Dutta, A.; Kollar, M. A universal gauge structure in the photon content of light-matter systems Unpublished (2026), arXiv: 2610.40446. @unpublished{dutta2026universalgaugestructurephoton,
title = {A universal gauge structure in the photon content of light-matter systems},
author = {A. Dutta and M. Kollar},
url = {https://arxiv.org/abs/2610.40446},
doi = {10.48550/arXiv.2610.40446},
year = {2026},
date = {2026-10-29},
urldate = {2026-10-01},
abstract = {After truncating the matter Hilbert space of a single-mode light-matter model, gauge transformations leave the spectrum invariant whereas the ground-state photon number is gauge dependent, hence, its meaning remains disputed. We show that this gauge dependence obeys an exact algebraic law whenever the gauge transformation acts as a displacement of the field coupled to a matter operator G. Namely, the ground-state photon number is a quadratic polynomial in the gauge parameter λ, ⟨a†a⟩λ = ⟨a†a⟩0 + Cλ + Qλ2. Here the coefficients are the correlator C=⟨Gp⟩ of G and the field momentum p, and Q=⟨G2⟩. Its minimum over all gauges is naturally gauge-invariant, and for the quantum Rabi model it is determined by the ground-state energy alone. We show that it is strictly positive for the quantum Rabi and Dicke models with level splitting, so that a residual photon content survives in every gauge. At weak coupling the gauge of minimal photon number is the Jaynes-Cummings gauge, and for the Dicke model it remains so at all couplings in the thermodynamic limit.},
note = {arXiv: 2610.40446},
keywords = {C4},
pubstate = {published},
tppubtype = {unpublished}
}
After truncating the matter Hilbert space of a single-mode light-matter model, gauge transformations leave the spectrum invariant whereas the ground-state photon number is gauge dependent, hence, its meaning remains disputed. We show that this gauge dependence obeys an exact algebraic law whenever the gauge transformation acts as a displacement of the field coupled to a matter operator G. Namely, the ground-state photon number is a quadratic polynomial in the gauge parameter λ, ⟨a†a⟩λ = ⟨a†a⟩0 + Cλ + Qλ2. Here the coefficients are the correlator C=⟨Gp⟩ of G and the field momentum p, and Q=⟨G2⟩. Its minimum over all gauges is naturally gauge-invariant, and for the quantum Rabi model it is determined by the ground-state energy alone. We show that it is strictly positive for the quantum Rabi and Dicke models with level splitting, so that a residual photon content survives in every gauge. At weak coupling the gauge of minimal photon number is the Jaynes-Cummings gauge, and for the Dicke model it remains so at all couplings in the thermodynamic limit. |
Meier, T. N. G.; Back, C. Thermal stability of skyrmions Journal Article J. Phys.: Condens. Matter 38, 423004 (2026), datadoi: 10.14459/2026mp1859939. @article{meierThermalStabilitySkyrmions2026,
title = {Thermal stability of skyrmions},
author = {T. N. G. Meier and C. Back},
url = {http://dx.doi.org/10.1088/1361-648X/aeb085},
doi = {10.1088/1361-648x/aeb085},
issn = {1361-648X},
year = {2026},
date = {2026-10-23},
urldate = {2026-10-01},
journal = {J. Phys.: Condens. Matter},
volume = {38},
number = {42},
pages = {423004},
publisher = {IOP Publishing},
abstract = {Magnetic skyrmions are vortex-like magnetic textures with a topologically nontrivial spin structure making a continuous transformation from the skyrmion to the uniform state impossible. Hence skyrmions are regarded as topologically protected attracting large interest concerning applications in data storage, neuromorphic computing, and digital logic. For such kind of applications an extraordinary stability of the skyrmion state against thermal fluctuations is required. In real matter, due to its discrete nature, topological protection translates into the existence of an energy barrier between the skyrmion and the uniform state leading to a finite skyrmion lifetime. Therefore the experimental determination of the skyrmion lifetime becomes important. Within this work we focus on experimental studies of the skyrmion lifetime in a variety of magnetic systems ranging from monolayers to bulk samples. We discuss previous experimental studies on the thermal stability of magnetic skyrmions in systems based on Fe/Ir(111) and in B20-materials. New experimental results on skyrmion nucleation and decay are obtained for thin Ni/Fe-films on Cu(001) using threshold photoemission magnetic circular dichroism in combination with photoemission electron microscopy (TP-MCD-PEEM). The results are interpreted using an Arrhenius-like activation behavior and evaluated regarding energy barriers and pre-exponential factors of the skyrmion dynamics.},
note = {datadoi: 10.14459/2026mp1859939},
keywords = {C2},
pubstate = {published},
tppubtype = {article}
}
Magnetic skyrmions are vortex-like magnetic textures with a topologically nontrivial spin structure making a continuous transformation from the skyrmion to the uniform state impossible. Hence skyrmions are regarded as topologically protected attracting large interest concerning applications in data storage, neuromorphic computing, and digital logic. For such kind of applications an extraordinary stability of the skyrmion state against thermal fluctuations is required. In real matter, due to its discrete nature, topological protection translates into the existence of an energy barrier between the skyrmion and the uniform state leading to a finite skyrmion lifetime. Therefore the experimental determination of the skyrmion lifetime becomes important. Within this work we focus on experimental studies of the skyrmion lifetime in a variety of magnetic systems ranging from monolayers to bulk samples. We discuss previous experimental studies on the thermal stability of magnetic skyrmions in systems based on Fe/Ir(111) and in B20-materials. New experimental results on skyrmion nucleation and decay are obtained for thin Ni/Fe-films on Cu(001) using threshold photoemission magnetic circular dichroism in combination with photoemission electron microscopy (TP-MCD-PEEM). The results are interpreted using an Arrhenius-like activation behavior and evaluated regarding energy barriers and pre-exponential factors of the skyrmion dynamics. |
Ranaut, D.; Unnikrishnan, G.; M, Samraj; Channarayappa, S. K.; Mukharjee, P. K.; Saravanan, P.; Gegenwart, P.; Jaiswal-Nagar, D. Frustrated Garnet Gd3Te2Li3O12 for High-Performance Sub-Kelvin Adiabatic Demagnetization Refrigeration Journal Article ACS Appl. Electron. Mater. (2026). @article{ranautFrustratedGarnetGd3Te2Li3O122026,
title = {Frustrated Garnet Gd_{3}Te_{2}Li_{3}O_{12} for High-Performance Sub-Kelvin Adiabatic Demagnetization Refrigeration},
author = {D. Ranaut and G. Unnikrishnan and Samraj M and S. K. Channarayappa and P. K. Mukharjee and P. Saravanan and P. Gegenwart and D. Jaiswal-Nagar},
url = {https://doi.org/10.1021/acsaelm.6c01619},
doi = {10.1021/acsaelm.6c01619},
issn = {2637-6113},
year = {2026},
date = {2026-10-22},
urldate = {2026-10-01},
journal = {ACS Appl. Electron. Mater.},
abstract = {Owing to the limited availability and high cost of 3He, adiabatic demagnetization refrigeration (ADR) has gained considerable attention as an efficient technology for ultralow-temperature applications. Here, we demonstrate the frustrated garnet Gd3Te2Li3O12 (GTLG) as a high-performance ADR material with substantially improved cooling capability over the commercial benchmark Gd3Ga5O12 (GGG). Direct ADR measurements reveal a minimum temperature of 165 mK for GTLG, nearly two-fold lower than that of GGG (320 mK). Under a cryostat bath temperature of 1.8 K, GTLG maintains temperatures below 1 K for more than 12 h and below the 3He-temperature (0.4 K) for over 3 h, demonstrating excellent temperature stability for prolonged cryogenic operation. Magnetic and thermodynamic measurements show that GTLG undergoes antiferromagnetic ordering at TN ∼ 0.25 K while retaining strong magnetic frustration and a large magnetic entropy reservoir favorable for efficient demagnetization cooling. Field-dependent heat capacity and magnetic Grüneisen parameter measurements further reveal the presence of field-induced critical behavior that enables cooling below TN through enhanced entropy accumulation near the critical region. These findings establish GTLG as a promising next-generation magnetic refrigerant and highlight field-induced criticality as an effective design principle for developing advanced materials for ultralow-temperature refrigeration.},
keywords = {B1, B3},
pubstate = {published},
tppubtype = {article}
}
Owing to the limited availability and high cost of 3He, adiabatic demagnetization refrigeration (ADR) has gained considerable attention as an efficient technology for ultralow-temperature applications. Here, we demonstrate the frustrated garnet Gd3Te2Li3O12 (GTLG) as a high-performance ADR material with substantially improved cooling capability over the commercial benchmark Gd3Ga5O12 (GGG). Direct ADR measurements reveal a minimum temperature of 165 mK for GTLG, nearly two-fold lower than that of GGG (320 mK). Under a cryostat bath temperature of 1.8 K, GTLG maintains temperatures below 1 K for more than 12 h and below the 3He-temperature (0.4 K) for over 3 h, demonstrating excellent temperature stability for prolonged cryogenic operation. Magnetic and thermodynamic measurements show that GTLG undergoes antiferromagnetic ordering at TN ∼ 0.25 K while retaining strong magnetic frustration and a large magnetic entropy reservoir favorable for efficient demagnetization cooling. Field-dependent heat capacity and magnetic Grüneisen parameter measurements further reveal the presence of field-induced critical behavior that enables cooling below TN through enhanced entropy accumulation near the critical region. These findings establish GTLG as a promising next-generation magnetic refrigerant and highlight field-induced criticality as an effective design principle for developing advanced materials for ultralow-temperature refrigeration. |
