† Corresponding author. E-mail:
Project supported by the Ministry of Science and Technology of China (Grant Nos. 2016YFA0300504 and 2017YFA0302904) and the National Natural Science Foundation of China (Grant Nos. 11474357, 11774419, 11604383, and 11704401). Y. Y. was supported by the Scientific Equipment Development Project of Chinese Academy of Sciences (Grant No. YJKYYQ20170027).
Raman scattering is a versatile and powerful technique and has been widely used in modern scientific research and vast industrial applications. It is one of the fundamental experimental techniques in condensed matter physics, since it can sensitively probe the basic elementary excitations in solids like electron, phonon, magnon, etc. The application of extreme conditions (low temperature, high magnetic field, high pressure, etc.) to Raman scattering, will push its capability up to an unprecedented level, because this enables us to look into new quantum phases driven by extreme conditions, trace the evolution of the excitations and their coupling, and hence uncover the underlying physics. This review contains two topics. In the first part, we will introduce the Raman facility under extreme conditions, belonging to the optical spectroscopy station of Synergetic Extreme Condition User Facilities (SECUF), with emphasis on the system design and the capability the facility can provide. Then in the second part we will focus on the applications of Raman scattering under extreme conditions to a variety of condensed matter systems such as superconductors, correlated electron systems, charge density waves (CDW) materials, etc. Finally, as a rapidly developing technique, time-resolved Raman scattering will be highlighted here.
Raman scattering is an inelastic light scattering process, i.e., one photon in one photon out. The differences in momentum and energy between incident and scattered photons are transferred to sample and excite quasiparticles in solids. It is basically analogical to inelastic neutron scattering. In fact, both processes share the similar phenomenological description. The Raman effect was discovered in 1928 by Raman. It is a quite tiny and subtle light scattering effect, and is usually over five orders of magnitude smaller than elastic scattering in intensity. In the early stage of Raman scattering history, it is hard to observe the effect due to the lack of high-gain detectors, particularly high-intensity light sources. The invention of laser in 1960s thoroughly changes the situation. The significant progresses in detectors (charge-coupled devices), holographic gratings, and other optical/electronic aspects, eventually push this technique to become a popular experimental tool in modern scientific research and industrial applications. Besides the conventional one, many technical branches such as resonant Raman scattering, tip-enhanced Raman scattering, surface-enhanced Raman scattering, time-resolved Raman scattering, etc., have been extensively developed in the last few decades. All these branches contribute to form a large Raman scattering family nowadays.
In most of the extensive applications, Raman scattering is employed as a fast, convenient, and non-destructive sample-characterization tool, just like x-ray diffraction. On the other hand, Raman scattering plays a vital role in the fundamental research, particularly in the vast field of condensed matter physics. Unlike many other techniques, Raman can probe various excitations from different degrees of freedom, as incident light is an optical-frequency electromagnetic radiation which easily interacts with substance. Basically, the so-called extreme conditions such as low temperature, high pressure, high magnetic field, ultrafast process, etc., can drive condensed matter into new quantum phases in which the novel excitations may be found and the exciting new physics may be established. Thus, Raman technique combined with extreme conditions is expected to tell us new knowledge of novel states of matter and bring us into a colorful physics world.
The review contains five sections and is organized as follows. We give a short introduction to Raman scattering in the first section. In Section
As a long-term national strategy aiming at the frontiers in condensed matter physics and material science, SECUF involves various top experimental techniques under extreme conditions. Raman facility is one of the key facilities of the optical spectroscopy station.
There are several guiding principles for designing such a Raman system. First, the system needs to highlight the feature of extreme conditions. It means that the system works not only in a single extreme condition. In some cases, we need to make Raman measurements under multiple extreme conditions combined together. For instance, usually we can collect Raman spectra at different temperatures and make a careful analysis on temperature dependence of one or more modes. On the other hand, it is also possible that the novel phases or excitations we are interested in, may be hidden behind more extreme conditions, not only low temperatures. In these cases, we have to simultaneously employ multi-extreme conditions like low temperatures, high magnetic fields, high pressure, etc., to probe unusual excitations. The simultaneous application of multi-extreme conditions will perhaps be a big challenge for Raman measurements, since Raman signal in this case may be dramatically reduced. This requires a careful and thorough optimization for the optical components along laser beam path. Second, the rapid development in optics and electronics is changing and shaping the future of Raman technique. Some of the important progresses in recent years should be reflected in the system. These include volume Bragg gratings, high-sensitivity detectors, broadband white laser sources, time-resolved measurement technique, etc. The system should also preserve the interfaces for future extensions, like tip/surface enhanced Raman scattering. Finally, this is a facility for the users worldwide, not just a laboratory setup for one or two professional groups. The efficiency, ease of use, maintainability, and extendibility of the system are real concerns and should be taken into account.
The sketch drawing of the Raman facility is shown in Fig.
There are some reasons for employing double Raman subsystems for the facility. One important reason is to guarantee the facility efficiency. Time-resolved Raman measurement is a very time-consuming work. Integration of multiple extreme conditions makes it much harder. If there is only one spectrometer, which could be occupied by time-resolved measurements for a long period, the requests for continuous-wave Raman measurements will be seriously blocked. As mentioned above, the two subsystems can share the light sources. They are complementary in this sense.
According to the frequently-used light sources, the two Raman subsystems are defined as continuous-wave and time-resolved subsystems, respectively. We will have detailed descriptions for them in the following.
With this subsystem, one can make conventional Raman measurements under low temperature (4 K), magnetic field (9 T), and high pressure (20 GPa). The vertical backscattering configuration is adopted to fit the magnet and cryostat.
Time-resolved Raman scattering is an emerging and rapidly developing branch of Raman scattering. It presents us an alternative degree of freedom beyond temperature and magnetic field to look into Raman process. It can tell us the important information on the non-equilibrium dynamics of electrons or other excitations involved in Raman process. We can expect that the time-resolved technique under low temperatures and high magnetic fields will open a fascinating and unique window for exploring novel quantum phases.
There are different scenarios to realize time-resolved Raman scattering. A direct way is to excite the sample with a fast laser and collect Raman spectra with a high-speed gated detector synchronized with the excitation pulse. However, it is limited by available high-speed detectors and their sensitivity. Here we employ the standard pump-probe technique (Fig.
In our case, the original intense pulse coming out of the ultrafast laser (∼ 100 fs) is spilt into two beams by a 90/10 beam splitter. One beam is taken as the pump and the other as the probe after frequency-doubled in an OPO box. The large difference in frequency between the pump and probe allow us to clearly distinguish the Raman signal excited by the pump from the one by the probe. The orthogonal polarization configuration for the pump and the probe can further reduce the influence from the pump. The idler light emitted out of the OPO can be taken as a backup of pump sources, which effectively extends the frequency range of pump light. The optical delay setup consists of two motorized stages and two mirrors mounted on them. The programmable stages allow to be driven in a micrometer scale and hence can realize a precise time delay between the pump and probe (better than 50 fs).
Besides the ultrafast laser, the subsystem is also equipped with a broadband white light source developed very recently. It is particularly helpful for resonance studies. Therefore, we use the triple-grating spectrometer here (T64000), whose subtractive mode ideally matches the white light source.
Extreme conditions applied to this subsystem are quite similar to the ones for the CW Raman subsystem. Both systems have the similar optical inserts which is composed of many non-magnetic and low-thermal-expansion components like objective lens, precise XYZ stages and rotator. The maximum magnetic field here is 14 T rather than 9 T. It should be pointed out that high pressure up to 20 GPa is designed just to be applied to CW measurements using white light source. At present it is not clear if high pressure cell can work well with time-resolved measurements, since the Raman signal may be extremely weak in this case.
Now we can have a summary for the Raman facility. The extreme conditions applied to the facility include low temperature (4 K), high magnetic field (14 T), high pressure (20 GPa), and high time resolution (< 50 fs). In principles, one can combine one or more of these conditions to realize multi-dimensional Raman measurements and reveal novel condensed matter phases and excitations. According to the incident light source, the available Raman measurement modes can be divided into conventional CW Raman/photoluminescence mode, resonance mode and time-resolved mode. We put all these modes in the following table.
Raman scattering has been proved to be an excellent method of characterizing the vibrational, electronic, and magnetic subsystems by probing the corresponding elementary excitations,[1,2] like phonon, magnon, electron, etc. The unique feature of this technique is that it can tell us symmetry, energy, and lifetime information about these excitations. When used in conjunction with pressure and magnetic field, Raman scattering can further provide pressure- and magnetic-field-dependent information about these excitations. These are proven valuable for understanding underlying physics and developing multiple applications. In the following we will present the applications of Raman scattering under high pressure and high magnetic field to a variety of condensed matter systems.
Under compression most solids eventually undergo one or more structural phase transformations. The scope extends from drastic changes in volume, symmetry, and electronic properties to subtle shifts of lattice parameters. Raman scattering can catch the symmetry of a high-pressure phase through the assignment of new features and/or excitations. Thus, it has been widely applied to probe structural properties of solids under high pressures and to identify pressure-induced phase transitions.[3–6] A good example is to resolve pressure-induced successive structural changes in bromine (iodine).[7] Bromine shows a structural phase transformation from the molecular phase I to the intermediate phase V near 80 GPa (20 GPa for Iodine), and then to the monatomic phase II around 118 GPa (31 GPa for Iodine), as illustrated in Fig.
Figures
The pressure dependence of the observed bands for iodine and bromine are shown in Figs.
A more detailed discussion about the pressure-induced phase transformations in Bromine and Iodine is beyond the scope of this review, but the above results clearly demonstrate that Raman scattering is powerful to probe structural properties of solids under high pressures and to identify pressure-induced phase transitions.
In addition to the structural phase transition, pressure can also drastically alter the properties of the electronic subsystem and induce an electronic transition. This can be observed by electronic Raman scattering via light scattering from electronic fluctuations at Fermi surface. A nice demonstration is seen in Bi1.98Sr2.06Y0.68Cu2O8+δ,[9] a slightly doped (δ ∼ 0.03) and insulating parent compound of cuprates. Raman scattering studies reveal a pressure-tuned electronic phase transition at ∼ 21 GPa.
Figure
In general, Raman cross section is related to optical conductivity in the following way: χ″ (Ω) ∝ Ωσ′(Ω), and thus a change in optical conductivity causes a change of low-frequency linear background of Raman cross section. Figure
On the other hand, two-magnon peak positions deviate from the power behavior (1/dCu−o)α at ∼ 21 GPa (Fig.
The above example clearly demonstrates that electronic Raman scattering under high pressure is a sensitive probe for pressure-induced electronic phase transition.
In Bi1.98Sr2.06Y0.68Cu2O8+δ, electronic phase transition is evidenced by electronic Raman signals. In fact, electronic information can also be obtained through its coupling to the lattice, i.e., phonons, particularly for the systems involving a strong electron–lattice coupling. Numerous studies have been conducted in this aspect.[13–16] A good example is 1T-TiSe2, which is a semimetal or small-gap semiconductor in the normal state and eventually develops into a commensurate CDW (inset of Fig.
Figure
Figures
(i) Crystalline CDW regime. From P = 0 to 5 kbar, the intensity of A1g-CDW slightly decreases, but its energy increases linearly with increasing pressure. This is similar to that of 203 cm−1 and 134 cm−1 modes (Figs.
(ii) Soft CDW regime. From P = 5 to 25 kbar, A1g-CDW mode exhibits a softening with a rapid decrease in intensity and linewidth (inset of Fig.
(iii) Disordered CDW regime. Above 25 kbar, both Eg-CDW and A1g-CDW modes are completely suppressed, which means that the CDW state has melted into a metallic or semi-metallic phase.
A detailed discussion on the pressure-induced phases of 1T-TiSe2 is beyond the scope of this review. Anyway, the results shown here demonstrate that the Raman scattering high pressure is a powerful tool to study electronic phase transition through electron–phonon coupling.
The application of magnetic field to Raman scattering allows us to study a lot of magnetic systems, particularly spin-lattice coupling systems. Magnetic-field-dependent Raman scattering has been widely used to study structural phase transitions.[17–23] Here a nice example is Mn3O4.[17] Raman scattering under magnetic fields offers field-dependent structural phase diagrams, and many microscopic details of field-dependent phase changes.
Mn3O4 has a spinel structure,
Figure
Electronic Raman scattering in metals results from electron–hole or multi-quasi-particle excitations around Fermi surface and may offer us charge and spin dynamics in different regions of Brillouin zone. For strongly correlated systems, incoherent quasiparticle scattering can usually lead to finite Raman intensity over a broad frequency range.[26,27] For the superconducting cuprates a flat continuum extending to at least 2 eV has been observed. On the other hand, the opening of a superconducting gap, Δ(
Figure
Generally, external magnetic fields have two main effects that can be detected by Raman scattering. One is single spin effect, Zeeman splitting[34–36] and the other one is energy band effect, the formation of Landau levels.[37–39] Magnetic fields can continuously tune excitation energies and thus offer us an opportunity to manipulate multiple interactions. In the following we take Raman studies of Co[N(CN)2]2 under magnetic fields[34] as a good example.
Figure
At 0 T, the intensity of the crystal-field excitation is too low to be observed, and there are only two phonons located at 147 and 158 cm−1. The excitation energy continuously increases with increasing magnetic fields due to Zeeman splitting. When the excitation energy is close to 147 cm−1 phonon frequency, it will strongly interacts with the phonon mode and becomes observable by transferring intensity from the phonon mode. With further increasing magnetic fields, the electronic excitation moves close to and interacts with the second phonon mode, and repeats a similar behavior as the first one. In this process, the energies of the three modes can be well described by the crystal-field-phonon coupling theory (Fig.
In addition to the structural and electronic phase transition, Raman scattering is also an excellent technique for studying magnetic excitations,[40–44] including one- and two-magnon excitations. As a combined application of low temperatures, magnetic fields, and pressure to light scattering studies of magnons, here we consider Ca3Ru2O7,[45,46] a bilayered, Ruddlesden-Popper-phase compound that exhibits a magnetic transition from a paramagnetic to an A-type antiferromagnetic state below TN = 56 K, and a structural transition below T0 ∼ 48 K.
As shown in Fig.
Figure
More interestingly, Raman spectra of the magnon mode has an anisotropic magnetic field dependence, as shown in Fig.
The interplay between different excitations in condensed matter, like phonons, magnons, electron–hole excitations, etc., plays a crucial role in determining their physical properties. Optically induced excitations typically leads to the creation of excited electron–hole pairs, which relax to a thermal equilibrium state in a fast time scale. For a modest level of optically induced excitations, the process of electron–hole excitation and subsequent relaxation can be considered as a near-equilibrium phenomenon and the structural and electronic properties of materials do not change much. However, optically induced phase transitions may occur if the density of optically induced excitations is high enough. Here we will discuss an example, A7 semimetal antimony,[53] which undergoes a structural phase transition when the pump excitation density exceeds 5 mJ/cm2.
Most elemental metals crystallize into a cubic or hexagonal closed structure. Crystalline Sb may be described as a distorted simple cubic structure (Fig.
The difference between A7 and “cubic” structures is that (111) plane of A7 structure has an alternating displacement along the [111] direction. This corresponds to Raman active A1g phonon mode (Fig.
One of the most intriguing open problems in condensed matter is the origin of high-temperature superconductivity. So far, the superconducting phase has been widely studied at thermal equilibrium through numerous techniques. However, few experiments have been performed to investigate their non-equilibrium properties. The development of Ti:Sapphire pulsed laser makes it possible. It was proposed that time-resolved Raman spectroscopy can be employed to probe the non-equilibrium properties of high-temperature superconductors (HTSCs).[55–57] The non-equilibrium properties in the superconducting phase has been first reported by Saichu et al.[55] They studied the superconducting order parameter in slightly over-doped Bi2Sr2CaCu2O8+δ(Bi-2212, Tc = 82 K) and found two different coupling mechanisms that contribute equally to the pair-breaking peak.
Figure
(i) The high-energy part of pair-breaking peak (∼ 420–600 cm−1, above the dashed line in Fig.
(ii) At the same time, the low-energy part of pair-breaking peak (∼ 220–420 cm−1 in Fig.
The work by Saichu et al.[55] have demonstrated that time-resolved Raman spectroscopy is an effective tool to probe the non-equilibrium electronic properties of HTSCs and shed light on the origin of superconductivity.
In this review, we have presented a brief introduction to Raman scattering, followed by a detailed description of the Raman facility belonging to the SECUF, including the system design and the capability the facility can provide. We have demonstrated that Raman scattering under extreme conditions (e.g., low temperature, high magnetic field, and high pressure), can effectively detect many elementary excitations including electron, phonon, magnon, etc., in a variety of correlated electron systems. The present review offers a glimpse of SECUF Raman facility and its possible applications. We hope that this may stimulate increased and multidisciplinary collaborations based on the facility in the future.
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