ell^1$-Bounded Sets

Authors: Christopher Heil, Pu-Ting Yu

Abstract: A subset $M$ of a separable Hilbert space $H$ is $\ell^1$-bounded if there exists a Riesz basis $\mathcal{F} = \{e_n\}_{n \in \mathbb{N}}$ for $H$ such that $\sup_{x \in M} \sum_{n \in \mathbb{N}} |\langle x, e_n\rangle| < \infty.$ A similar definition for $\ell^1$-frame-bounded sets is made by replacing Riesz bases with frames. This paper derives properties of $\ell^1$-bounded sets, operations on… ▽ More A subset $M$ of a separable Hilbert space $H$ is $\ell^1$-bounded if there exists a Riesz basis $\mathcal{F} = \{e_n\}_{n \in \mathbb{N}}$ for $H$ such that $\sup_{x \in M} \sum_{n \in \mathbb{N}} |\langle x, e_n\rangle| < \infty.$ A similar definition for $\ell^1$-frame-bounded sets is made by replacing Riesz bases with frames. This paper derives properties of $\ell^1$-bounded sets, operations on the collection of $\ell^1$-bounded sets, and the relation between $\ell^1$-boundedness and $\ell^1$-frame-boundedness. Some open problems are stated, several of which have intriguing implications. △ Less

Submitted 8 July, 2023; originally announced July 2023.

Temperature and quantum anharmonic lattice effects on stability and superconductivity in lutetium trihydride

Authors: Roman Lucrezi, Pedro P. Ferreira, Markus Aichhorn, Christoph Heil

Abstract: In this work, we resolve conflicting experimental and theoretical findings related to the dynamical stability and superconducting properties of $Fm\overline{3}m$-LuH$_3$, which was recently suggested as the parent phase harboring room-temperature superconductivity at near-ambient pressures. Including temperature and quantum anharmonic lattice effects in our calculations, we demonstrate that the th… ▽ More In this work, we resolve conflicting experimental and theoretical findings related to the dynamical stability and superconducting properties of $Fm\overline{3}m$-LuH$_3$, which was recently suggested as the parent phase harboring room-temperature superconductivity at near-ambient pressures. Including temperature and quantum anharmonic lattice effects in our calculations, we demonstrate that the theoretically predicted structural instability of the $Fm\overline{3}m$ phase near ambient pressures is suppressed for temperatures above $200\,\text{K}$. We provide a $p\,\unicode{x2013}\,T$ phase diagram for stability up to pressures of $6\,\text{GPa}$, where the required temperature for stability is reduced to $T>80\,\text{K}$. We also determine the superconducting critical temperature $T_\text{c}$ of $Fm\overline{3}m$-LuH$_3$ within the Migdal-Eliashberg formalism, using temperature- and quantum-anharmonically-corrected phonon dispersions, finding that the expected $T_\text{c}$ for electron-phonon mediated superconductivity is in the range of $50$ $\unicode{x2013}$ $60\,\text{K}$, i.e., well below the temperatures required to stabilize the lattice. When considering moderate doping based on rigidly shifting the Fermi level, $T_\text{c}$ decreases for both hole and electron doping. Our results thus provide evidence that any observed room-temperature superconductivity in pure or doped $Fm\overline{3}m$-LuH$_3$, if confirmed, cannot be explained by a conventional electron-phonon mediated pairing mechanism. △ Less

Submitted 11 January, 2024; v1 submitted 13 April, 2023; originally announced April 2023.

Comments: 9 pages, 5 figures. Revised manuscript. Supplemental Information available on the journal website (open access)

Journal ref: Nat Commun 15, 441 (2024)

Search for ambient superconductivity in the Lu-N-H system

Authors: Pedro P. Ferreira, Lewis J. Conway, Alessio Cucciari, Simone Di Cataldo, Federico Giannessi, Eva Kogler, Luiz T. F. Eleno, Chris J. Pickard, Christoph Heil, Lilia Boeri

Abstract: Motivated by the recent report of room-temperature superconductivity at near-ambient pressure in N-doped lutetium hydride by Dasenbrock et al. [Nature 615, 244 (2023)], we performed a comprehensive, detailed study of the phase diagram of the Lu-N-H system, looking for superconducting phases. We combined ab initio crystal structure prediction with ephemeral data-derived interatomic potentials to sa… ▽ More Motivated by the recent report of room-temperature superconductivity at near-ambient pressure in N-doped lutetium hydride by Dasenbrock et al. [Nature 615, 244 (2023)], we performed a comprehensive, detailed study of the phase diagram of the Lu-N-H system, looking for superconducting phases. We combined ab initio crystal structure prediction with ephemeral data-derived interatomic potentials to sample over 200,000 different structures. Out of the more than 150 structures predicted to be metastable within $\sim$ 50 meV from the convex hull we identify 52 viable candidates for conventional superconductivity, for which we computed their superconducting properties from Density Functional Perturbation Theory. Although for some of these structures we do predict a finite superconducting $T_{\text{c}}$, none is even remotely compatible with room-temperature superconductivity as reported by Dasenbrock et al. Our work joins the broader community effort that has followed the report of near-ambient superconductivity, confirming beyond reasonable doubt that no conventional mechanism can explain the reported $T_{\text{c}}$ in Lu-N-H. △ Less

Submitted 6 September, 2023; v1 submitted 10 April, 2023; originally announced April 2023.

Journal ref: Nat Commun 14, 5367 (2023)

Quantum lattice dynamics and their importance in ternary superhydride clathrates

Authors: Roman Lucrezi, Eva Kogler, Simone Di Cataldo, Markus Aichhorn, Lilia Boeri, Christoph Heil

Abstract: The quantum nature of the hydrogen lattice in superconducting hydrides can have crucial effects on the material's properties. Taking a detailed look at the dynamic stability of the recently predicted BaSiH$_8$ phase, we find that the inclusion of anharmonic quantum ionic effects leads to an increase in the critical dynamical pressure to $20\,\text{GPa}$ as compared to $5\,\text{GPa}$ within the ha… ▽ More The quantum nature of the hydrogen lattice in superconducting hydrides can have crucial effects on the material's properties. Taking a detailed look at the dynamic stability of the recently predicted BaSiH$_8$ phase, we find that the inclusion of anharmonic quantum ionic effects leads to an increase in the critical dynamical pressure to $20\,\text{GPa}$ as compared to $5\,\text{GPa}$ within the harmonic approximation. We identify the change in the crystal structure due to quantum ionic effects to be the main driving force for this increase and demonstrate that this can already be understood at the harmonic level by considering zero-point energy corrections to the total electronic energy. In fact, the previously determined critical pressure of kinetic stability $p_\text{kin} = 30\,\text{GPa}$ still poses a stricter bound for the synthesizability of BaSiH$_8$ and similar hydride materials than the dynamical stability and therefore constitutes a more rigorous and accurate estimate for the experimental realizability of these structures. △ Less

Submitted 16 October, 2023; v1 submitted 19 December, 2022; originally announced December 2022.

Comments: 10 pages, 5 figures. Revised manuscript with minor changes. Supplemental Material available on the journal website (open access)

Journal ref: Commun Phys 6, 298 (2023)

Computational Reverse-Engineering Analysis for Scattering Experiments for Form Factor and Structure Factor Determination ('P(q) and S(q) CREASE')

Authors: Christian M. Heil, Yingzhen Ma, Bhuvnesh Bharti, Arthi Jayaraman

Abstract: In this paper we present an open-source machine learning (ML) accelerated computational method to analyze small-angle scattering profiles [I(q) vs. q] from concentrated macromolecular solutions to simultaneously obtain the form factor P(q) (e.g., dimensions of a micelle) and structure factor S(q) (e.g., spatial arrangement of the micelles) without relying on analytical models. This method builds o… ▽ More In this paper we present an open-source machine learning (ML) accelerated computational method to analyze small-angle scattering profiles [I(q) vs. q] from concentrated macromolecular solutions to simultaneously obtain the form factor P(q) (e.g., dimensions of a micelle) and structure factor S(q) (e.g., spatial arrangement of the micelles) without relying on analytical models. This method builds on our recent work on Computational Reverse Engineering Analysis for Scattering Experiments (CREASE) that has either been applied to obtain P(q) from dilute macromolecular solutions (where S(q) ~1) or to obtain S(q) from concentrated particle solution when the P(q) is known (e.g., sphere form factor). This paper's newly developed CREASE that calculates P(q) and S(q), termed as 'P(q) and S(q) CREASE' is validated by taking as input I(q) vs. q from in silico structures of known polydisperse core(A)-shell(B) micelles in solutions at varying concentrations and micelle-micelle aggregation. We demonstrate how 'P(q) and S(q) CREASE' performs if given two or three of the relevant scattering profiles - Itotal(q), IA(q), and IB(q) - as inputs; this demonstration is meant to guide experimentalists who may choose to do small-angle X-ray scattering (for total scattering from the micelles) and/or small-angle neutron scattering with appropriate contrast matching to get scattering solely from one or the other component (A or B). After validation of 'P(q) and S(q) CREASE' on in silico structures, we present our results analyzing small-angle neutron scattering profiles from a solution of core-shell type surfactant coated nanoparticles with varying extents of aggregation. △ Less

Submitted 6 December, 2022; originally announced December 2022.

Comments: 41 pages, 7 figures, 1 ToC figure

Superconductivity in Te-deficient ZrTe$_2$

Authors: L. E. Correa, P. P. Ferreira, L. R. de Faria, V. M. Fim, M. S. da Luz, M. S. Torikachvili, C. Heil, L. T. F. Eleno, A. J. S. Machado

Abstract: We present structural, electrical, and thermoelectric potential measurements on high-quality single crystals of ZrTe$_{1.8}$ grown from isothermal chemical vapor transport. These measurements show that the Te-deficient ZrTe$_{1.8}$, which forms the same structure as the non-superconducting ZrTe$_2$, is superconducting below 3.2\,K. The temperature dependence of the upper critical field (H$_{c2}$)… ▽ More We present structural, electrical, and thermoelectric potential measurements on high-quality single crystals of ZrTe$_{1.8}$ grown from isothermal chemical vapor transport. These measurements show that the Te-deficient ZrTe$_{1.8}$, which forms the same structure as the non-superconducting ZrTe$_2$, is superconducting below 3.2\,K. The temperature dependence of the upper critical field (H$_{c2}$) deviates from the behavior expected in conventional single-band superconductors, being best described by an electron-phonon two-gap superconducting model with strong intraband coupling. For the ZrTe$_{1.8}$ single crystals, the Seebeck potential measurements suggest that the charge carriers are predominantly negative, in agreement with the ab initio calculations. Through first-principles calculations within DFT, we show that the slight reduction of Te occupancy in ZrTe$_2$ unexpectedly gives origin to density of states peaks at the Fermi level due to the formation of localized Zr-$d$ bands, possibly promoting electronic instabilities at the Fermi level and an increase at the critical temperature according to the standard BCS theory. These findings highlight that the Te deficiency promotes the electronic conditions for the stability of the superconducting ground state, suggesting that defects can fine-tune the electronic structure to support superconductivity. △ Less

Submitted 10 April, 2023; v1 submitted 5 December, 2022; originally announced December 2022.

Journal ref: J. Phys. Chem. C 2023, 127, 10, 5162-5168

Mechanism of Structural Colors in Binary Mixtures of Nanoparticle-based Supraballs

Authors: Christian M. Heil, Anvay Patil, Bram Vanthournout, Saranshu Singla, Markus Bleuel, Jing-Jin Song, Ziying Hu, Nathan C. Gianneschi, Matthew D. Shawkey, Sunil K. Sinha, Arthi Jayaraman, Ali Dhinojwala

Abstract: Inspired by structural colors in avian species, various synthetic strategies have been developed to produce non-iridescent, saturated colors using nanoparticle assemblies. Mixtures of nanoparticles varying in particle chemistry (or complex refractive indices) and particle size have additional emergent properties that impact the color produced. For such complex multi-component systems, an understan… ▽ More Inspired by structural colors in avian species, various synthetic strategies have been developed to produce non-iridescent, saturated colors using nanoparticle assemblies. Mixtures of nanoparticles varying in particle chemistry (or complex refractive indices) and particle size have additional emergent properties that impact the color produced. For such complex multi-component systems, an understanding of assembled structure along with a robust optical modeling tool can empower scientists to perform intensive structure-color relationship studies and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments (CREASE) method and then use the reconstructed structure in finite-difference time-domain (FDTD) calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing melanin nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach presented in this work is useful for engineering synthetic materials with desired colors without laborious trial and error experiments. △ Less

Submitted 9 October, 2022; originally announced October 2022.

Comments: 23 Pages, 5 Figures, 1 ToC Figure

Modeling Structural Colors from Disordered One-Component Colloidal Nanoparticle-based Supraballs using Combined Experimental and Simulation Techniques

Authors: Anvay Patil, Christian M. Heil, Bram Vanthournout, Saranshu Singla, Ziying Hu, Jan Ilavsky, Nathan C. Gianneschi, Matthew D. Shawkey, Sunil K. Sinha, Arthi Jayaraman, Ali Dhinojwala

Abstract: Bright, saturated structural colors in birds have inspired synthesis of self-assembled, disordered arrays of assembled nanoparticles with varied particle spacings and refractive indices. However, predicting colors of assembled nanoparticles, and thereby guiding their synthesis, remains challenging due to the effects of multiple scattering and strong absorption. Here, we use a computational approac… ▽ More Bright, saturated structural colors in birds have inspired synthesis of self-assembled, disordered arrays of assembled nanoparticles with varied particle spacings and refractive indices. However, predicting colors of assembled nanoparticles, and thereby guiding their synthesis, remains challenging due to the effects of multiple scattering and strong absorption. Here, we use a computational approach to first reconstruct the nanoparticles' assembled structures from small-angle scattering measurements and then input the reconstructed structures to a finite-difference time-domain method to predict their color and reflectance. This computational approach is successfully validated by comparing its predictions against experimentally measured reflectance and provides a pathway for reverse engineering colloidal assemblies with desired optical and photothermal properties. △ Less

Submitted 7 October, 2022; v1 submitted 25 May, 2022; originally announced May 2022.

Comments: 14 pages, 3 figures, 1 ToC figure

Journal ref: ACS Materials Letters 2022

Convergence of frame series

Authors: Christopher Heil, Pu-Ting Yu

Abstract: If $\{x_n\}_{n \in \mathbb{N}}$ is a frame for a Hilbert space $H,$ then there exists a canonical dual frame $\{\tilde{x_n}\}_{n \in \mathbb{N}}$ such that for every $x \in H$ we have $x = \sum \langle x, \tilde{x_n} \rangle \, x_n,$ with unconditional convergence of this series. However, if the frame is not a Riesz basis, then there exist alternative duals $\{y_n\}_{n \in \mathbb{N}}$ and synthes… ▽ More If $\{x_n\}_{n \in \mathbb{N}}$ is a frame for a Hilbert space $H,$ then there exists a canonical dual frame $\{\tilde{x_n}\}_{n \in \mathbb{N}}$ such that for every $x \in H$ we have $x = \sum \langle x, \tilde{x_n} \rangle \, x_n,$ with unconditional convergence of this series. However, if the frame is not a Riesz basis, then there exist alternative duals $\{y_n\}_{n \in \mathbb{N}}$ and synthesis-pseudo duals $\{z_n\}_{n \in \mathbb{N}}$ such that $x = \sum \langle x, y_n \rangle \, x_n,$ and $x = \sum \langle x, x_n \rangle \, z_n,$ for every $x.$ We characterize the frames for which the frame series ($x = \sum \langle x, y_n \rangle \, x_n,$) converges unconditionally for every $x$ for every alternative dual, and similarly for synthesis-pseudo duals. In particular, we prove that if $\{x_n\}_{n \in \mathbb{N}}$ does not contain infinitely many zeros then the frame series converge unconditionally for every alternative dual (or synthesis-pseudo dual) if and only if $\{x_n\}_{n \in \mathbb{N}}$ is a near-Riesz basis. We also prove that all alternative duals and synthesis-pseudo duals have the same excess as their associated frame. △ Less

Submitted 16 January, 2023; v1 submitted 14 March, 2022; originally announced March 2022.

Comments: Final version

MSC Class: 40A05; 42C15; 46C99

In-silico synthesis of lowest-pressure high-$T_\text{c}$ ternary superhydrides

Authors: Roman Lucrezi, Simone Di Cataldo, Wolfgang von der Linden, Lilia Boeri, Christoph Heil

Abstract: We report the theoretical prediction of two high-performing hydride superconductors BaSiH$_8$ and SrSiH$_8$. They are thermodynamically stable above pressures of $130$ and $174\,\text{GPa}$, respectively, and metastable below that. Employing anharmonic phonon calculations, we determine the minimum pressures of dynamical stability to be around $3\,\text{GPa}$ for BaSiH$_8$ and $27\,\text{GPa}$ for… ▽ More We report the theoretical prediction of two high-performing hydride superconductors BaSiH$_8$ and SrSiH$_8$. They are thermodynamically stable above pressures of $130$ and $174\,\text{GPa}$, respectively, and metastable below that. Employing anharmonic phonon calculations, we determine the minimum pressures of dynamical stability to be around $3\,\text{GPa}$ for BaSiH$_8$ and $27\,\text{GPa}$ for SrSiH$_8$, and using the fully anisotropic Migdal-Eliashberg theory, we predict $T_\text{c}$'s around $71$ and $126\,\text{K}$, respectively. We also introduce a method to estimate the lowest pressure of synthesis, based on the calculation of the enthalpy barriers protecting the BaSiH$_8$ $Fm\bar{3}m$ structure from decomposition at various pressures. This $kinetic$ pressure threshold is sensibly higher than the one based on $dynamic$ stability, but gives a much more rigorous limit for synthesizability. △ Less

Submitted 30 May, 2022; v1 submitted 3 December, 2021; originally announced December 2021.

Comments: 9 pages, 6 figures. Revised manuscript with minor changes. Supplemental Material available on the journal website (open access)

Journal ref: npj Comput Mater 8, 119 (2022)

Structural Color Production in Melanin-based Disordered Colloidal Nanoparticle Assemblies in Spherical Confinement

Authors: Anvay Patil, Christian M. Heil, Bram Vanthournout, Markus Bleuel, Saranshu Singla, Ziying Hu, Nathan C. Gianneschi, Matthew D. Shawkey, Sunil K. Sinha, Arthi Jayaraman, Ali Dhinojwala

Abstract: Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material, melanin, affects the generated color is unknown. Using a combined molecular dynamics and finite-difference time-domain computational approach, this paper investigates structural color generation in one-component… ▽ More Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material, melanin, affects the generated color is unknown. Using a combined molecular dynamics and finite-difference time-domain computational approach, this paper investigates structural color generation in one-component melanin nanoparticle-based supra-assemblies (called supraballs) as well as binary mixtures of melanin and silica (non-absorbing) nanoparticle-based supraballs. Experimentally produced one-component melanin and one-component silica supraballs, with thoroughly characterized primary particle characteristics using neutron scattering, produce reflectance profiles similar to the computational analogues, confirming that the computational approach correctly simulates both absorption and multiple scattering from the self-assembled nanoparticles. These combined approaches demonstrate that melanin's broadband absorption increases the primary reflectance peak wavelength, increases saturation, and decreases lightness factor. In addition, the dispersity of nanoparticle size more strongly influences the optical properties of supraballs than packing fraction, as evidenced by production of a larger range of colors when size dispersity is varied versus packing fraction. For binary melanin and silica supraballs, the chemistry-based stratification allows for more diverse color generation and finer saturation tuning than does the degree of mixing/demixing between the two chemistries. △ Less

Submitted 4 July, 2022; v1 submitted 20 November, 2021; originally announced November 2021.

Comments: 40 pages, Figure 6

LaBH$_{8}$: the first high-T$_{c}$ low-pressure superhydride

Authors: Simone Di Cataldo, Christoph Heil, Wolfgang von der Linden, Lilia Boeri

Abstract: In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures which are way too high to meet any practical application. In this work, we report the computational prediction of a hydride superconductor, LaBH$_{8}$, with a T$_{c}$ of 126 K at a pressure of 50 GPa, thermodynamically stable abo… ▽ More In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures which are way too high to meet any practical application. In this work, we report the computational prediction of a hydride superconductor, LaBH$_{8}$, with a T$_{c}$ of 126 K at a pressure of 50 GPa, thermodynamically stable above 100 GPa, and dynamically stable down to 40 GPa, an unprecedentedly low pressure for high-T$_{c}$ hydrides. LaBH$_{8}$ can be seen as a ternary sodalite-like hydride, in which a metallic hydrogen sublattice is stabilized by the chemical pressure exerted by the guest elements. The combination of two elements with different atomic sizes in LaBH$_{8}$ realizes a more efficient packing of atoms than in binary sodalite hydrides. A suitable choice of elements may be exploited to further reduce the stabilization pressure to ambient conditions. △ Less

Submitted 24 February, 2021; v1 submitted 22 February, 2021; originally announced February 2021.

Comments: 6 pages, 4 figures, 1 table

Journal ref: Phys. Rev. B 104, 020511 (2021)

The 2021 Quantum Materials Roadmap

Authors: Feliciano Giustino, Jin Hong Lee, Felix Trier, Manuel Bibes, Stephen M Winter, Roser Valentí, Young-Woo Son, Louis Taillefer, Christoph Heil, Adriana I. Figueroa, Bernard Plaçais, QuanSheng Wu, Oleg V. Yazyev, Erik P. A. M. Bakkers, Jesper Nygård, Pol Forn-Diaz, Silvano De Franceschi, J. W. McIver, L. E. F. Foa Torres, Tony Low, Anshuman Kumar, Regina Galceran, Sergio O. Valenzuela, Marius V. Costache, Aurélien Manchon , et al. (4 additional authors not shown)

Abstract: In recent years, the notion of Quantum Materials has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and cold atom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topologi… ▽ More In recent years, the notion of Quantum Materials has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and cold atom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moire materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. △ Less

Submitted 4 February, 2021; originally announced February 2021.

Comments: 93 pages, 27 figures

Journal ref: J. Phys. Mater. 3 042006 (2020)

Superconductivity and strong anharmonicity in novel Nb-S phases

Authors: Roman Lucrezi, Christoph Heil

Abstract: In this work we explore the phase diagram of the binary Nb-S system from ambient pressures up to 250 GPa using $ab$ $initio$ evolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pre… ▽ More In this work we explore the phase diagram of the binary Nb-S system from ambient pressures up to 250 GPa using $ab$ $initio$ evolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pressure phases of pure sulfur, are metals with low electron-phonon coupling strengths and critical superconducting temperatures below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on lattice dynamics, electron-phonon interactions, and superconductivity for the novel high-pressure phase of $\mathrm{Nb}_2\mathrm{S}$, demonstrating that the inclusion of anharmonicity stabilizes the lattice and enhances the electron-phonon interaction. △ Less

Submitted 12 January, 2021; originally announced January 2021.

Comments: 9 pages, 6 figures and Supplemental Material

Journal ref: J. Phys.: Condens. Matter (2021)

Magnetic surface reconstruction in the van-der-Waals antiferromagnet Fe$_{1+x}$Te

Authors: C. Trainer, M. Songvilay, N. Qureshi, A. Stunault, C. M. Yim, E. E. Rodriguez, C. Heil, V. Tsurkan, M. A. Green, A. Loidl, P. Wahl, C. Stock

Abstract: Fe$_{1+x}$Te is a two dimensional van der Waals antiferromagnet that becomes superconducting on anion substitution on the Te site. The parent phase of Fe$_{1+x}$Te is sensitive to the amount of interstitial iron situated between the iron-tellurium layers displaying collinear magnetic order coexisting with low temperature metallic resistivity for small concentrations of interstitial iron $x$ and he… ▽ More Fe$_{1+x}$Te is a two dimensional van der Waals antiferromagnet that becomes superconducting on anion substitution on the Te site. The parent phase of Fe$_{1+x}$Te is sensitive to the amount of interstitial iron situated between the iron-tellurium layers displaying collinear magnetic order coexisting with low temperature metallic resistivity for small concentrations of interstitial iron $x$ and helical magnetic order for large values of $x$. While this phase diagram has been established through scattering [see for example E. E. Rodriguez $\textit{et al.}$ Phys. Rev. B ${\bf{84}}$, 064403 (2011) and S. Rössler $\textit{et al.}$ Phys. Rev. B ${\bf{84}}$, 174506 (2011)], recent scanning tunnelling microscopy measurements [C. Trainer $\textit{et al.}$ Sci. Adv. ${\bf{5}}$, eaav3478 (2019)] have observed a different magnetic structure for small interstitial iron concentrations $x$ with a significant canting of the magnetic moments along the crystallographic $c$ axis of $θ$=28 $\pm$ 3$^{\circ}$. In this paper, we revisit the magnetic structure of Fe$_{1.09}$Te using spherical neutron polarimetry and scanning tunnelling microscopy to search for this canting in the bulk phase and compare surface and bulk magnetism. The results show that the bulk magnetic structure of Fe$_{1.09}$Te is consistent with collinear in-plane order ($θ=0$ with an error of $\sim$ 5$^{\circ}$). Comparison with scanning tunnelling microscopy on a series of Fe$_{1+x}$Te samples reveals that the surface exhibits a magnetic surface reconstruction with a canting angle of the spins of $θ=29.8^{\circ}$. We suggest that this is a consequence of structural relaxation of the surface layer resulting in an out-of-plane magnetocrystalline anisotropy. The magnetism in Fe$_{1+x}$Te displays different properties at the surface when the symmetry constraints of the bulk are removed. △ Less

Submitted 3 January, 2021; originally announced January 2021.

Comments: 11 pages, 6 figures, to be Published in Phys. Rev. B

Journal ref: Phys. Rev. B 103, 024406 (2021)

Electronic, vibrational, and electron-phonon coupling properties in SnSe$_2$ and SnS$_2$ under pressure

Authors: Gyanu Prasad Kafle, Christoph Heil, Hari Paudyal, Elena R. Margine

Abstract: The tin-selenide and tin-sulfide classes of materials undergo multiple structural transitions under high pressure leading to periodic lattice distortions, superconductivity, and topologically non-trivial phases, yet a number of controversies exist regarding the structural transformations in these systems. We perform first-principles calculations within the framework of density functional theory an… ▽ More The tin-selenide and tin-sulfide classes of materials undergo multiple structural transitions under high pressure leading to periodic lattice distortions, superconductivity, and topologically non-trivial phases, yet a number of controversies exist regarding the structural transformations in these systems. We perform first-principles calculations within the framework of density functional theory and a careful comparison of our results with available experiments on SnSe$_2$ reveals that the apparent contradictions among high-pressure results can be attributed to differences in experimental conditions. We further demonstrate that under hydrostatic pressure a $\sqrt{3} \times \sqrt{3} \times 1$ superstructure can be stabilized above 20 GPa in SnS$_2$ via a periodic lattice distortion as found recently in the case of SnSe$_2$, and that this pressure-induced phase transition is due to the combined effect of Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}$ = $(1/3, 1/3, 0)$. In addition, we investigate the contribution of nonadiabatic corrections on the calculated phonon frequencies, and show that the quantitative agreement between theory and experiment for the high-energy $A_{1g}$ phonon mode is improved when these effects are taken into account. Finally, we examine the nature of the superconducting state recently observed in SnSe$_2$ under nonhydrostatic pressure and predict the emergence of superconductivity with a comparable critical temperature in SnS$_2$ under similar experimental conditions. Interestingly, in the periodic lattice distorted phases, the critical temperature is found to be reduced by an order of magnitude due to the restructuring of the Fermi surface. △ Less

Submitted 22 October, 2020; originally announced October 2020.

Comments: accepted manuscript, 15 pages, 11 figures, and supplemental material

Journal ref: Journal of Materials Chemistry C, 2020

Electronic structure and superconductivity of the non-centrosymmetric Sn$_4$As$_3$

Authors: C. A. Marques, M. J. Neat, C. M. Yim, M. D. Watson, L. C. Rhodes, C. Heil, K. S. Pervakov, V. A. Vlasenko, V. M. Pudalov, A. V. Muratov, T. K. Kim, P. Wahl

Abstract: In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin-orbit coupling are a platform to realize these po… ▽ More In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin-orbit coupling are a platform to realize these possibilities. Here, we report the synthesis and characterisation of high quality crystals of Sn$_4$As$_3$, with non-centrosymmetric unit cell ($R3m$). We have characterised the normal and superconducting state using a range of methods. Angle-resolved photoemission spectroscopy shows a multiband Fermi surface and the presence of two surface states, confirmed by Density-functional theory calculations. Specific heat measurements reveal a superconducting critical temperature of $T_c\sim 1.14$ K and an upper critical magnetic field of $H_c\gtrsim 7$ mT, which are both confirmed by ultra-low temperature scanning tunneling microscopy and spectroscopy. Scanning tunneling spectroscopy shows a fully formed superconducting gap, consistent with conventional $s$-wave superconductivity. △ Less

Submitted 29 June, 2020; v1 submitted 13 December, 2019; originally announced December 2019.

Comments: 11 pages, 7 figures

Journal ref: New J. Phys. 22 (2020) 063049

Superconductivity in sodalite-like yttrium hydride clathrates

Authors: Christoph Heil, Simone di Cataldo, Giovanni B. Bachelet, Lilia Boeri, ITP-CP, Graz University of Technology, Austria, Dipartimento di Fisica, Sapienza Universita' di Roma, Italy

Abstract: We report ab-initio calculations of the superconducting properties of two high-Tc sodalite-like clathrate yttrium hydrides, YH6 and YH10, within the fully anisotropic ME theory, including Coulomb corrections. For both compounds we find almost isotropic superconducting gaps, resulting from a uniform distribution of the electron-phonon coupling over phonon modes and electronic states of mixed Y and… ▽ More We report ab-initio calculations of the superconducting properties of two high-Tc sodalite-like clathrate yttrium hydrides, YH6 and YH10, within the fully anisotropic ME theory, including Coulomb corrections. For both compounds we find almost isotropic superconducting gaps, resulting from a uniform distribution of the electron-phonon coupling over phonon modes and electronic states of mixed Y and H character. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential mu*= 0:11, at odds with claims of unusually large Tc in lanthanum hydrides. The corresponding critical temperatures at 300 GPa exceed room temperature (Tc = 290 K and 310 K for YH6 and YH10), in agreement with a previous isotropic-gap calculation. The different response of these two compounds to external pressure, along with a comparison to low-Tc superconducting YH3, may inspire strategies to improve the superconducting properties of this class of hydrides. △ Less

Submitted 13 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. B 99, 220502 (2019)

Quasiparticle $GW$ band structures and Fermi surfaces of bulk and monolayer NbS$_2$

Authors: Christoph Heil, Martin Schlipf, Feliciano Giustino

Abstract: In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level,… ▽ More In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level, and that our calculations are in good agreement with recent ARPES measurements. In the case of a free-standing monolayer NbS$_2$, we observe a strong increase of the screened Coulomb interaction and the quasiparticle corrections as compared to bulk. In this case we also perform calculations to include the effect of screening by a substrate. We report in detail the results of our convergence tests and computational parameters, to serve as a solid basis for future studies. △ Less

Submitted 10 August, 2018; v1 submitted 24 July, 2018; originally announced July 2018.

Comments: 15 pages, 18 figures

Journal ref: Phys. Rev. B 98, 075120 (2018)

Unusual pressure-induced periodic lattice distortion in SnSe$_2$

Authors: Jianjun Ying, Hari Paudyal, Christoph Heil, Xiao-Jia Chen, Viktor V. Struzhkin, Elena R. Margine

Abstract: We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition… ▽ More We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}=(1/3,1/3,1)$. In contrast, similar PLD transitions associated with charge density wave (CDW) orderings in transition metal dichalcogenides (TMDs) do not involve significant Fermi surface nesting. The discovered pressure-induced PLD is quite remarkable, as pressure usually suppresses CDW phases in related materials. Our findings, therefore, provide new playgrounds to study the intricate mechanisms governing the emergence of PLD in TMD-related materials. △ Less

Submitted 13 July, 2018; originally announced July 2018.

Comments: 6 pages, 4 figures + Supplemental (12 pages, 14 figures)

Journal ref: Phys. Rev. Lett. 121, 027003 (2018)

Absence of superconductivity in iron polyhydrides at high pressures

Authors: Christoph Heil, Giovanni B. Bachelet, Lilia Boeri

Abstract: Recently, C. M. Pépin \textit{et al.} [Science \textbf{357}, 382 (2017)] reported the formation of several new iron polyhydrides FeH$_x$ at pressures in the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a potential high-\tc \ superconductor, because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A.~Majumdar \textit{et al.} [Phys. Rev. B \textbf{96… ▽ More Recently, C. M. Pépin \textit{et al.} [Science \textbf{357}, 382 (2017)] reported the formation of several new iron polyhydrides FeH$_x$ at pressures in the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a potential high-\tc \ superconductor, because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A.~Majumdar \textit{et al.} [Phys. Rev. B \textbf{96}, 201107 (2017)] and A.~G.~Kvashnin \textit{et al.} [J. Phys. Chem. C \textbf{122}, 4731 (2018)] based on {\em ab initio} Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither FeH$_5$ nor its precursor, FeH$_3$, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the FeH$_3$ structure type by adding more electrons to partially fill one of the Fe--H hybrid bands (as, e.g., in NiH$_3$). Critical temperatures, however, will remain low because the $d$--metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds. △ Less

Submitted 14 June, 2018; v1 submitted 10 April, 2018; originally announced April 2018.

Comments: 8 pages, 8 figures

Journal ref: Phys. Rev. B 97, 214510 (2018)

Manipulating surface magnetic order in iron telluride

Authors: Christopher Trainer, Chi Ming Yim, Christoph Heil, Feliciano Giustino, Dorina Croitori, Vladimir Tsurkan, Alois Loidl, Efrain Rodriguez, Chris Stock, Peter Wahl

Abstract: Control of emergent magnetic orders in correlated electron materials promises new opportunities for applications in spintronics. For their technological exploitation, it is important to understand the role of surfaces and interfaces to other materials and their impact on the emergent magnetic orders. Here, we demonstrate for iron telluride, the nonsuperconducting parent compound of the iron chalco… ▽ More Control of emergent magnetic orders in correlated electron materials promises new opportunities for applications in spintronics. For their technological exploitation, it is important to understand the role of surfaces and interfaces to other materials and their impact on the emergent magnetic orders. Here, we demonstrate for iron telluride, the nonsuperconducting parent compound of the iron chalcogenide superconductors, determination and manipulation of the surface magnetic structure by low-temperature spin-polarized scanning tunneling microscopy. Iron telluride exhibits a complex structural and magnetic phase diagram as a function of interstitial iron concentration. Several theories have been put forward to explain the different magnetic orders observed in the phase diagram, which ascribe a dominant role either to interactions mediated by itinerant electrons or to local moment interactions. Through the controlled removal of surface excess iron, we can separate the influence of the excess iron from that of the change in the lattice structure. △ Less

Submitted 4 March, 2019; v1 submitted 16 February, 2018; originally announced February 2018.

Comments: 8 pages, 6 figures, published version

Journal ref: Sci. Adv. 5, eaav3478 (2019)

Origin of superconductivity and latent charge density wave in NbS$_2$

Authors: Christoph Heil, Samuel Poncé, Henry Lambert, Martin Schlipf, Elena R. Margine, Feliciano Giustino

Abstract: We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place… ▽ More We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide definitive evidence for two-gap superconductivity in 2$H$-NbS$_2$, and show that the low- and high-energy peaks observed in tunneling spectra correspond to the $Γ$- and $K$-centered Fermi surface pockets, respectively. The present findings call for further efforts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides. △ Less

Submitted 4 August, 2017; originally announced August 2017.

Comments: 6 pages, 5 figures and Supplemental Material

Journal ref: Phys. Rev. Lett. 119, 087003 (2017)

Search for high-Tc conventional superconductivity at megabar pressures in the lithium-sulfur system

Authors: Christian Kokail, Christoph Heil, Lilia Boeri

Abstract: Motivated by the recent report of superconductivity above 200 K in ultra-dense hydrogen sulfide, we search for high-\tc\ conventional superconductivity in the phase diagram of the binary Li-S system, using {\em ab-initio} methods for crystal structure prediction and linear response calculations for the electron-phonon coupling. We find that at pressures higher than 20 GPa, several new compositions… ▽ More Motivated by the recent report of superconductivity above 200 K in ultra-dense hydrogen sulfide, we search for high-\tc\ conventional superconductivity in the phase diagram of the binary Li-S system, using {\em ab-initio} methods for crystal structure prediction and linear response calculations for the electron-phonon coupling. We find that at pressures higher than 20 GPa, several new compositions, besides the known Li$_2$S, are stabilized; many exhibit electride-like {\em interstitial} charge localization observed in other alkali metal compounds. Of all predicted phases, only Li$_3$S at P > 640 GPa displays a sizable \tc, in contrast to what is observed in sulfur and phosphorus hydrides, where several stoichiometries lead to high \tc. We attribute this difference to 2$s$-2$p$ hybridization and avoided core overlap, and predict similar behavior for other alkali metal compounds. △ Less

Submitted 24 May, 2016; originally announced May 2016.

Journal ref: Phys. Rev. B 94, 060502 (2016)

Superconductivity in metastable phases of phosphorus-hydride compounds under high pressure

Authors: José A. Flores-Livas, Maximilian Amsler, Christoph Heil, Antonio Sanna, Lilia Boeri, Gianni Profeta, Chris Wolverton, Stefan Goedecker, E. K. U. Gross

Abstract: Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur-hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed phosphine w… ▽ More Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur-hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed phosphine was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (T$_{c}$) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. In this work the phase diagram of PH$_n$ ($n=1,2,3,4,5,6$) was extensively explored by means of {\it ab initio} crystal structure predictions using the Minima Hopping Method (MHM). The results do not support the existence of thermodynamically stable PH$_n$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH$_{1,2,3}$ display T$_{c}$'s comparable to experiments, it remains uncertain if the measured values of T$_{c}$ can be fully attributed to a phase-pure compound of PH$_n$. △ Less

Submitted 7 December, 2015; originally announced December 2015.

Journal ref: Phys. Rev. B 93, 020508 (2016)

Influence of bonding on superconductivity in high-pressure hydrides

Authors: Christoph Heil, Lilia Boeri

Abstract: The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated the interest in dense hydrogen-rich solids. In this paper, we investigate a possible way to optimize the critical temperature in these compounds using first-principles linear-response calculations. We construct hy… ▽ More The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated the interest in dense hydrogen-rich solids. In this paper, we investigate a possible way to optimize the critical temperature in these compounds using first-principles linear-response calculations. We construct hypothetical alchemical atoms to smoothly interpolate between elements of the chalcogen group and study their bonding and superconducting properties. Our results show that the already remarkable critical temperatures of H$_3$S could be improved even further by increasing the ionic character of the relevant bonds, i.e. replacing sulfur partially with more electronegative elements. △ Less

Submitted 31 August, 2015; v1 submitted 9 July, 2015; originally announced July 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 92, 060508 (2015)