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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Scaling whole-chip QAOA for higher-order ising spin glass models on heavy-hex graphs

Abstract We show that the quantum approximate optimization algorithm (QAOA) for higher-order, random coefficient, heavy-hex compatible spin glass Ising models has strong parameter concentration across problem sizes from 16 up to 127 qubits for p = 1 up to p = 5, which allows for computationally efficient parameter transfer of QAOA angles. Matrix product state (MPS) simulation is used to compute noise-free QAOA performance. Hardware-compatible short-depth QAOA circuits are executed on ensembles of 100 higher-order Ising models on noisy IBM quantum superconducting processors with 16, 27, and 127 qubits using QAOA angles learned from a single 16-qubit instance using the JuliQAOA tool. We show that the best quantum processors find lower energy solutions up to p = 2 or p = 3, and find mean energies that are about a factor of two off from the noise-free distribution. We show that p = 1 QAOA energy landscapes remain very similar as the problem size increases using NISQ hardware gridsearches with up to a 414 qubit processor.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Disorder-induced spin excitation continuum and spin-glass ground state in the inverse spinel CuGa 2 O 4

Spinel-structured compounds serve as prototypical examples of highly frustrated systems and are promising candidates for realizing the long-sought quantum spin liquid (QSL) state. However, structural disorder is inevitable in many real QSL candidates and its impact remains a topic of intense debate. In this work, we conduct comprehensive investigations on CuGa 2 ⁢ O 4 , a spinel compound with significant structural disorder, focusing on its thermodynamic properties and spectroscopic behaviors. No long-range magnetic order is observed down to ∼80 mK, as evidenced by magnetic susceptibility, specific-heat, and elastic neutron scattering measurements. More intriguingly, inelastic neutron scattering experiments reveal a broad gapless continuum of magnetic excitations around the Brillouin zone boundary, resembling the magnetic excitation spectra expected for a QSL. Nevertheless, a spin-freezing transition at 𝑇 f ≈ 0.88 K is identified from the cusp in the dc susceptibility curves, where a bifurcation between zero-field-cooling and field-cooling curves occurs. Furthermore, ac susceptibility measurements show a peak close to 𝑇 f at low frequency, which shifts to higher temperature with increasing frequency. These results show that CuGa 2 ⁢ O 4 has a spin-glass ground state, consistent with the establishment of short-range order inferred from the specific-heat measurements. Collectively, these results illustrate the crucial role of disorder in defining the excitation spectrum out of the disordered ground state. Furthermore, our findings shed light onto the broader class of 𝐴⁢𝐵 2 ⁢O 4 spinels and advance our understanding of the spin dynamics in magnetically disordered systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1991

Spin glass freezing and superconductivity in YBa2(Cu(1-x)Fe(x))3O7 alloys

The dynamics were studied of the iron spins in superconducting YBa2(Cu(0.94)Fe(0.06))3O7 by neutron time of flight measurements. Two samples were studied with slightly different characteristics, as shown by resistivity and neutron diffraction measurements. The same dynamical anomalies are observed by neutrons in both samples. Differences appear qualitative but not quantitative. In the whole temperature range, the q-dependence of the magnetic intensity mainly reflects the magnetic form factor of iron which shows that the iron spins are almost uncorrelated. The elastic and quasielastic intensities strongly vary with temperature. A spin glass like freezing is revealed at low temperature by a sharp decrease of the quasielastic intensity, an increase of the 'elastic' or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature (T sub f - 18 K) corresponds to that already determined by a magnetic splitting in Mossbauer experiments. Above T sub f, the relaxation of the iron spins in the paramagnetic state is modified by the occurrence of superconductivity. An increase was observed of the quasielastic intensity and of the quasielastic width at the superconducting transition.

Mirebeau, I.↗
DOE OSTI2025

Iterative quantum optimization of spin glass problems with rapidly oscillating transverse fields

In this work, we introduce a new iterative quantum algorithm, called Iterative Symphonic Tunneling for Satisfiability problems (IST-SAT), which solves quantum spin glass optimization problems using high-frequency oscillating transverse fields. IST-SAT operates as a sequence of iterations, in which bitstrings returned from one iteration are used to set spin-dependent phases in oscillating transverse fields in the next iteration. Over several iterations, the novel mechanism of the algorithm steers the system toward the problem ground state. We benchmark IST-SAT on sets of hard MAX-3-XORSAT problem instances with exact state vector simulation, and report polynomial speedups over Trotterized adiabatic quantum computation and the best known semi-greedy classical algorithm. When IST-SAT is seeded with a sufficiently good initial approximation, the algorithm converges to exact solution(s) in a polynomial number of iterations. Our numerical results identify a critical Hamming radius, or quality of initial approximation, where the time-to-solution crosses from exponential to polynomial scaling in problem size. This work proposes IST-SAT a new quantum algorithm, which improves upon solutions obtained from initial classical or quantum optimization algorithms. The steering mechanism we introduce through IST-SAT presents a new path toward achieving quantum advantage in optimization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Spin-glass state in nickelate superconductors

Magneto-optical measurements in La 0.8 Sr 0.2 NiO 2 and Nd 0.825 Sr 0.175 NiO 2 reveal an intriguing new facet of infinite-layer nickelate superconductors: the onset of spin-glass behavior at a temperature far exceeding the superconducting critical temperature T c . This discovery sharply contrasts with copper oxide superconductors, where magnetism and superconductivity remain largely exclusive. Moreover, the magnitude and onset temperature of the polar Kerr effect in Nd 0.825 Sr 0.175 NiO 2 fabricated on SrTiO 3 and (LaAlO 3 ) 0.3 (Sr 2 TaAlO 6 ) 0.7 substrates differ dramatically, while T c does not.

FOS: Physical sciences↗
DOE OSTI2025

Spin Glass Behavior and Giant Magnetoresistance via Aliovalent Fe/Ni Alloying in Amorphous Tetrathiafulvalene-Tetrathiolate Coordination Polymers

The discovery of materials with programmable combinations of charge transport and magnetic properties is a major goal in synthetic chemistry. Molecular materials, such as coordination polymers (CPs), are emerging candidates in this field due to their synthetic modularity. While there are many conductive or magnetic CPs, single-component examples with both properties are still emerging. Here, in this work, we demonstrate how alloying paramagnetic Fe(III) centers into a highly conductive Ni-based CP results in novel amorphous materials with high conductivity and giant magnetoresistance. Aliovalent doping with paramagnetic Fe centers engenders spin-glass transitions, while Ni and the strong π-stacking interactions of tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt) ligands support conductivity. Ni 0.69 Fe 0.31 TTFtt has σ ≈ 200 S/cm and a 1.8 K magnetoresistance of −52% at 5 T, among the largest for any coordination solid. This work demonstrates not only how magnetic properties can be rationally incorporated into conductive CPs, but also an unexpected potential for amorphous materials in spintronic applications.

amorphous materials↗
NASA NTRS1987

Simulated annealing in networks for computing possible arrangements for red and green cones

Attention is given to network models in which each of the cones of the retina is given a provisional color at random, and then the cones are allowed to determine the colors of their neighbors through an iterative process. A symmetric-structure spin-glass model has allowed arrays to be generated from completely random arrangements of red and green to arrays with approximately as much disorder as the parafoveal cones. Simulated annealing has also been added to the process in an attempt to generate color arrangements with greater regularity and hence more revealing moirepatterns than than the arrangements yielded by quenched spin-glass processes. Attention is given to the perceptual implications of these results.

Ahumada, Albert J., Jr.↗
NASA NTRS1989

Studies of superconducting materials with muon spin rotation

The muon spin rotation/relaxation technique was found to be an exceptionally effective means of measuring the magnetic properties of superconductors, including the new high temperature superconductor materials, at the microscopic level. The technique directly measures the magnetic penetration depth (type II superconductors (SC's)) and detects the presence of magnetic ordering (antiferromagnetism or spin-glass ordering were observed in some high temperature superconductor (HTSC's) and in many closely related compounds). Extensive studies of HTSC materials were conducted by the Virginia State University - College of William and Mary - Columbia University collaboration at Brookhaven National Laboratory and TRIUMF (Vancouver). A survey of LaSrCuO and YBaCaCuO systems shows an essentially linear relationship between the transition temperature T(sub c) and the relaxation rate. This appears to be a manifestation of the proportionality between T(sub c) and the Fermi energy, which suggests a high energy scale for the SC coupling, and which is not consistent with the weak coupling of phonon-mediated SC. Studies of LaCuO and YBaCuO parent compounds show clear evidence of antiferromagnetism. YBa2Cu(3-x)CO(x)O7 shows the simultaneous presence of spin-glass magnetic ordering and superconductivity. Three-dimensional SC, (Ba, K) BiO3, unlike the layered CuO-based compounds, shows no suggestion of magnetic ordering. Experimental techniques and theoretical implications are discussed.

Davis, Michael R.↗
NASA NTRS1988

Coexisting static magnetic ordering and superconductivity in CeCu2.1Si2 found by muon spin relaxation

Zero- and longitudinal-field muon spin relaxation measurements on a heavy fermion system CeCu2.1 Si2 have revealed an onset of static magnetic ordering below T(M) approximately 0.8 K, which coexists with superconductivity below T(c) = 0.7 K. The line shapes of the observed muon spin depolarization functions suggest an ordering in either spin glass or incommensurate spin-density-wave state, with a small averaged static moment of the order of 0.1 micro-B per formula unit at T approaches 0.

Uemura, Y. J.↗
NASA NTRS1988

Static magnetic ordering of CeCu2.1Si2 found by muon spin relaxation

Zero- and longitudinal-field muon spin relaxation measurements on a polycrystal sample of a heavy fermion superconductor CeCu2.1 Si2 (T(c) = 0.7 K) have revealed an onset of static magnetic ordering below T approximately 0.8 K. The line shapes of the observed spectra in zero field indicate a wide distribution of static random local fields at muon sites, suggesting that the ordering is either spin glass or incommensurate spin-density-wave state. The observed width of the random local field at T = 0.05 K corresponds to a small averaged static moment of the order of 0.1 micro-B per formula unit.

Uemura, Y. J.↗
DOE OSTI2025

Scattering neutrinos, spin models, and permutations

We consider a class of Heisenberg all-to-all coupled spin models inspired by neutrino interactions in a supernova with N degrees of freedom. These models are characterized by a coupling matrix that is relatively simple in the sense that there are only a few, relative to N , nontrivial eigenvalues, in distinction to the classic Heisenberg spin-glass models, leading to distinct behavior in both the high-temperature and low-temperature regimes. When the momenta of the neutrinos are uniform and random in directions, we can calculate the large- N partition function for the quantum Heisenberg model. In particular, the high-temperature partition function predicts a non-Gaussian density of states, providing interesting counterexamples showing the limits of general theorems on the density of states for quantum spin models. We can repeat the same argument for classical Heisenberg models, also known as rotor models, and we find the high-temperature expansion is completely controlled by the eigenvalues of the coupling matrix, and again predicts non-Gaussian behavior for the density of states as long as the number of eigenvalues does not scale linearly with N . Indeed, we derive the amusing fact that these partition functions are essentially the generating function for counting permutations in the high-temperature regime. Finally, for the case relevant to neutrinos in a supernova, we identify the low-temperature phase as a unique state with the direction of the momenta of the neutrino dictating its coherent state in flavor-space, a state we dub the “flavor-momentum-locked” state. Published by the American Physical Society 2025

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Accelerating multicanonical sampling with irreversibility

Flat-histogram Monte Carlo simulations are well-established, robust methods to perform random walks in a physical observable or parameter space, making them suitable for finding ground states or studying phase transitions in complex systems in statistical physics. However, their efficiency can be limited by the time to attain the desired flat distribution, which is generally unknown prior to the simulations. In particular, they might suffer from slowing down towards the end of a simulation due to the diffusive nature of random walks. In this work we apply irreversibility to the multicanonical Monte Carlo method via the lifting approach to alleviate this behavior. We achieve a 2–4 times speedup in ground-state search for a two-dimensional (2D) Ising model, and up to an order of magnitude of speedup for finding the ground-state energy in an Edwards–Anderson spin glass, compared to traditional multicanonical sampling. In conclusion, the round-trip times between ground states show a narrower distribution and are significantly shorter compared to the reversible counterpart, suggesting that a lower convergence time with a smaller time variance is feasible.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Biased degenerate ground-state sampling of small Ising models with converged quantum approximate optimization algorithm

The quantum alternating operator ansatz, a generalization of the quantum approximate optimization algorithm (QAOA), is a quantum algorithm used for approximately solving combinatorial optimization problems. QAOA typically uses the transverse field mixer as the driving Hamiltonian. One of the interesting properties of the transverse field driving Hamiltonian is that it results in nonuniform sampling of degenerate ground states of optimization problems. In this study, we numerically examine the fair sampling properties of the transverse field mixer QAOA, and Grover mixer QAOA (GM-QAOA), which provides theoretical guarantees of fair sampling of degenerate optimal solutions, up to a large enough p such that the mean expectation value converges to an optimal approximation ratio of 1. This comparison is performed with high-quality heuristically computed, but not necessarily optimal, QAOA angles, which give strictly monotonically improving solution quality as p increases. These angles are computed using the Julia based numerical simulation software JuliQAOA. Fair sampling of degenerate ground states is quantified using the Shannon entropy of the ground-state amplitudes distribution. The fair sampling properties are reported on several quantum signature Hamiltonians from previous quantum annealing fair sampling studies. Small random fully connected spin glasses are shown, which exhibit exponential suppression of some degenerate ground states with transverse field mixer QAOA. The transverse field mixer QAOA simulations show that some problem instances clearly saturate the Shannon entropy of 0 with a maximally biased distribution that occurs when the learning converges to an approximation ratio of 1 while other problem instances never deviate from a maximum Shannon entropy (uniform distribution) at any p step. Published by the American Physical Society 2025

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Emergent Epitaxial Configuration of Pr 3 IrO 7 Domains via YSZ (111) Substrate

The 5d rare Earth iridate is an intriguing material with exhibiting exotic electronic and magnetic phases due to spin-orbit coupled states. Ternary iridium oxides Ln 3 IrO 7 contain an unusual Ir 5+ (5d 4 ) system, which remain a subject of active research. Fabricating epitaxial Ln 3 IrO 7 films is challenging due to substrate compatibility, but it offers a valuable platform to explore electronic and magnetic behaviors under reduced dimensionality and substrate interactions, revealing novel phenomena based on Ir 5+ (5d 4 ). In this regard, this demonstrates that Pr 3 IrO 7 with its highly anisotropic orthorhombic structure can be epitaxially grown on a cubic (111)-oriented yttrium-stabilized ZrO 2 (YSZ) substrate. Pr 3 IrO 7 film exhibits six epitaxial domains, where the (220) and (202) planes aligning epitaxially to YSZ (111) with the threefold symmetry. This diverse domain configuration in Pr 3 IrO 7 film leads to unique magnetic properties, exhibiting spin-glass-like behavior. Pr 3 IrO 7 thin film offers a platform for exploring unconventional magnetic states, and their successful heteroepitaxy on YSZ substrates opens new avenues for discovering novel physical phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Switching of Magnetic Order via Non‐Magnetic Al Addition in FeCoNiMnAl x Films

Magnetic high entropy alloys (HEAs) consisting of 3 d transition metals offer an exciting platform to explore novel magnetic phases as they often house competing exchange interactions in combination with random site disorders. In this work, a sensitive and tunable magnetic order is demonstrated in sputtered single-layer FeCoNiMnAl x films, as a function of non-magnetic Al addition, along with an unexpected exchange bias effect. Thin films of 50 nm FeCoNiMn exhibit a face-centered-cubic ( fcc ) phase, reentrant spin glass (SG) transition near 100 K, and a large exchange bias of over 500 Oe after field-cooling to 5 K. The exchange bias is increased to 930 Oe through a small addition of 5 at.% Al. Further Al addition to 12 at.% results in a body-centered-cubic ( bcc ) phase, coinciding with a large increase in the saturation magnetization, decrease of exchange bias to 50 Oe, and suppression of SG behavior. The change in magnetic order across the Al-induced structural transformation is mediated by the switching of Mn ground state from AF to FM, which is supported by first-principles calculations and experimentally confirmed via X-ray magnetic circular dichroism. These results open up new HEA strategies for explorations of novel magnetic phases.

competing exchange interactions↗
DOE OSTI2025

New precious metal containing normal spinels: LiRhRu 1-x Ir x O 4 , LiFeIr 1-x Ru x O 4 , and LiCoIr 1-x Ru x O 4

Large spin-orbit-coupled cations in geometrically frustrated crystal structures have the most suitable setting for exploring novel exotic states of matter. Spinel oxides (AM 2 O 4 ) are well-known examples of geometrically frustrated systems. In this study, we report for the first time the synthesis of compositions LiRhRu 1-x Ir x O 4 (x = 0–0.5), LiFeIr 1-x Ru x O 4 (x = 0–0.5), and LiCoIr 1-x Ru x O 4 (x = 0–0.3) containing precious metal cations on edge-sharing octahedral M-sites, and systematically investigate their magnetic and electrical properties. 57 Fe Mössbauer spectroscopy revealed that iron is trivalent in all LiFeIr 1-x Ru x O 4 solid solutions. Magnetic measurements indicate deviations from theoretical spin-only magnetic moment values, indicating the influence of spin-orbit coupling owing to the presence of 4d and 5d block elements. The LiFeIr 1-x Ru x O 4 series shows spin-glass-like freezing behavior with T g ≈ 20 K, and a small frustration index (f ≈ 1-2), indicating that the frustration originates from site disorder. LiRhRu 1-x Ir x O 4 and LiCoIr 1-x Ru x O 4 exhibit strongly geometrically frustrated magnetism. Electrical resistivity measurements as a function of temperature indicate that all phases are semiconducting. Seebeck coefficient measurements show that LiRhRu 1-x Ir x O 4 and LiFeIr 1-x Ru x O 4 are p-type semiconductors with holes as the major charge carriers. A sign reversal of the Seebeck coefficient indicates both holes and electrons as carriers for LiCoIr 1-x RuxO 4 (x = 0–0.2), but only holes as major carriers for x = 0.3. Here, the Seebeck coefficient and power factor increase drastically in the LiRhRu 1-x Ir x O 4 solid solution with Ir substitution, reaching a maximum of ≈ +125 μV/K and ≈2.3×10 -6 W/mK 2 at ∼650 K for x = 0.5.

Charge carriers↗
DOE OSTI2026

Electronic glasses from a broken gauge symmetry in disorder-free systems

Glass phases can be stabilized by quenched disorders, as in most spin-glass materials, or self-generated through kinetic freezing in disorder-free systems. A canonical example of the latter is structural glasses, which have been extensively studied for many decades. Yet, how the rugged energy landscape of a glass phase is spontaneously generated in disorder-free systems remains one of the key questions in glass physics. Here, in this work, we present a general electronic mechanism for the emergence of glassy phase using the example of itinerant electrons coupled to XY spins on a lattice. This model can also be viewed as the mean-field theory of a superconducting system with attractive density-density interactions. Intriguingly, the electron gauge symmetry in the strong pairing limit gives rise to a macroscopic degeneracy of XY spins. In the presence of electron hopping that breaks the gauge symmetry, the lifting of the extensive degeneracy leads to a glass phase with disordered pairings. Our findings highlight a scenario in which a glassy state originates from the breaking of quantum gauge symmetry without quenched disorders.

XY model↗
DOE OSTI2025

Pyrochlore NaYbO2: A Potential Quantum Spin Liquid Candidate

The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice β-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered α-NaYbO2 (∼250 μm on edge) and octahedral β-NaYbO2 (∼50 μm on edge), were grown for the first time. Synchrotron X-ray single-crystal diffraction unambiguously determined that the newfound β-NaYbO2 belongs to the three-dimensional pyrochlore structure characterized by the R3̅m space group, corroborated by synchrotron X-ray and neutron powder diffraction and pair distribution function. Magnetic measurements revealed no long-range magnetic order or spin glass behavior down to 0.4 K with a low boundary spin frustration factor of 17.5, suggesting a potential QSL ground state. Under high magnetic fields, the potential QSL state was broken and spins order. Our findings reveal that NaYbO2 is a fertile playground for studying novel quantum states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗