Ultra solid state is a novel quantum state that emerges near absolute zero,possessing two seemingly contradictory characteristics:solid and superfluid.Since the theoretical speculation of ultra solid state was proposed in the 1970s,scientists have not yet found reliable experimental evidence for the existence of ultra solid state in solid matter,except for simulation experiments of cold atomic gas.The joint research team composed of Su Gang,professor of the University of Chinese Academy of Sciences,Sun Peijie,researcher of the Institute of Physics of the Chinese Academy of Sciences,Li Wei,researcher of the Institute of Theoretical Physics of the Chinese Academy of Sciences,and Jin Wentao,associate professor of Beijing University of Aeronautics and Astronautics,for the first time found the existence of supersolids(spin supersolids)in frustrated quantum magnets in cobalt based triangular lattice quantum magnetic materials through theoretical and experimental research.Furthermore,the study investigated the spin induced quantum phase transition of materials through magnetic field control,and discovered the extremely low temperature giant magnetic card effect.Under adiabatic conditions,an extremely low temperature of 94 mK was achieved,achieving a sub open temperature range of ultra-low temperature refrigeration without liquid helium.The discovery of this new state of matter and new effects is an important breakthrough in basic research,providing new solutions for China's extremely low temperature refrigeration problems in fields such as deep space exploration,quantum technology,and material science research.
Can solid substances simultaneously possess superfluidity?This is from 1970 by A Leggett's scientific question.Leggett first proposed using the non classical behavior of moment of inertia in helium 4 solid to detect this novel quantum material.In 2004,a research team in the United States reported the observation of elemental helium in a super solid state,but it was later confirmed to lack conclusive evidence.In 1962,Yang Zhenning proposed the introduction of non diagonal long programs to characterize macroscopic quantum states such as superfluidity and superconductivity,in order to distinguish them from classical diagonal long programs such as ordered atomic arrangements.According to this definition,ultra solid is a state of matter in which diagonal and non diagonal long programs coexist.For half a century,scientists have yet to find conclusive evidence of the existence of ultra solids in condensed matter systems,and the search for this unique quantum state has long been a research goal in disciplines such as supercooled atomic gas simulation.
In recent years,the research on frustrated quantum magnetism has flourished,providing a new platform for the search for ultra solid states.For the Heisenberg model with triangular lattice easy axis,multi-body calculations indicate that its spin out of plane component disrupts lattice translational symmetry,forming a three sublattice"solid"order,i.e.a diagonal long program;The in-plane spin component disrupts the rotational symmetry of U(1),forming a"superfluid"sequence(non diagonal long program).This is precisely the correspondence between supersolids in quantum magnetism-spin supersolids.However,in what practical system can this spin supersolid be demonstrated,and whether there are experimentally measurable novel effects,are important issues that need to be explored.
Na2BaCo(PO4)2 is a newly synthesized cobalt based triangular lattice quantum antiferromagnet.Early studies have observed strong low-energy spin fluctuations in materials,suggesting the possibility of achieving quantum spin liquid states.Theoretical research by the Institute of Theoretical Physics and the Institute of Physics has shown that the Heisenberg model of the easy axis triangular lattice can well describe the cobalt based triangular magnet and predict the existence of spin supersolids in the material.How to confirm the existence of ultra solid state is a challenging problem.Through theoretical experimental collaboration,the joint team discovered for the first time spin metasolids in an actual quantum magnet;The neutron diffraction experiment revealed the existence of a three lattice"solid"order,supporting the existence of a non diagonal"superfluid"order,and providing microscopic evidence of the spin supersolid quantum phase transition in Na2BaCo(PO4)2.
Furthermore,research has found a significant magnetic card effect in spin metasolids at extremely low temperatures.By regulating the magnetic field under adiabatic conditions,researchers observed a sharp decrease in the temperature of the material near the spin ultra solid quantum phase transition point,reaching the lowest cooling temperature of 94 mK.The adiabatic temperature change rate exhibits a high peak,which is four times higher than the current commonly used refrigerant Gd3Ga5O12.In addition,in the spin metastable phase,Na2BaCo(PO4)2 can maintain a very low cooling temperature due to strong spin fluctuations,forming a sharp contrast with other spin ordered substances.These characteristics make cobalt based magnetic crystals with giant magnetic card effect a promising quantum material for ultra-low temperature refrigeration in the sub Kelvin temperature range.
At present,the main technologies for extremely low temperature refrigeration include helium refrigeration and magnetic refrigeration.The former relies on the special low-temperature characteristics brought about by the strong quantum fluctuations and weak interactions of helium,a rare element,while the latter currently mainly relies on the magnetic card effect of hydrated paramagnetic salt working fluids.The magnetic card effect refers to the phenomenon of significant temperature changes in magnetic materials under the action of a magnetic field.By utilizing the nearly free magnetic moment in paramagnetic salts,scientists have achieved for the first time an extremely low temperature significantly below 1 on through adiabatic demagnetization.However,the distribution of magnetic ions in hydrated paramagnetic salts is sparse,and they have inherent drawbacks such as low magnetic entropy density,poor stability,and low thermal conductivity.The spin metasolid refrigeration discovered in this work,based on the multi-body effect,regulates collective excitation to achieve entropy change,and based on paramagnetism,regulates free magnetic moment to achieve entropy change.There is a fundamental difference in the refrigeration principle and mechanism,which can effectively overcome the inherent limitations of the latter.In the context of global helium supply shortages,the development of high-performance new refrigeration technologies is of great significance in high-tech fields such as space applications and quantum technology that urgently require extremely low temperature environments.
On January 11th,the related research results were published in the journal Nature,titled Giant magnetallic effect in spin supersolid candidate Na2BaCo(PO4)2.Nature also published a research brief titled Spin supersolid with giant magnetic effect promises a new route to extreme cooling,and evaluated the paper as"remarkable because it reports evidence of supersolids in single crystal repulsive magnets and the magnetic card effect originating from fundamental physical discoveries that can be used for sub open temperature cooling.Two important developments were reported in one paper.".
The research work was supported by the National Natural Science Foundation for Distinguished Young Scientists Fund,the Chinese Academy of Sciences Ministry of Finance Basic Research Field Stable Support Youth Team"Liquid helium free extreme low temperature refrigeration based on new principles"project,etc.
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Research Briefing
Spin ultra solid state and its giant magnetic card effect.(Left)Spin supersolids are novel quantum states in which"solid"(diagonal long program)and"superfluid"(non diagonal long program)coexist.The adiabatic refrigeration curve of spin metasolid state(right)exhibits different characteristics compared to traditional paramagnetic materials,with a higher cooling rate during demagnetization.The illustration shows the cobalt based triangular lattice structure diagram(top left)and crystal photo(bottom right).
